Attachment reactions. Types of chemical reactions in organic chemistry Electrophilic addition reactions
hydrocarbons
Lecture No. 13
The enol formed during the hydration of the alkyne cannot be isolated, since the tautomeric equilibrium is always strongly shifted towards the keto form.
The rearrangement of the enol into the keto form proceeds due to the high polarity of the O-H bond, which leads to easy elimination of the proton of the hydroxyl group.
The proton can then attach either back to the oxygen of the enolate anion or to carbon. If it joins a carbon atom, then a less acidic compound is formed in which the proton does not show a pronounced tendency to dissociate. Thus, the keto form accumulates in the reaction mixture.
There is only one case when an aldehyde is formed in the Kucherov reaction - the hydration of acetylene itself. All other reactions produce ketones.
Hydration of propyne results in acetone rather than propionaldehyde.
Nucleophilic addition reactions. Alkynes are capable of adding nucleophilic reagents in the presence of a catalyst. As a result, vinyl derivatives of different classes are formed. These reactions are called vinylation reactions.
Some reactions of nucleophilic addition to alkynes are shown above. They are often used to prepare compounds that serve as monomers in HMC synthesis. For example, the reaction of acetylene with n-butanol leads to butylvinyl ether, polymerization of which gives vinyline (Shostakovsky's balm) is a valuable drug.
Oxidation. Like all organic compounds containing multiple carbon-carbon bonds, alkynes are easily oxidized with a variety of oxidizing agents, such as potassium permanganate or potassium dichromate. The reaction proceeds with a complete rupture of the triple bond and the formation of carboxylic acids (alkynes with a terminal triple bond form a carboxylic acid and carbon dioxide).
These reactions can be used to determine the structure of alkynes.
Some oxidizing agents, such as selenium dioxide, allow selective oxidation of alkynes, during which only p-bonds are cleaved. In this case, disubstituted alkynes are converted into a-diketones, and monosubstituted ones into a-keto acids.
Polymerization of alkynes. In the series of alkynes, the most interesting are the reactions of di-, trimerization, as well as cyclotri- and cyclotetramerization.
Linear di- and trimers of acetylene, which are of great industrial importance, can be obtained in the presence of monovalent copper salts.
Vinylacetylene is the starting compound for the synthesis of chloroprene (2-chlorobutadiene-1,3), polymerization of which produces chloroprene rubber.
The cyclotrimerization of acetylene, leading to benzene, was discovered in 1866 by M. Berthelot and modified by N.D. Zelinsky and B.A. Kazansky (1922).
Cyclotetramerization was discovered by W. Reppe (1949).
) bonds of another chemical compound. Accession can be carried out as a connection carbon-carbon, and by communication carbon heteroatom. Addition reactions are denoted by English letters "Ad".
General view of addition reactions by bond carbon-carbon:
General view of addition reactions by bond carbon-oxygen:
Usually, the reagent to which the addition occurs is called substrate, and the other ( "X-Y") - attack reagent.
An example of an addition reaction is the bromination of ethylene:
Addition reactions are often reversible, pairing with elimination reactions, so it should be borne in mind that the mechanism for such a "paired" addition-elimination reaction is common.
Depending on the nature of the attacking particle and the reaction mechanism, a distinction is made between nucleophilic, electrophilic, radical, or synchronous addition.
Nucleophilic addition reactions
In nucleophilic addition reactions, the attacking particle is the nucleophile, that is, a negatively charged particle or a particle with a free electron pair.
General view of nucleophilic addition reactions:
Nucleophilic addition reactions are denoted "AdN".
Nucleophilic addition reactions at a bond C=C are quite rare, the most widespread and practical value is connection by connection C=O :
Among the reactions of nucleophilic addition, the most common is the above two-stage bimolecular mechanism Ad N 2: In the beginning, the nucleophile slowly adds at a multiple bond to form a carbanion, which in the second step is rapidly attacked by an electrophilic species.
Electrophilic addition reactions
In electrophilic addition reactions, the attacking particle is electrophile, that is, a positively charged particle, most often a proton H+, or an electron-deficient particle.
General view of electrophilic addition reactions:
Electrophilic addition reactions are denoted "Ad e".
Electrophilic addition reactions are widespread among the reactions of unsaturated hydrocarbons: alkenes, alkynes and dienes.
An example of such reactions is the hydration of alkenes:
Electrophilic bonding carbon heteroatom is also quite common, and most often such a connection is C=O:
Among the reactions of electrophilic addition, the most common is the above two-stage bimolecular mechanism Ad E 2: In the beginning, the electrophile slowly adds at a multiple bond to form a carbocation, which undergoes nucleophilic attack in the second step.
