What bonds can be between carbon atoms. Concepts about the types of bonds between atoms, the valence states of carbon and the mechanism of organic reactions
Continuation. For the beginning, see № 15, 16/2004
Lesson 5
atomic orbitals of carbon
A covalent chemical bond is formed using common bonding electron pairs of the type:
Form a chemical bond, i.e. only unpaired electrons can create a common electron pair with a “foreign” electron from another atom. When writing electronic formulas, unpaired electrons are located one by one in the orbital cell.
atomic orbital is a function that describes the density of the electron cloud at each point in space around the nucleus of an atom. An electron cloud is a region of space in which an electron can be found with a high probability.
To harmonize the electronic structure of the carbon atom and the valency of this element, the concepts of excitation of the carbon atom are used. In the normal (unexcited) state, the carbon atom has two unpaired 2 R 2 electrons. In an excited state (when energy is absorbed) one of 2 s 2-electrons can pass to free R-orbital. Then four unpaired electrons appear in the carbon atom:
Recall that in the electronic formula of an atom (for example, for carbon 6 C - 1 s 2 2s 2 2p 2) large numbers in front of the letters - 1, 2 - indicate the number of the energy level. Letters s and R indicate the shape of the electron cloud (orbitals), and the numbers to the right above the letters indicate the number of electrons in a given orbital. All s- spherical orbitals:
At the second energy level except 2 s-there are three orbitals 2 R-orbitals. These 2 R-orbitals have an ellipsoidal shape, similar to dumbbells, and are oriented in space at an angle of 90 ° to each other. 2 R-Orbitals denote 2 p x, 2r y and 2 pz according to the axes along which these orbitals are located.
When chemical bonds are formed, the electron orbitals acquire the same shape. So, in saturated hydrocarbons, one s-orbital and three R-orbitals of a carbon atom to form four identical (hybrid) sp 3-orbitals:
It - sp 3 - hybridization.
Hybridization– alignment (mixing) of atomic orbitals ( s and R) with the formation of new atomic orbitals, called hybrid orbitals.
Hybrid orbitals have an asymmetric shape, elongated towards the attached atom. Electron clouds repel each other and are located in space as far as possible from each other. At the same time, the axes of four sp 3-hybrid orbitals turn out to be directed to the vertices of the tetrahedron (regular triangular pyramid).
Accordingly, the angles between these orbitals are tetrahedral, equal to 109°28".
The tops of electron orbitals can overlap with the orbitals of other atoms. If electron clouds overlap along a line connecting the centers of atoms, then such a covalent bond is called sigma()-bond. For example, in a C 2 H 6 ethane molecule, a chemical bond is formed between two carbon atoms by overlapping two hybrid orbitals. This is a connection. In addition, each of the carbon atoms with its three sp 3-orbitals overlap with s-orbitals of three hydrogen atoms, forming three -bonds.
In total, three valence states with different types of hybridization are possible for a carbon atom. Except sp 3-hybridization exists sp 2 - and sp-hybridization.
sp 2 -Hybridization- mixing one s- and two R-orbitals. As a result, three hybrid sp 2 -orbitals. These sp 2 -orbitals are located in the same plane (with axes X, at) and are directed to the vertices of the triangle with an angle between the orbitals of 120°. unhybridized
R-orbital is perpendicular to the plane of the three hybrid sp 2 orbitals (oriented along the axis z). Upper half R-orbitals are above the plane, the lower half is below the plane.
Type of sp 2-hybridization of carbon occurs in compounds with a double bond: C=C, C=O, C=N. Moreover, only one of the bonds between two atoms (for example, C=C) can be a bond. (The other bonding orbitals of the atom are directed in opposite directions.) The second bond is formed as a result of the overlap of non-hybrid R-orbitals on both sides of the line connecting the nuclei of atoms.
Covalent bond formed by lateral overlap R-orbitals of neighboring carbon atoms is called pi()-bond.
Education
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Due to less overlap of orbitals, the -bond is less strong than the -bond.
sp-Hybridization is a mixing (alignment in form and energy) of one s- and one
R-orbitals with the formation of two hybrid sp-orbitals. sp- Orbitals are located on the same line (at an angle of 180 °) and directed in opposite directions from the nucleus of the carbon atom. Two
R-orbitals remain unhybridized. They are placed perpendicular to each other.
directions - connections. On the image sp-orbitals are shown along the axis y, and the unhybridized two
R-orbitals - along the axes X and z.
The triple carbon-carbon bond CC consists of a -bond that occurs when overlapping
sp-hybrid orbitals, and two -bonds.
The relationship between such parameters of the carbon atom as the number of attached groups, the type of hybridization and the types of chemical bonds formed is shown in Table 4.
Table 4
Covalent bonds of carbon
| Number of groups related with carbon |
Type of hybridization |
Types participating chemical bonds |
Examples of compound formulas |
|---|---|---|---|
| 4 | sp 3 | Four - connections | |
| 3 | sp 2 | Three - connections and one is connection |
|
| 2 | sp | Two - connections and two connections |
H-CC-H |
Exercises.
1. What electrons of atoms (for example, carbon or nitrogen) are called unpaired?
2. What does the concept of "shared electron pairs" mean in compounds with a covalent bond (for example, CH 4 or H 2 S )?
3. What are the electronic states of atoms (for example, C or N ) are called basic, and which are excited?
4. What do the numbers and letters mean in the electronic formula of an atom (for example, C or N )?
5. What is an atomic orbital? How many orbitals are in the second energy level of a C atom and how do they differ?
6. What is the difference between hybrid orbitals and the original orbitals from which they were formed?
7. What types of hybridization are known for the carbon atom and what are they?
8. Draw a picture of the spatial arrangement of orbitals for one of the electronic states of the carbon atom.
9. What chemical bonds are called and what? Specify-and-connections in connections:
10. For the carbon atoms of the compounds below, indicate: a) the type of hybridization; b) types of its chemical bonds; c) bond angles.
Answers to exercises for topic 1
Lesson 5
1. Electrons that are one per orbital are called unpaired electrons. For example, in the electron diffraction formula of an excited carbon atom, there are four unpaired electrons, and the nitrogen atom has three:
2. Two electrons participating in the formation of one chemical bond are called common electron pair. Usually, before the formation of a chemical bond, one of the electrons of this pair belonged to one atom, and the other electron belonged to another atom:
3. The electronic state of the atom, in which the order of filling of electronic orbitals is observed: 1 s 2 , 2s 2 , 2p 2 , 3s 2 , 3p 2 , 4s 2 , 3d 2 , 4p 2 etc. are called main state. AT excited state one of the valence electrons of the atom occupies a free orbital with a higher energy, such a transition is accompanied by the separation of paired electrons. Schematically it is written like this:
Whereas in the ground state there were only two valence unpaired electrons, in the excited state there are four such electrons.
5.