Radical addition reactions
In radical addition reactions, free radicals are the attacking species.
Radical addition reactions are denoted "Ad R".
Radical addition reactions usually proceed instead of electrophilic addition reactions in the presence of a source of free radicals:
Synchronous addition reactions
In some cases, addition by a multiple bond occurs with the simultaneous attack of both atoms, which does not allow one to determine the priority of the attack. Such a mechanism is called synchronous connection. Synchronous addition reactions lead to the formation of cyclic products, so they are often called cycloaddition.
Notes
| Chemical reactions in organic chemistry | |
|---|---|
| Substitution reactions | Nucleophilic substitution reactions Electrophilic substitution reactions Radical substitution reactions |
| Addition reactions | Nucleophilic addition reactions Electrophilic addition reactions Radical addition reactions Simultaneous addition reactions |
| Elimination reactions | Heterolytic elimination reactions Pericyclic elimination reactions Radical elimination reactions |
| rearrangement reactions | Nucleophilic rearrangements Electrophilic rearrangements Radical rearrangements |
| Oxidation and reduction reactions | Oxidation reactions Reduction reactions |
| Other | Nominal reactions in organic chemistry |
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See what "Addition reactions" are in other dictionaries:
addition reaction polymer- - EN addition polymer A polymer formed by the chain addition of unsaturated monomer molecules, such as olefins, with one another without the formation of a by product, as water;… … Technical Translator's Handbook
- (English addition electrophilic reaction) addition reactions, in which the attack at the initial stage is carried out by an electrophile particle, positively charged or having a deficit of electrons. At the final stage, the resulting ... ... Wikipedia
- (English addition nucleophilic reaction) addition reactions in which the attack at the initial stage is carried out by a nucleophile particle, negatively charged or having a free electron pair. At the final stage, the resulting ... ... Wikipedia
- (eng. addition radical reaction) addition reactions in which the attack is carried out by free radicals of a particle containing one or more unpaired electrons. At the same time, radicals can attack both other radicals and ... ... Wikipedia
Addition reactions in which both atoms of a multiple bond are attacked simultaneously. Another name for reactions of this type is cycloaddition reactions, since the end products of such reactions are cyclic substrates. There are two ... ... Wikipedia
- (English nucleophilic substitution reaction) substitution reactions in which the attack is carried out by a nucleophile reagent carrying an unshared electron pair. The leaving group in nucleophilic substitution reactions is called a nucleofug. All ... Wikipedia
Characteristic chemical properties of hydrocarbons: alkanes, alkenes, dienes, alkynes, aromatic hydrocarbons
Alkanes
Alkanes are hydrocarbons in whose molecules the atoms are linked by single bonds and which correspond to the general formula $C_(n)H_(2n+2)$.
Homologous series of methane
As you already know, homologues are substances that are similar in structure and properties and differ by one or more $CH_2$ groups.
Limit hydrocarbons make up the homologous series of methane.
Isomerism and nomenclature
Alkanes are characterized by the so-called structural isomerism. Structural isomers differ from each other in the structure of the carbon skeleton. As you already know, the simplest alkane, which is characterized by structural isomers, is butane:
Let us consider in more detail for alkanes the basics of the IUPAC nomenclature:
1. Choice of the main circuit.
The formation of the name of a hydrocarbon begins with the definition of the main chain - the longest chain of carbon atoms in the molecule, which is, as it were, its basis.
2.
The atoms of the main chain are assigned numbers. The numbering of atoms of the main chain starts from the end closest to the substituent (structures A, B). If the substituents are at an equal distance from the end of the chain, then the numbering starts from the end at which there are more of them (structure B). If different substituents are at an equal distance from the ends of the chain, then the numbering starts from the end to which the older one is closer (structure D). The seniority of hydrocarbon substituents is determined by the order in which the letter with which their name begins follows in the alphabet: methyl (—$CH_3$), then propyl ($—CH_2—CH_2—CH_3$), ethyl ($—CH_2—CH_3$ ) etc.
Note that the name of the substitute is formed by replacing the suffix -en to suffix -silt in the name of the corresponding alkane.
3. Name formation.
Numbers are indicated at the beginning of the name - the numbers of carbon atoms at which the substituents are located. If there are several substituents at a given atom, then the corresponding number in the name is repeated twice, separated by commas ($2.2-$). After the number, a hyphen indicates the number of substituents ( di- two, three- three, tetra- four, penta- five) and the name of the deputy ( methyl, ethyl, propyl). Then without spaces and hyphens - the name of the main chain. The main chain is called as a hydrocarbon - a member of the homologous series of methane ( methane, ethane, propane, etc.).
The names of the substances whose structural formulas are given above are as follows:
- structure A: $2$ -methylpropane;
- Structure B: $3$ -ethylhexane;
- Structure B: $2,2,4$ -trimethylpentane;
- structure Г: $2$ -methyl$4$-ethylhexane.
Physical and chemical properties of alkanes
physical properties. The first four representatives of the homologous series of methane are gases. The simplest of them is methane - a colorless, tasteless and odorless gas (the smell of gas, upon smelling which you need to call $104$, is determined by the smell of mercaptans - sulfur-containing compounds specially added to methane used in household and industrial gas appliances so that people those near them could smell the leak).
Hydrocarbons of composition from $С_5Н_(12)$ to $С_(15)Н_(32)$ are liquids; heavier hydrocarbons are solids.
The boiling and melting points of alkanes gradually increase with increasing carbon chain length. All hydrocarbons are poorly soluble in water; liquid hydrocarbons are common organic solvents.
Chemical properties.
1. substitution reactions. The most characteristic of alkanes are free radical substitution reactions, during which a hydrogen atom is replaced by a halogen atom or some group.
Let us present the equations of the most typical reactions.
Halogenation:
$CH_4+Cl_2→CH_3Cl+HCl$.
In the case of an excess of halogen, chlorination can go further, up to the complete replacement of all hydrogen atoms by chlorine:
$CH_3Cl+Cl_2→HCl+(CH_2Cl_2)↙(\text"dichloromethane(methylene chloride)")$,
$CH_2Cl_2+Cl_2→HCl+(CHСl_3)↙(\text"trichloromethane(chloroform)")$,
$CHCl_3+Cl_2→HCl+(CCl_4)↙(\text"tetrachloromethane(carbon tetrachloride)")$.
The resulting substances are widely used as solvents and starting materials in organic synthesis.
2. Dehydrogenation (elimination of hydrogen). During the passage of alkanes over the catalyst ($Pt, Ni, Al_2O_3, Cr_2O_3$) at a high temperature ($400-600°C$), a hydrogen molecule is split off and an alkene is formed:
$CH_3—CH_3→CH_2=CH_2+H_2$
3. Reactions accompanied by the destruction of the carbon chain. All saturated hydrocarbons are burning with the formation of carbon dioxide and water. Gaseous hydrocarbons mixed with air in certain proportions can explode. The combustion of saturated hydrocarbons is a free radical exothermic reaction, which is of great importance when using alkanes as a fuel:
$CH_4+2O_2→CO_2+2H_2O+880 kJ.$
In general, the combustion reaction of alkanes can be written as follows:
$C_(n)H_(2n+2)+((3n+1)/(2))O_2→nCO_2+(n+1)H_2O$
Thermal breakdown of hydrocarbons:
$C_(n)H_(2n+2)(→)↖(400-500°C)C_(n-k)H_(2(n-k)+2)+C_(k)H_(2k)$
The process proceeds according to the free radical mechanism. An increase in temperature leads to a homolytic rupture of the carbon-carbon bond and the formation of free radicals:
$R—CH_2CH_2:CH_2—R→R—CH_2CH_2+CH_2—R$.
These radicals interact with each other, exchanging a hydrogen atom, with the formation of an alkane molecule and an alkene molecule:
$R—CH_2CH_2+CH_2—R→R—CH=CH_2+CH_3—R$.
Thermal splitting reactions underlie the industrial process - hydrocarbon cracking. This process is the most important stage of oil refining.
When methane is heated to a temperature of $1000°C$, pyrolysis of methane begins - decomposition into simple substances:
$CH_4(→)↖(1000°C)C+2H_2$
When heated to a temperature of $1500°C$, the formation of acetylene is possible:
$2CH_4(→)↖(1500°C)CH=CH+3H_2$
4. Isomerization. When linear hydrocarbons are heated with an isomerization catalyst (aluminum chloride), substances with a branched carbon skeleton are formed:
5. Aromatization. Alkanes with six or more carbon atoms in the chain in the presence of a catalyst are cyclized to form benzene and its derivatives:
What is the reason that alkanes enter into reactions proceeding according to the free radical mechanism? All carbon atoms in alkane molecules are in the $sp^3$ hybridization state. The molecules of these substances are built using covalent nonpolar $C—C$ (carbon—carbon) bonds and weakly polar $C—H$ (carbon—hydrogen) bonds. They do not contain areas with high and low electron density, easily polarizable bonds, i.e. such bonds, the electron density in which can be shifted under the influence of external factors (electrostatic fields of ions). Therefore, alkanes will not react with charged particles, because bonds in alkane molecules are not broken by a heterolytic mechanism.