An atomic orbital is a function that describes the density of an electron cloud at each point in space around the nucleus of a given atom. There are four orbitals on the second energy level of the carbon atom - 2 s, 2p x, 2r y,
2pz. These orbitals are:
a) the shape of the electron cloud ( s- ball, R- dumbbell);
b) R-orbitals have different orientations in space - along mutually perpendicular axes x, y and z, they are denoted p x, r y,
pz.
6.
Hybrid orbitals differ from the original (non-hybrid) orbitals in shape and energy. For example, s-orbital - the shape of a sphere, R- symmetrical figure eight, sp-hybrid orbital - asymmetric figure eight.
Energy Differences: E(s) < E(sp) < E(R). Thus, sp-orbital - an orbital averaged in shape and energy, obtained by mixing the initial s-
and p-orbitals.
7. Three types of hybridization are known for the carbon atom: sp 3 , sp 2 and sp (see the text of lesson 5).
9.
-bond - a covalent bond formed by frontal overlapping of orbitals along a line connecting the centers of atoms.
-bond - a covalent bond formed by lateral overlap R-orbitals on either side of the line connecting the centers of atoms.
- Bonds are shown by the second and third lines between the connected atoms.
In the ground state, the carbon atom C (1s 2 2s 2 2p 2) has two unpaired electrons, due to which only two common electron pairs can be formed. However, in most of its compounds, carbon is tetravalent. This is due to the fact that the carbon atom, absorbing a small amount of energy, goes into an excited state in which it has 4 unpaired electrons, i.e. able to form four covalent bonds and take part in the formation of four common electron pairs:
6 C 1s 2 2s 2 2p 2 6 C * 1s 2 2s 1 2p 3 .
| ↓ | ||||||||||||
| 1 | ↓ | p | ↓ | p | ||||||||
| s | s | |||||||||||
The excitation energy is compensated by the formation of chemical bonds, which occurs with the release of energy.
Carbon atoms have the ability to form three types of hybridization of electron orbitals ( sp 3, sp 2, sp) and the formation of multiple (double and triple) bonds between themselves (Table 2.2).
Table 2.2
Types of hybridization and geometry of molecules
A simple (single) s-bond occurs when sp 3-hybridization, in which all four hybrid orbitals are equivalent and have a spatial orientation at an angle of 109 ° 29 ’ to each other and are oriented to the vertices of a regular tetrahedron (Fig. 2.8).
Rice. 2.8. The formation of a methane CH 4 molecule
If hybrid orbitals of carbon overlap with spherical s-orbitals of the hydrogen atom, then the simplest organic compound methane CH 4 is formed - a saturated hydrocarbon.
Of great interest is the study of the bonds of carbon atoms with each other and with atoms of other elements. Consider the structure of the molecules of ethane, ethylene and acetylene.
The angles between all bonds in the ethane molecule are almost exactly equal to each other (Fig. 2.9) and do not differ from the C - H angles in the methane molecule.
Therefore, the carbon atoms are in the state sp 3-hybridization.
Rice. 2.9. Ethane molecule C 2 H 6
Hybridization of electron orbitals of carbon atoms can be incomplete, i.e. it can involve two sp 2-hybridization) or one ( sp-hybridization) of three R-orbitals. In this case, between the carbon atoms are formed multiple bonds (double or triple). Hydrocarbons with multiple bonds are called unsaturated or unsaturated. A double bond (C=C) is formed when sp 2-hybridization.
In this case, each of the carbon atoms has one of three R-orbitals are not involved in hybridization, resulting in the formation of three sp 2- hybrid orbitals located in the same plane at an angle of 120 ° to each other, and non-hybrid 2 R-orbital is perpendicular to this plane. Two carbon atoms are connected to each other, forming one s-bond due to the overlap of hybrid orbitals and one p-bond due to overlap R-orbitals.
Interaction of free hybrid orbitals of carbon with 1 s-orbitals of hydrogen atoms leads to the formation of an ethylene molecule C 2 H 4 (Fig. 2.10) - the simplest representative of unsaturated hydrocarbons.
Rice. 2.10. The formation of an ethylene molecule C 2 H 4
The overlap of electron orbitals in the case of a p-bond is less and the zones with increased electron density lie farther from the nuclei of atoms, so this bond is less strong than the s-bond.
A triple bond is formed by one s-bond and two p-bonds. In this case, the electron orbitals are in a state of sp-hybridization, the formation of which occurs due to one s- and one R-orbitals (Fig. 2.11).
The two hybrid orbitals are located at an angle of 180° relative to each other, and the remaining two non-hybrid R-orbitals are located in two mutually perpendicular planes. The formation of a triple bond takes place in the C 2 H 2 acetylene molecule (see Fig. 2.11).
Rice. 2.11. The formation of an acetylene molecule C 2 H 2
A special type of bond arises during the formation of a benzene molecule (C 6 H 6) - the simplest representative of aromatic hydrocarbons.
Benzene contains six carbon atoms linked together in a cycle (benzene ring), while each carbon atom is in a state of sp 2 hybridization (Fig. 2.12).
Rice. 2.12. sp 2 - orbitals of the benzene molecule C 6 H 6
All carbon atoms included in the benzene molecule are located in the same plane. Each carbon atom in the sp 2 hybridization state has another non-hybrid p-orbital with an unpaired electron, which forms a p-bond (Fig. 2.13).
Axis like this R-orbital is located perpendicular to the plane of the benzene molecule.
All six non-hybrid R-orbitals form a common bonding molecular p-orbital, and all six electrons are combined into a p-electron sextet.
The boundary surface of such an orbital is located above and below the carbon s-skeleton plane. As a result of circular overlap, a single delocalized p-system arises, covering all carbon atoms of the cycle (Fig. 2.13).
Benzene is schematically depicted as a hexagon with a ring inside, which indicates that there is a delocalization of electrons and the corresponding bonds.
Rice. 2.13. -bonds in the benzene molecule C 6 H 6
Ionic chemical bond
Ionic bond- a chemical bond formed as a result of mutual electrostatic attraction of oppositely charged ions, in which a stable state is achieved by a complete transition of the total electron density to an atom of a more electronegative element.
A purely ionic bond is the limiting case of a covalent bond.
In practice, a complete transition of electrons from one atom to another atom through a bond is not realized, since each element has a greater or lesser (but not zero) EO, and any chemical bond will be covalent to some extent.
Such a bond arises in the case of a large difference in the ER of atoms, for example, between cations s-metals of the first and second groups of the periodic system and anions of non-metals of groups VIA and VIIA (LiF, NaCl, CsF, etc.).
Unlike a covalent bond, ionic bond has no direction . This is explained by the fact that the electric field of the ion has spherical symmetry, i.e. decreases with distance according to the same law in any direction. Therefore, the interaction between ions is independent of direction.
The interaction of two ions of opposite sign cannot lead to complete mutual compensation of their force fields. Because of this, they retain the ability to attract ions of the opposite sign in other directions. Therefore, unlike a covalent bond, ionic bond is also characterized by unsaturability .