Alkenes
Unsaturated hydrocarbons include hydrocarbons containing multiple bonds between carbon atoms in molecules. Unlimited are alkenes, alkadienes (polyenes), alkynes. Cyclic hydrocarbons containing a double bond in the cycle (cycloalkenes), as well as cycloalkanes with a small number of carbon atoms in the cycle (three or four atoms) also have an unsaturated character. The property of unsaturation is associated with the ability of these substances to enter into addition reactions, primarily hydrogen, with the formation of saturated, or saturated, hydrocarbons - alkanes.
Alkenes are acyclic hydrocarbons containing in the molecule, in addition to single bonds, one double bond between carbon atoms and corresponding to the general formula $C_(n)H_(2n)$.
Its second name olefins- alkenes were obtained by analogy with unsaturated fatty acids (oleic, linoleic), the remains of which are part of liquid fats - oils (from lat. oleum- oil).
Homologous series of ethene
Unbranched alkenes make up the homologous series of ethene (ethylene):
$C_2H_4$ is ethene, $C_3H_6$ is propene, $C_4H_8$ is butene, $C_5H_(10)$ is pentene, $C_6H_(12)$ is hexene, etc.
Isomerism and nomenclature
For alkenes, as well as for alkanes, structural isomerism is characteristic. Structural isomers differ from each other in the structure of the carbon skeleton. The simplest alkene, which is characterized by structural isomers, is butene:
A special type of structural isomerism is the double bond position isomerism:
$CH_3—(CH_2)↙(butene-1)—CH=CH_2$ $CH_3—(CH=CH)↙(butene-2)—CH_3$
Almost free rotation of carbon atoms is possible around a single carbon-carbon bond, so alkane molecules can take on a wide variety of shapes. Rotation around the double bond is impossible, which leads to the appearance of another type of isomerism in alkenes - geometric, or cis-trans isomerism.
cis- isomers are different from trance- isomers by the spatial arrangement of fragments of the molecule (in this case, methyl groups) relative to the $π$-bond plane, and, consequently, by properties.
Alkenes are isomeric to cycloalkanes (interclass isomerism), for example:
The nomenclature of alkenes developed by IUPAC is similar to the nomenclature of alkanes.
1. Choice of the main circuit.
The formation of the name of a hydrocarbon begins with the definition of the main chain - the longest chain of carbon atoms in a molecule. In the case of alkenes, the main chain must contain a double bond.
2. Atom numbering of the main chain.
The numbering of the atoms of the main chain starts from the end to which the double bond is closest. For example, the correct connection name is:
$5$-methylhexene-$2$, not $2$-methylhexene-$4$, as might be expected.
If it is impossible to determine the beginning of the numbering of atoms in the chain by the position of the double bond, then it is determined by the position of the substituents, just as for saturated hydrocarbons.
3. Name formation.
The names of alkenes are formed in the same way as the names of alkanes. At the end of the name indicate the number of the carbon atom at which the double bond begins, and the suffix indicating that the compound belongs to the class of alkenes - -en.
For example:
Physical and chemical properties of alkenes
physical properties. The first three representatives of the homologous series of alkenes are gases; substances of the composition $C_5H_(10)$ - $C_(16)H_(32)$ are liquids; higher alkenes are solids.
The boiling and melting points naturally increase with an increase in the molecular weight of the compounds.
Chemical properties.
Addition reactions. Recall that a distinctive feature of the representatives of unsaturated hydrocarbons - alkenes is the ability to enter into addition reactions. Most of these reactions proceed by the mechanism
1. hydrogenation of alkenes. Alkenes are able to add hydrogen in the presence of hydrogenation catalysts, metals - platinum, palladium, nickel:
$CH_3—CH_2—CH=CH_2+H_2(→)↖(Pt)CH_3—CH_2—CH_2—CH_3$.
This reaction proceeds at atmospheric and elevated pressure and does not require high temperature, because is exothermic. With an increase in temperature on the same catalysts, the reverse reaction, dehydrogenation, can occur.