The lack of orientation and saturation of the ionic bond causes the tendency of ionic molecules to associate. All ionic compounds in the solid state have an ionic crystal lattice in which each ion is surrounded by several ions of the opposite sign. In this case, all bonds of a given ion with neighboring ions are equivalent.
metal connection
Metals are characterized by a number of special properties: electrical and thermal conductivity, characteristic metallic luster, malleability, high ductility, and high strength. These specific properties of metals can be explained by a special type of chemical bond called metallic .
A metallic bond is the result of overlapping delocalized orbitals of atoms approaching each other in the crystal lattice of a metal.
Most metals have a significant number of vacant orbitals and a small number of electrons at the outer electronic level.
Therefore, it is energetically more favorable that the electrons are not localized, but belong to the entire metal atom. At the lattice sites of a metal, there are positively charged ions that are immersed in an electron "gas" distributed throughout the metal:
Me ↔ Me n + + n .
Between positively charged metal ions (Me n +) and non-localized electrons (n) there is an electrostatic interaction that ensures the stability of the substance. The energy of this interaction is intermediate between the energies of covalent and molecular crystals. Therefore, elements with a purely metallic bond ( s-, and p-elements) are characterized by relatively high melting points and hardness.
The presence of electrons, which can freely move around the volume of the crystal, and provide specific properties of the metal
hydrogen bond
hydrogen bond – a special type of intermolecular interaction. Hydrogen atoms that are covalently bonded to an atom of an element that has a high electronegativity value (most commonly F, O, N, but also Cl, S, and C) carry a relatively high effective charge. As a result, such hydrogen atoms can electrostatically interact with the atoms of these elements.
So, the H d + atom of one water molecule is oriented and accordingly interacts (as shown by three points) with the O d atom - another water molecule:
The bonds formed by an H atom located between two atoms of electronegative elements are called hydrogen bonds:
d- d+ d-
A − H × × × B
The energy of a hydrogen bond is much less than the energy of an ordinary covalent bond (150–400 kJ / mol), but this energy is sufficient to cause the aggregation of molecules of the corresponding compounds in a liquid state, for example, in liquid hydrogen fluoride HF (Fig. 2.14). For fluorine compounds, it reaches about 40 kJ/mol.
Rice. 2.14. Aggregation of HF molecules due to hydrogen bonds
The length of the hydrogen bond is also less than the length of the covalent bond. So, in the polymer (HF) n, the F−H bond length is 0.092 nm, and the F∙∙∙H bond is 0.14 nm. For water, the O−H bond length is 0.096 nm, and the O∙∙∙H bond length is 0.177 nm.
The formation of intermolecular hydrogen bonds leads to a significant change in the properties of substances: an increase in viscosity, dielectric constant, boiling and melting points.
Similar information.
Most organic compounds have a molecular structure. Atoms in substances with a molecular type of structure always form only covalent bonds with each other, which is also observed in the case of organic compounds. Recall that a covalent bond is such a type of bond between atoms, which is realized due to the fact that atoms socialize part of their outer electrons in order to acquire the electronic configuration of a noble gas.
According to the number of socialized electron pairs, covalent bonds in organic substances can be divided into single, double and triple. These types of connections are indicated in the graphic formula, respectively, by one, two or three lines:
The multiplicity of the bond leads to a decrease in its length, so a single C-C bond has a length of 0.154 nm, a double C=C bond - 0.134 nm, a triple C≡C bond - 0.120 nm.
Types of bonds according to the way the orbitals overlap
As is known, orbitals can have different shapes, for example, s-orbitals are spherical, and p-dumbbell-shaped. For this reason, bonds can also differ in the way electron orbitals overlap:
ϭ-bonds - are formed when the orbitals overlap in such a way that the region of their overlap is intersected by a line connecting the nuclei. Examples of ϭ-bonds:
π-bonds - are formed when the orbitals overlap, in two areas - above and below the line connecting the nuclei of atoms. Examples of π bonds:
How to know when there are π- and ϭ-bonds in a molecule?
With a covalent type of bond, there is always a ϭ-bond between any two atoms, and it has a π-bond only in the case of multiple (double, triple) bonds. Wherein:
- Single bond - always a ϭ-bond
- A double bond always consists of one ϭ- and one π-bond
- A triple bond is always formed by one ϭ and two π bonds.
Let us indicate these types of bonds in the propinoic acid molecule:
Hybridization of carbon atom orbitals
Orbital hybridization is a process in which orbitals that originally have different shapes and energies are mixed, forming in return the same number of hybrid orbitals, equal in shape and energy.
For example, when mixing one s- and three p- four orbitals are formed sp 3-hybrid orbitals:
In the case of carbon atoms, hybridization always takes part s- orbital, and the number p-orbitals that can take part in hybridization varies from one to three p- orbitals.
How to determine the type of hybridization of a carbon atom in an organic molecule?
Depending on how many other atoms a carbon atom is bonded to, it is either in the state sp 3, or in the state sp 2, or in the state sp- hybridization:
Let's practice determining the type of hybridization of carbon atoms using the example of the following organic molecule:
The first carbon atom is bonded to two other atoms (1H and 1C), so it is in the state sp-hybridization.
- The second carbon atom is bonded to two atoms - sp-hybridization
- The third carbon atom is bonded to four other atoms (two C and two H) - sp 3-hybridization
- The fourth carbon atom is bonded to three other atoms (2O and 1C) - sp 2-hybridization.
Radical. Functional group
The term "radical" most often means a hydrocarbon radical, which is the remainder of a molecule of any hydrocarbon without one hydrogen atom.
The name of the hydrocarbon radical is formed based on the name of the corresponding hydrocarbon by replacing the suffix –an to suffix –silt .
Functional group - a structural fragment of an organic molecule (a certain group of atoms), which is responsible for its specific chemical properties.
Depending on which of the functional groups in the molecule of the substance is the eldest, the compound is assigned to one or another class.
R is the designation of a hydrocarbon substituent (radical).
Radicals can contain multiple bonds, which can also be considered as functional groups, since multiple bonds contribute to the chemical properties of the substance.
If an organic molecule contains two or more functional groups, such compounds are called polyfunctional.
Variety of inorganic and organic substances
Organic chemistry is chemistry carbon compounds. Inorganic carbon compounds include: carbon oxides, carbonic acid, carbonates and bicarbonates, carbides. Organic matter other than carbon contain hydrogen, oxygen, nitrogen, phosphorus, sulfur and other elements. Carbon atoms can form long unbranched and branched chains, rings, attach other elements, so the number of organic compounds has approached 20 million, while there are a little more than 100 thousand inorganic substances.
The basis for the development of organic chemistry is the theory of the structure of organic compounds by A. M. Butlerov. An important role in describing the structure of organic compounds belongs to the concept of valency, which characterizes the ability of atoms to form chemical bonds and determines their number. Carbon in organic compounds always tetravalent. The main postulate of the theory of A. M. Butlerov is the position on the chemical structure of matter, that is, the chemical bond. This order is displayed using structural formulas. Butlerov's theory states the idea that every substance has certain chemical structure and the properties of substances depend on the structure.
Theory of the chemical structure of organic compounds A. M. Butlerova
Just as for inorganic chemistry the basis of development is the Periodic law and the Periodic system of chemical elements of D. I. Mendeleev, for organic chemistry it has become fundamental.
Theory of the chemical structure of organic compounds A. M. Butlerova The main postulate of Butlerov's theory is the position on the chemical structure of a substance, which is understood as the order, the sequence of the mutual combination of atoms into molecules, i.e. chemical bond.
Chemical structure- the order of connection of atoms of chemical elements in a molecule according to their valency.
This order can be displayed using structural formulas, in which the valences of atoms are indicated by dashes: one dash corresponds to the unit of valence of an atom of a chemical element. For example, for the organic substance methane, which has the molecular formula CH 4, the structural formula looks like this:
The main provisions of the theory of A. M. Butlerov:
The atoms in the molecules of organic substances are connected to each other according to their valency. Carbon in organic compounds is always tetravalent, and its atoms are able to combine with each other, forming various chains.
The properties of substances are determined not only by their qualitative and quantitative composition, but also by the order of connection of atoms in a molecule, i.e. the chemical structure of matter.
The properties of organic compounds depend not only on the composition of the substance and the order of connection of atoms in its molecule, but also on mutual influence of atoms and groups of atoms to each other.
The theory of the structure of organic compounds is a dynamic and developing doctrine. With the development of knowledge about the nature of the chemical bond, about the influence of the electronic structure of the molecules of organic substances, they began to use, in addition to empirical and structural, electronic formulas. These formulas show the direction shifts of electron pairs in a molecule.
Quantum chemistry and the chemistry of the structure of organic compounds confirmed the theory of the spatial direction of chemical bonds (cis- and trans isomerism), studied the energy characteristics of mutual transitions in isomers, made it possible to judge the mutual influence of atoms in the molecules of various substances, created the prerequisites for predicting the types of isomerism and directions and mechanisms of chemical reactions.
Organic substances have a number of features.
The composition of all organic substances includes carbon and hydrogen, therefore, when burned, they form carbon dioxide and water.
・Organic matter built complex and can have a huge molecular weight (proteins, fats, carbohydrates).
Organic substances can be arranged in rows similar in composition, structure and properties homologues.
For organic substances, it is characteristic isomerism.
Isomerism and homology of organic substances
The properties of organic substances depend not only on their composition, but also on order of connection of atoms in a molecule.
isomerism- this is the phenomenon of the existence of different substances - isomers with the same qualitative and quantitative composition, i.e. with the same molecular formula.
There are two types of isomerism: structural and spatial(stereoisomerism). Structural isomers differ from each other in the order of bonding of atoms in a molecule; stereoisomers - the arrangement of atoms in space with the same order of bonds between them.
The main types of isomerism:
Structural isomerism - substances differ in the order of bonding of atoms in molecules:
1) isomerism of the carbon skeleton;
2) position isomerism:
- multiple bonds;
- deputies;
- functional groups;
3) isomerism of homological series (interclass).
· Spatial isomerism - molecules of substances differ not in the order of connection of atoms, but in their position in space: cis-, trans-isomerism (geometric).
Classification of organic substances
It is known that the properties of organic substances are determined by their composition and chemical structure. Therefore, it is not surprising that the classification of organic compounds is based on the theory of structure - the theory of A. M. Butlerov. Classify organic substances by the presence and order of connection of atoms in their molecules. The most durable and least changeable part of an organic substance molecule is its skeleton - a chain of carbon atoms. Depending on the order of connection of carbon atoms in this chain, substances are divided into acyclic, not containing closed chains of carbon atoms in molecules, and carbocyclic containing such chains (cycles) in molecules.
In addition to carbon and hydrogen atoms, molecules of organic substances may contain atoms of other chemical elements. Substances in the molecules of which these so-called heteroatoms are included in a closed chain are classified as heterocyclic compounds.
heteroatoms(oxygen, nitrogen, etc.) can be part of molecules and acyclic compounds, forming functional groups in them, for example,
hydroxyl
carbonyl
,
carboxyl
,
amino group
.
Functional group- a group of atoms that determines the most characteristic chemical properties of a substance and its belonging to a certain class of compounds.
Nomenclature of organic compounds
At the beginning of the development of organic chemistry, discovered compounds were assigned trivial names, often associated with the history of their production: acetic acid (which is the basis of wine vinegar), butyric acid (formed in butter), glycol (i.e. "sweet"), etc. As the number of new discovered substances increased, the need arose associate names with their structure. This is how rational names appeared: methylamine, diethylamine, ethyl alcohol, methyl ethyl ketone, which are based on the name of the simplest compound. For more complex compounds, rational nomenclature is unsuitable.
The theory of structure of A. M. Butlerov provided the basis for the classification and nomenclature of organic compounds according to structural elements and the arrangement of carbon atoms in a molecule. Currently, the most used is the nomenclature developed by International Union of Pure and Applied Chemistry (IUPAC), which is called the nomenclature IUPAC. IUPAC rules recommend several principles for the formation of names, one of them is the principle of substitution. Based on this, a replacement nomenclature has been developed, which is the most universal. Here are a few basic rules of substitution nomenclature and consider their application using the example of a heterofunctional compound containing two functional groups - the amino acid leucine:
1. The name of the compounds is based on the parent structure (the main chain of an acyclic molecule, a carbocyclic or heterocyclic system). The name of the ancestral structure is the basis of the name, the root of the word.
In this case, the parent structure is a chain of five carbon atoms linked by single bonds. Thus, the root part of the name is pentane.
2. Characteristic groups and substituents (structural elements) are denoted by prefixes and suffixes. Characteristic groups are subdivided according to seniority. The order of precedence of the main groups:
The senior characteristic group is identified, which is designated in the suffix. All other substituents are named in the prefix in alphabetical order.
In this case, the senior characteristic group is carboxyl, i.e. this compound belongs to the class of carboxylic acids, so we add -oic acid to the root part of the name. The second most senior group is the amino group, which is denoted by the prefix amino-. In addition, the molecule contains a hydrocarbon substituent methyl-. Thus, the basis of the name is aminomethylpentanoic acid.
3. The name includes the designation of a double and triple bond, which comes immediately after the root.
The compound under consideration does not contain multiple bonds.
4. The atoms of the parent structure are numbered. Numbering starts from the end of the carbon chain, which is closer to the highest characteristic group:
The chain numbering starts from the carbon atom that is part of the carboxyl group, it is assigned the number 1. In this case, the amino group will be at carbon 2, and methyl at carbon 4.
Thus, the natural amino acid leucine, according to the IUPAC nomenclature rules, is called 2-amino-4-methylpentanoic acid.
Hydrocarbons. Hydrocarbon classification
hydrocarbons are compounds that consist only of hydrogen and carbon atoms.
Depending on the structure of the carbon chain, organic compounds are divided into compounds with an open chain - acyclic(aliphatic) and cyclic- with a closed chain of atoms.