2. Halogenation (addition of halogens). The interaction of an alkene with bromine water or a solution of bromine in an organic solvent ($CCl_4$) leads to a rapid discoloration of these solutions as a result of the addition of a halogen molecule to the alkene and the formation of dihalogen alkanes:
$CH_2=CH_2+Br_2→CH_2Br—CH_2Br$.
3.
$CH_3-(CH)↙(propene)=CH_2+HBr→CH_3-(CHBr)↙(2-bromopropene)-CH_3$
This reaction is subject to Markovnikov's rule:
When a hydrogen halide is added to an alkene, hydrogen is attached to a more hydrogenated carbon atom, i.e. the atom at which there are more hydrogen atoms, and the halogen - to the less hydrogenated one.
Hydration of alkenes leads to the formation of alcohols. For example, the addition of water to ethene underlies one of the industrial methods for producing ethyl alcohol:
$(CH_2)↙(ethene)=CH_2+H_2O(→)↖(t,H_3PO_4)CH_3-(CH_2OH)↙(ethanol)$
Note that a primary alcohol (with a hydroxo group at the primary carbon) is formed only when ethene is hydrated. When propene or other alkenes are hydrated, secondary alcohols are formed.
This reaction also proceeds in accordance with Markovnikov's rule - the hydrogen cation is attached to the more hydrogenated carbon atom, and the hydroxo group to the less hydrogenated one.
5. Polymerization. A special case of addition is the polymerization reaction of alkenes:
$nCH_2(=)↙(ethene)CH_2(→)↖(UV light,R)(...(-CH_2-CH_2-)↙(polyethylene)...)_n$
This addition reaction proceeds by a free radical mechanism.
6. Oxidation reaction.
Like any organic compounds, alkenes burn in oxygen to form $CO_2$ and $H_2O$:
$CH_2=CH_2+3O_2→2CO_2+2H_2O$.
In general:
$C_(n)H_(2n)+(3n)/(2)O_2→nCO_2+nH_2O$
Unlike alkanes, which are resistant to oxidation in solutions, alkenes are easily oxidized by the action of potassium permanganate solutions. In neutral or alkaline solutions, alkenes are oxidized to diols (dihydric alcohols), and hydroxyl groups are attached to those atoms between which a double bond existed before oxidation:
Alkadienes (diene hydrocarbons)
Alkadienes are acyclic hydrocarbons containing in the molecule, in addition to single bonds, two double bonds between carbon atoms and corresponding to the general formula $C_(n)H_(2n-2)$.
Depending on the mutual arrangement of double bonds, there are three types of dienes:
- alkadienes with cumulated arrangement of double bonds:
- alkadienes with conjugated double bonds;
$CH_2=CH—CH=CH_2$;
- alkadienes with isolated double bonds
$CH_2=CH—CH_2—CH=CH_2$.
All three types of alkadienes differ significantly from each other in structure and properties. The central carbon atom (an atom that forms two double bonds) in alkadienes with cumulated bonds is in the $sp$-hybridization state. It forms two $σ$-bonds lying on the same straight line and directed in opposite directions, and two $π$-bonds lying in perpendicular planes. $π$-bonds are formed due to unhybridized p-orbitals of each carbon atom. The properties of alkadienes with isolated double bonds are very specific, because conjugated $π$-bonds significantly affect each other.
p-Orbitals forming conjugated $π$-bonds make up practically a single system (it is called a $π$-system), because p-orbitals of neighboring $π$-bonds partially overlap.
Isomerism and nomenclature
Alkadienes are characterized by both structural isomerism and cis- and trans-isomerism.
Structural isomerism.
— isomerism of the carbon skeleton:
— isomerism of the position of multiple bonds:
$(CH_2=CH—CH=CH_2)↙(butadiene-1,3)$ $(CH_2=C=CH—CH_3)↙(butadiene-1,2)$
cis-, trans- isomerism (spatial and geometric)
For example:
Alkadienes are isomeric compounds of the classes of alkynes and cycloalkenes.
When forming the name of the alkadiene, the numbers of double bonds are indicated. The main chain must necessarily contain two multiple bonds.
For example:
Physical and chemical properties of alkadienes
physical properties.
Under normal conditions, propandien-1,2, butadiene-1,3 are gases, 2-methylbutadiene-1,3 is a volatile liquid. Alkadienes with isolated double bonds (the simplest of them is pentadiene-1,4) are liquids. Higher dienes are solids.
Chemical properties.