Cycles are divided into two groups: carbocyclic compounds(cycles are formed only by carbon atoms) and heterocyclic(the cycles also include other atoms, such as oxygen, nitrogen, sulfur).
Carbocyclic compounds, in turn, include two series of compounds: alicyclic and aromatic.
Aromatic compounds in the basis of the structure of molecules have planar carbon-containing cycles with a special closed system of p-electrons, forming a common π-system (a single π-electron cloud). Aromaticity is also characteristic of many heterocyclic compounds.
All other carbocyclic compounds belong to the alicyclic series.
Both acyclic (aliphatic) and cyclic hydrocarbons can contain multiple (double or triple) bonds. These hydrocarbons are called unlimited(unsaturated), in contrast to the limiting (saturated), containing only single bonds.
Limit aliphatic hydrocarbons are called alkanes, they have the general formula C n H 2n+2, where n is the number of carbon atoms. Their old name is often used today - paraffins:
Unsaturated aliphatic hydrocarbons containing one double bond are called alkenes. They have the general formula C n H 2n:
Unsaturated aliphatic hydrocarbons with two double bonds are called alkadienes. Their general formula is C n H 2n-2:
Unsaturated aliphatic hydrocarbons with one triple bond are called alkynes. Their general formula is C n H 2n - 2:
Limit alicyclic hydrocarbons - cycloalkanes, their general formula C n H 2n:
A special group of hydrocarbons, aromatic, or arenes(with a closed common n-electron system), known from the example of hydrocarbons with the general formula C n H 2n - 6:
Thus, if in their molecules one or more hydrogen atoms are replaced by other atoms or groups of atoms (halogens, hydroxyl groups, amino groups, etc.), hydrocarbon derivatives are formed: halogen derivatives, oxygen-containing, nitrogen-containing and other organic compounds.
Homologous series of hydrocarbons
Hydrocarbons and their derivatives with the same functional group form homologous series.
Homologous series a number of compounds belonging to the same class (homologues) are called, arranged in ascending order of their relative molecular weights, similar in structure and chemical properties, where each member differs from the previous one by the homological difference CH 2 . For example: CH 4 - methane, C 2 H 6 - ethane, C 3 H 8 - propane, C 4 H 10 - butane, etc. The similarity of the chemical properties of homologues greatly simplifies the study of organic compounds.
Isomers of hydrocarbons
Those atoms or groups of atoms that determine the most characteristic properties of a given class of substances are called functional groups.
Halogen derivatives of hydrocarbons can be considered as products of substitution in hydrocarbons of one or more hydrogen atoms by halogen atoms. In accordance with this, there can be limiting and unlimiting mono-, di-, tri- (in the general case, poly-) halogen derivatives.
The general formula of monohalogen derivatives of saturated hydrocarbons:
and the composition is expressed by the formula
where R is the remainder of the saturated hydrocarbon (alkane), hydrocarbon radical (this designation is used further when considering other classes of organic substances), Г is a halogen atom (F, Cl, Br, I).
For example:
Here is one example of a dihalogen derivative:
To oxygenated organic matter include alcohols, phenols, aldehydes, ketones, carboxylic acids, ethers and esters. Alcohols are derivatives of hydrocarbons in which one or more hydrogen atoms are replaced by hydroxyl groups.
Alcohols are called monohydric if they have one hydroxyl group, and limiting if they are derivatives of alkanes.
General formula for limit monohydric alcohols:
and their composition is expressed by the general formula:
For example:
Known examples polyhydric alcohols, i.e. having several hydroxyl groups:
Phenols- derivatives of aromatic hydrocarbons (benzene series), in which one or more hydrogen atoms in the benzene ring are replaced by hydroxyl groups.
The simplest representative with the formula C 6 H 5 OH or
called phenol.
Aldehydes and ketones- derivatives of hydrocarbons containing a carbonyl group of atoms
(carbonyl).
in molecules aldehydes one bond of the carbonyl goes to the connection with the hydrogen atom, the other - with the hydrocarbon radical. General formula of aldehydes:
For example:
When ketones the carbonyl group is bonded to two (generally different) radicals, the general formula of ketones is:
For example:
The composition of limiting aldehydes and ketones is expressed by the formula C 2n H 2n O.
carboxylic acids- derivatives of hydrocarbons containing carboxyl groups
(or -COOH).
If there is one carboxyl group in the acid molecule, then the carboxylic acid is monobasic. The general formula of saturated monobasic acids:
Their composition is expressed by the formula C n H 2n O 2 .
For example:
Ethers are organic substances containing two hydrocarbon radicals connected by an oxygen atom: R-O-R or R 1 -O-R 2 .
The radicals may be the same or different. The composition of ethers is expressed by the formula C n H 2n+2 O.
For example:
Esters- compounds formed by replacing the hydrogen atom of the carboxyl group in carboxylic acids with a hydrocarbon radical.
General formula of esters:
For example:
Nitro compounds- derivatives of hydrocarbons in which one or more hydrogen atoms are replaced by a nitro group -NO 2 .
General formula of limiting mononitro compounds:
and the composition is expressed by the general formula C n H 2n+1 NO 2 .
For example:
Arene nitro derivatives:
Amines- compounds that are considered as derivatives of ammonia (NH 3), in which hydrogen atoms are replaced by hydrocarbon radicals. Depending on the nature of the radical, amines can be aliphatic, for example:
and aromatic, for example:
Depending on the number of hydrogen atoms replaced by radicals, there are:
primary amines with the general formula:
secondary- with the general formula:
tertiary- with the general formula:
In a particular case, secondary as well as tertiary amines may have the same radicals.
Primary amines can also be considered as derivatives of hydrocarbons (alkanes), in which one hydrogen atom is replaced by an amino group -NH 2 . The composition of limiting primary amines is expressed by the formula C n H 2n + 3 N.
For example:
Amino acids contain two functional groups connected to a hydrocarbon radical: the amino group -NH 2 and carboxyl -COOH.
The general formula of α-amino acids (they are most important for building the proteins that make up living organisms):
The composition of limiting amino acids containing one amino group and one carboxyl is expressed by the formula C n H 2n+1 NO 2.
For example:
Other important organic compounds are known that have several different or identical functional groups, long linear chains associated with benzene rings. In such cases, a strict definition of whether a substance belongs to a particular class is impossible. These compounds are often isolated into specific groups of substances: carbohydrates, proteins, nucleic acids, antibiotics, alkaloids, etc.
Currently, there are also many compounds that can be classified as both organic and inorganic. x are called organoelement compounds. Some of them can be considered as derivatives of hydrocarbons.
For example:
There are compounds that have the same molecular formula expressing the composition of substances.
The phenomenon of isomerism consists in the fact that there can be several substances with different properties that have the same composition of molecules, but different structures. These substances are called isomers.
In our case, these are interclass isomers: cycloalkanes and alkanes, alkadienes and alkynes, saturated monohydric alcohols and ethers, aldehydes and ketones, saturated monobasic carboxylic acids and esters.