The chemical properties of alkadienes with isolated double bonds differ little from those of alkenes. Alkadienes with conjugated bonds have some special features.
1. Addition reactions. Alkadienes are capable of adding hydrogen, halogens, and hydrogen halides.
A feature of addition to alkadienes with conjugated bonds is the ability to attach molecules both in positions 1 and 2, and in positions 1 and 4.
The ratio of the products depends on the conditions and method of carrying out the corresponding reactions.
2.polymerization reaction. The most important property of dienes is the ability to polymerize under the influence of cations or free radicals. The polymerization of these compounds is the basis of synthetic rubbers:
$nCH_2=(CH—CH=CH_2)↙(butadiene-1,3)→((... —CH_2—CH=CH—CH_2— ...)_n)↙(\text"synthetic butadiene rubber")$ .
The polymerization of conjugated dienes proceeds as 1,4-addition.
In this case, the double bond turns out to be central in the link, and the elementary link, in turn, can take both cis-, and trance- configuration.
Alkynes
Alkynes are acyclic hydrocarbons containing in the molecule, in addition to single bonds, one triple bond between carbon atoms and corresponding to the general formula $C_(n)H_(2n-2)$.
Homologous series of ethine
Unbranched alkynes make up the homologous series of ethyne (acetylene):
$C_2H_2$ - ethyne, $C_3H_4$ - propyne, $C_4H_6$ - butyne, $C_5H_8$ - pentine, $C_6H_(10)$ - hexine, etc.
Isomerism and nomenclature
For alkynes, as well as for alkenes, structural isomerism is characteristic: isomerism of the carbon skeleton and isomerism of the position of the multiple bond. The simplest alkyne, which is characterized by structural isomers of the multiple bond position of the alkyne class, is butyne:
$CH_3—(CH_2)↙(butyn-1)—C≡CH$ $CH_3—(C≡C)↙(butyn-2)—CH_3$
The isomerism of the carbon skeleton in alkynes is possible, starting from pentyn:
Since the triple bond assumes a linear structure of the carbon chain, the geometric ( cis-, trans-) isomerism is not possible for alkynes.
The presence of a triple bond in hydrocarbon molecules of this class is reflected by the suffix -in, and its position in the chain - the number of the carbon atom.
For example:
Alkynes are isomeric compounds of some other classes. So, hexine (alkyne), hexadiene (alkadiene) and cyclohexene (cycloalkene) have the chemical formula $С_6Н_(10)$:
Physical and chemical properties of alkynes
physical properties. The boiling and melting points of alkynes, as well as alkenes, naturally increase with an increase in the molecular weight of the compounds.
Alkynes have a specific smell. They are more soluble in water than alkanes and alkenes.
Chemical properties.
Addition reactions. Alkynes are unsaturated compounds and enter into addition reactions. Basically, these are reactions. electrophilic addition.
1. Halogenation (addition of a halogen molecule). Alkyne is able to attach two halogen molecules (chlorine, bromine):
$CH≡CH+Br_2→(CHBr=CHBr)↙(1,2-dibromoethane),$
$CHBr=CHBr+Br_2→(CHBr_2-CHBr_2)↙(1,1,2,2-tetrabromoethane)$
2. Hydrohalogenation (addition of hydrogen halide). The addition reaction of hydrogen halide, proceeding according to the electrophilic mechanism, also proceeds in two stages, and at both stages the Markovnikov rule is fulfilled:
$CH_3-C≡CH+Br→(CH_3-CBr=CH_2)↙(2-bromopropene),$
$CH_3-CBr=CH_2+HBr→(CH_3-CHBr_2-CH_3)↙(2,2-dibromopropane)$
3. Hydration (addition of water). Of great importance for the industrial synthesis of ketones and aldehydes is the water addition reaction (hydration), which is called Kucherov's reaction:
4. hydrogenation of alkynes. Alkynes add hydrogen in the presence of metal catalysts ($Pt, Pd, Ni$):
$R-C≡C-R+H_2(→)↖(Pt)R-CH=CH-R,$
$R-CH=CH-R+H_2(→)↖(Pt)R-CH_2-CH_2-R$
Since the triple bond contains two reactive $π$ bonds, alkanes add hydrogen in steps:
1) trimerization.
When ethyne is passed over activated carbon, a mixture of products is formed, one of which is benzene:
2) dimerization.
In addition to trimerization of acetylene, its dimerization is also possible. Under the action of monovalent copper salts, vinylacetylene is formed:
$2HC≡CH→(HC≡C-CH=CH_2)