Structural isomerism
There are the following varieties structural isomerism: carbon skeleton isomerism, position isomerism, isomerism of various classes of organic compounds (interclass isomerism).
The isomerism of the carbon skeleton is due to different bond order between carbon atoms that form the skeleton of the molecule. As already shown, the molecular formula C 4 H 10 corresponds to two hydrocarbons: n-butane and isobutane. For the hydrocarbon C 5 H 12, three isomers are possible: pentane, isopentane and neopentane.
With an increase in the number of carbon atoms in a molecule, the number of isomers increases rapidly. For the hydrocarbon C 10 H 22 there are already 75 of them, and for the hydrocarbon C 20 H 44 - 366 319.
Position isomerism is due to the different position of the multiple bond, substituent, functional group with the same carbon skeleton of the molecule:
The isomerism of various classes of organic compounds (interclass isomerism) is due to the different position and combination of atoms in the molecules of substances that have the same molecular formula, but belong to different classes. So, the molecular formula C 6 H 12 corresponds to the unsaturated hydrocarbon hexene-1 and the cyclic hydrocarbon cyclohexane.
The isomers are a hydrocarbon related to alkynes - butyne-1 and a hydrocarbon with two double bonds in the butadiene-1,3 chain:
Diethyl ether and butyl alcohol have the same molecular formula C 4 H 10 O:
Structural isomers are aminoacetic acid and nitroethane, corresponding to the molecular formula C 2 H 5 NO 2:
Isomers of this type contain different functional groups and belong to different classes of substances. Therefore, they differ in physical and chemical properties much more than carbon skeleton isomers or position isomers.
Spatial isomerism
Spatial isomerism divided into two types: geometric and optical.
Geometric isomerism is characteristic of compounds, containing double bonds, and cyclic compounds. Since the free rotation of atoms around a double bond or in a cycle is impossible, substituents can be located either on one side of the plane of the double bond or cycle (cis position), or on opposite sides (transposition). The designations cis- and trans- usually refer to a pair of identical substituents.
Geometric isomers differ in physical and chemical properties.
Optical isomerism occurs if the molecule is incompatible with its image in the mirror. This is possible when the carbon atom in the molecule has four different substituents. This atom is called asymmetric. An example of such a molecule is the α-aminopropionic acid (α-alanine) CH 3 CH(NH 2)OH molecule.
The α-alanine molecule cannot coincide with its mirror image under any movement. Such spatial isomers are called mirror, optical antipodes, or enantiomers. All physical and almost all chemical properties of such isomers are identical.
The study of optical isomerism is necessary when considering many reactions occurring in the body. Most of these reactions are under the action of enzymes - biological catalysts. The molecules of these substances must approach the molecules of the compounds on which they act like a key to a lock, therefore, the spatial structure, the relative position of the molecular regions and other spatial factors are of great importance for the course of these reactions. Such reactions are called stereoselective.
Most natural compounds are individual enantiomers, and their biological action (from taste and smell to medicinal action) differs sharply from the properties of their optical antipodes obtained in the laboratory. Such a difference in biological activity is of great importance, since it underlies the most important property of all living organisms - metabolism.
isomerism The electronic structure of the carbon atom
Carbon, which is part of organic compounds, exhibits a constant valency. The last energy level of the carbon atom contains 4 electrons, two of which occupy the 2s orbital, which has a spherical shape, and two electrons occupy the 2p orbitals, which have a dumbbell shape. When excited, one electron from the 2s orbital can go to one of the vacant 2p orbitals. This transition requires some energy costs (403 kJ/mol). As a result, the excited carbon atom has 4 unpaired electrons and its electronic configuration is expressed by the formula 2s 1 2p 3 .. Thus, in the case of the methane hydrocarbon (CH 4), the carbon atom forms 4 bonds with the s-electrons of hydrogen atoms. In this case, 1 bond of the s-s type (between the s-electron of the carbon atom and the s-electron of the hydrogen atom) and 3 p-s bonds (between 3 p-electrons of the carbon atom and 3 s-electrons of 3 hydrogen atoms) should have been formed. This leads to the conclusion that the four covalent bonds formed by the carbon atom are not equivalent. However, the practical experience of chemistry indicates that all 4 bonds in the methane molecule are absolutely equivalent, and the methane molecule has a tetrahedral structure with valence angles of 109.5 0, which could not be the case if the bonds were not equivalent. After all, only the orbitals of p-electrons are oriented in space along mutually perpendicular axes x, y, z, and the orbital of an s-electron has a spherical shape, so the direction of formation of a bond with this electron would be arbitrary. The theory of hybridization was able to explain this contradiction. L. Polling suggested that in any molecules there are no bonds isolated from each other. When bonds are formed, the orbitals of all valence electrons overlap. Several types are known hybridization of electron orbitals. It is assumed that in the molecule of methane and other alkanes 4 electrons enter into hybridization.
Hybridization of carbon atom orbitals
Hybridization of orbitals- this is a change in the shape and energy of some electrons during the formation of a covalent bond, leading to a more efficient overlap of orbitals and an increase in the strength of bonds. Orbital hybridization always occurs when electrons belonging to different types of orbitals participate in the formation of bonds.
1. sp 3 -hybridization(the first valence state of carbon). With sp 3 hybridization, 3 p-orbitals and one s-orbital of an excited carbon atom interact in such a way that orbitals are obtained that are absolutely identical in energy and symmetrically located in space. This transformation can be written like this:
During hybridization, the total number of orbitals does not change, but only their energy and shape change. It is shown that the sp 3 hybridization of the orbitals resembles a three-dimensional figure-eight, one of the blades of which is much larger than the other. Four hybrid orbitals are extended from the center to the vertices of a regular tetrahedron at angles of 109.5 0 . The bonds formed by hybrid electrons (for example, the s-sp 3 bond) are stronger than the bonds made by unhybridized p-electrons (for example, the s-p bond). Because the hybrid sp 3 orbital provides a larger area of electron orbital overlap than the unhybridized p orbital. Molecules in which sp 3 hybridization is carried out have a tetrahedral structure. In addition to methane, these include methane homologues, inorganic molecules such as ammonia. The figures show a hybridized orbital and a tetrahedral methane molecule.
Chemical bonds that arise in methane between carbon and hydrogen atoms are of the type of σ-bonds (sp 3 -s-bond). Generally speaking, any sigma bond is characterized by the fact that the electron density of two interconnected atoms overlaps along the line connecting the centers (nuclei) of atoms. σ-Bonds correspond to the maximum possible degree of overlap of atomic orbitals, so they are strong enough.
2. sp 2 -hybridization(the second valence state of carbon). Occurs as a result of the overlap of one 2s and two 2p orbitals. The resulting sp 2 hybrid orbitals are located in the same plane at an angle of 120 0 to each other, and the unhybridized p orbital is perpendicular to it. The total number of orbitals does not change - there are four of them.
The sp 2 hybridization state occurs in alkene molecules, in carbonyl and carboxyl groups, i.e. in compounds containing a double bond. So, in the ethylene molecule, the hybridized electrons of the carbon atom form 3 σ-bonds (two sp 2 -s type bonds between the carbon atom and hydrogen atoms and one sp 2 -sp 2 type bond between carbon atoms). The remaining unhybridized p-electron of one carbon atom forms a π-bond with the unhybridized p-electron of the second carbon atom. A characteristic feature of the π bond is that the overlap of electron orbitals goes beyond the line connecting the two atoms. Orbital overlap goes above and below the σ-bond connecting both carbon atoms. Thus, a double bond is a combination of σ- and π-bonds. The first two figures show that in the ethylene molecule the bond angles between the atoms that form the ethylene molecule are 120 0 (corresponding to the orientation of three sp 2 hybrid orbitals in space). The figures show the formation of a π bond.
Since the area of overlap of unhybridized p-orbitals in π-bonds is less than the area of overlapping of orbitals in σ-bonds, the π-bond is less strong than the σ-bond and is more easily broken in chemical reactions.
3. sp hybridization(the third valence state of carbon). In the state of sp-hybridization, the carbon atom has two sp-hybrid orbitals located linearly at an angle of 180 0 to each other and two unhybridized p-orbitals located in two mutually perpendicular planes. sp hybridization is characteristic of alkynes and nitriles; for compounds containing a triple bond.
So, in an acetylene molecule, the bond angles between atoms are 180 o. The hybridized electrons of a carbon atom form 2 σ-bonds (one sp-s bond between a carbon atom and a hydrogen atom and another sp-sp type bond between carbon atoms. Two unhybridized p-electrons of one carbon atom form two π-bonds with unhybridized p electrons of the second The overlap of p-electron orbitals goes not only above and below the σ-bond, but also in front and behind, and the total p-electron cloud has a cylindrical shape.Thus, a triple bond is a combination of one σ-bond and two π-bonds. The presence of less strong two π-bonds in the acetylene molecule ensures the ability of this substance to enter into addition reactions with the breaking of the triple bond.
Reference material for passing the test:
periodic table
Solubility table
169375 0
Each atom has a certain number of electrons.
Entering into chemical reactions, atoms donate, acquire, or socialize electrons, reaching the most stable electronic configuration. The configuration with the lowest energy is the most stable (as in noble gas atoms). This pattern is called the "octet rule" (Fig. 1).
Rice. one.
This rule applies to all connection types. Electronic bonds between atoms allow them to form stable structures, from the simplest crystals to complex biomolecules that eventually form living systems. They differ from crystals in their continuous metabolism. However, many chemical reactions proceed according to the mechanisms electronic transfer, which play an important role in the energy processes in the body.
A chemical bond is a force that holds together two or more atoms, ions, molecules, or any combination of them..
The nature of the chemical bond is universal: it is an electrostatic force of attraction between negatively charged electrons and positively charged nuclei, determined by the configuration of the electrons in the outer shell of atoms. The ability of an atom to form chemical bonds is called valence, or oxidation state. The concept of valence electrons- electrons that form chemical bonds, that is, those located in the most high-energy orbitals. Accordingly, the outer shell of an atom containing these orbitals is called valence shell. At present, it is not enough to indicate the presence of a chemical bond, but it is necessary to clarify its type: ionic, covalent, dipole-dipole, metallic.
The first type of connection isionic connection
According to Lewis and Kossel's electronic theory of valency, atoms can achieve a stable electronic configuration in two ways: first, by losing electrons, becoming cations, secondly, acquiring them, turning into anions. As a result of electron transfer, due to the electrostatic force of attraction between ions with charges of the opposite sign, a chemical bond is formed, called Kossel " electrovalent(now called ionic).
In this case, anions and cations form a stable electronic configuration with a filled outer electron shell. Typical ionic bonds are formed from cations of T and II groups of the periodic system and anions of non-metallic elements of groups VI and VII (16 and 17 subgroups - respectively, chalcogens and halogens). The bonds in ionic compounds are unsaturated and non-directional, so they retain the possibility of electrostatic interaction with other ions. On fig. 2 and 3 show examples of ionic bonds corresponding to the Kossel electron transfer model.
Rice. 2.
Rice. 3. Ionic bond in the sodium chloride (NaCl) molecule
Here it is appropriate to recall some of the properties that explain the behavior of substances in nature, in particular, to consider the concept of acids and grounds.
Aqueous solutions of all these substances are electrolytes. They change color in different ways. indicators. The mechanism of action of indicators was discovered by F.V. Ostwald. He showed that the indicators are weak acids or bases, the color of which in the undissociated and dissociated states is different.
Bases can neutralize acids. Not all bases are soluble in water (for example, some organic compounds that do not contain -OH groups are insoluble, in particular, triethylamine N (C 2 H 5) 3); soluble bases are called alkalis.
Aqueous solutions of acids enter into characteristic reactions:
a) with metal oxides - with the formation of salt and water;
b) with metals - with the formation of salt and hydrogen;
c) with carbonates - with the formation of salt, CO 2 and H 2 O.
The properties of acids and bases are described by several theories. In accordance with the theory of S.A. Arrhenius, an acid is a substance that dissociates to form ions H+ , while the base forms ions HE- . This theory does not take into account the existence of organic bases that do not have hydroxyl groups.
In line with proton Bronsted and Lowry's theory, an acid is a substance containing molecules or ions that donate protons ( donors protons), and the base is a substance consisting of molecules or ions that accept protons ( acceptors protons). Note that in aqueous solutions, hydrogen ions exist in a hydrated form, that is, in the form of hydronium ions H3O+ . This theory describes reactions not only with water and hydroxide ions, but also carried out in the absence of a solvent or with a non-aqueous solvent.
For example, in the reaction between ammonia NH 3 (weak base) and hydrogen chloride in the gas phase, solid ammonium chloride is formed, and in an equilibrium mixture of two substances there are always 4 particles, two of which are acids, and the other two are bases:
This equilibrium mixture consists of two conjugated pairs of acids and bases:
1)NH 4+ and NH 3
2) HCl and Cl ‑
Here, in each conjugated pair, the acid and base differ by one proton. Every acid has a conjugate base. A strong acid has a weak conjugate base, and a weak acid has a strong conjugate base.
The Bronsted-Lowry theory makes it possible to explain the unique role of water for the life of the biosphere. Water, depending on the substance interacting with it, can exhibit the properties of either an acid or a base. For example, in reactions with aqueous solutions of acetic acid, water is a base, and with aqueous solutions of ammonia, it is an acid.
1) CH 3 COOH + H 2 O ↔ H 3 O + + CH 3 SOO- . Here the acetic acid molecule donates a proton to the water molecule;
2) NH3 + H 2 O ↔ NH4 + + HE- . Here the ammonia molecule accepts a proton from the water molecule.
Thus, water can form two conjugated pairs:
1) H 2 O(acid) and HE- (conjugate base)
2) H 3 O+ (acid) and H 2 O(conjugate base).
In the first case, water donates a proton, and in the second, it accepts it.
Such a property is called amphiprotonity. Substances that can react as both acids and bases are called amphoteric. Such substances are often found in nature. For example, amino acids can form salts with both acids and bases. Therefore, peptides readily form coordination compounds with the metal ions present.
Thus, the characteristic property of an ionic bond is the complete displacement of a bunch of binding electrons to one of the nuclei. This means that there is a region between the ions where the electron density is almost zero.
The second type of connection iscovalent connection
Atoms can form stable electronic configurations by sharing electrons.
Such a bond is formed when a pair of electrons is shared one at a time. from each atom. In this case, the socialized bond electrons are distributed equally among the atoms. An example of a covalent bond is homonuclear diatomic H molecules 2 , N 2 , F 2. Allotropes have the same type of bond. O 2 and ozone O 3 and for a polyatomic molecule S 8 and also heteronuclear molecules hydrogen chloride Hcl, carbon dioxide CO 2, methane CH 4, ethanol FROM 2 H 5 HE, sulfur hexafluoride SF 6, acetylene FROM 2 H 2. All these molecules have the same common electrons, and their bonds are saturated and directed in the same way (Fig. 4).
For biologists, it is important that the covalent radii of atoms in double and triple bonds are reduced compared to a single bond.
Rice. four. Covalent bond in the Cl 2 molecule.
Ionic and covalent types of bonds are two limiting cases of many existing types of chemical bonds, and in practice most of the bonds are intermediate.
Compounds of two elements located at opposite ends of the same or different periods of the Mendeleev system predominantly form ionic bonds. As the elements approach each other within a period, the ionic nature of their compounds decreases, while the covalent character increases. For example, the halides and oxides of the elements on the left side of the periodic table form predominantly ionic bonds ( NaCl, AgBr, BaSO 4 , CaCO 3 , KNO 3 , CaO, NaOH), and the same compounds of the elements on the right side of the table are covalent ( H 2 O, CO 2, NH 3, NO 2, CH 4, phenol C6H5OH, glucose C 6 H 12 O 6, ethanol C 2 H 5 OH).
The covalent bond, in turn, has another modification.
In polyatomic ions and in complex biological molecules, both electrons can only come from one atom. It is called donor electron pair. An atom that socializes this pair of electrons with a donor is called acceptor electron pair. This type of covalent bond is called coordination (donor-acceptor, ordative) communication(Fig. 5). This type of bond is most important for biology and medicine, since the chemistry of the most important d-elements for metabolism is largely described by coordination bonds.
Pic. 5.
As a rule, in a complex compound, a metal atom acts as an electron pair acceptor; on the contrary, in ionic and covalent bonds, the metal atom is an electron donor.
The essence of the covalent bond and its variety - the coordination bond - can be clarified with the help of another theory of acids and bases, proposed by GN. Lewis. He somewhat expanded the semantic concept of the terms "acid" and "base" according to the Bronsted-Lowry theory. The Lewis theory explains the nature of the formation of complex ions and the participation of substances in nucleophilic substitution reactions, that is, in the formation of CS.
According to Lewis, an acid is a substance capable of forming a covalent bond by accepting an electron pair from a base. A Lewis base is a substance that has a lone pair of electrons, which, by donating electrons, forms a covalent bond with Lewis acid.
That is, the Lewis theory expands the range of acid-base reactions also to reactions in which protons do not participate at all. Moreover, the proton itself, according to this theory, is also an acid, since it is able to accept an electron pair.
Therefore, according to this theory, cations are Lewis acids and anions are Lewis bases. The following reactions are examples:
It was noted above that the subdivision of substances into ionic and covalent ones is relative, since there is no complete transition of an electron from metal atoms to acceptor atoms in covalent molecules. In compounds with an ionic bond, each ion is in the electric field of ions of the opposite sign, so they are mutually polarized, and their shells are deformed.
Polarizability determined by the electronic structure, charge and size of the ion; it is higher for anions than for cations. The highest polarizability among cations is for cations of larger charge and smaller size, for example, for Hg 2+ , Cd 2+ , Pb 2+ , Al 3+ , Tl 3+. Has a strong polarizing effect H+ . Since the effect of ion polarization is two-sided, it significantly changes the properties of the compounds they form.
The third type of connection -dipole-dipole connection
In addition to the listed types of communication, there are also dipole-dipole intermolecular interactions, also known as van der Waals .
The strength of these interactions depends on the nature of the molecules.
There are three types of interactions: permanent dipole - permanent dipole ( dipole-dipole attraction); permanent dipole - induced dipole ( induction attraction); instantaneous dipole - induced dipole ( dispersion attraction, or London forces; rice. 6).
Rice. 6.
Only molecules with polar covalent bonds have a dipole-dipole moment ( HCl, NH 3, SO 2, H 2 O, C 6 H 5 Cl), and the bond strength is 1-2 debye(1D \u003d 3.338 × 10 -30 coulomb meters - C × m).
In biochemistry, another type of bond is distinguished - hydrogen connection, which is a limiting case dipole-dipole attraction. This bond is formed by the attraction between a hydrogen atom and a small electronegative atom, most often oxygen, fluorine and nitrogen. With large atoms that have a similar electronegativity (for example, with chlorine and sulfur), the hydrogen bond is much weaker. The hydrogen atom is distinguished by one essential feature: when the binding electrons are pulled away, its nucleus - the proton - is exposed and ceases to be screened by electrons.
Therefore, the atom turns into a large dipole.
A hydrogen bond, unlike a van der Waals bond, is formed not only during intermolecular interactions, but also within one molecule - intramolecular hydrogen bond. Hydrogen bonds play an important role in biochemistry, for example, for stabilizing the structure of proteins in the form of an α-helix, or for the formation of a DNA double helix (Fig. 7).
Fig.7.
Hydrogen and van der Waals bonds are much weaker than ionic, covalent, and coordination bonds. The energy of intermolecular bonds is indicated in Table. one.
Table 1. Energy of intermolecular forces
Note: The degree of intermolecular interactions reflect the enthalpy of melting and evaporation (boiling). Ionic compounds require much more energy to separate ions than to separate molecules. The melting enthalpies of ionic compounds are much higher than those of molecular compounds.
The fourth type of connection -metallic bond
Finally, there is another type of intermolecular bonds - metal: connection of positive ions of the lattice of metals with free electrons. This type of connection does not occur in biological objects.
From a brief review of the types of bonds, one detail emerges: an important parameter of an atom or ion of a metal - an electron donor, as well as an atom - an electron acceptor is its the size.
Without going into details, we note that the covalent radii of atoms, the ionic radii of metals, and the van der Waals radii of interacting molecules increase as their atomic number in the groups of the periodic system increases. In this case, the values of the ion radii are the smallest, and the van der Waals radii are the largest. As a rule, when moving down the group, the radii of all elements increase, both covalent and van der Waals.
The most important for biologists and physicians are coordination(donor-acceptor) bonds considered by coordination chemistry.
Medical bioinorganics. G.K. Barashkov
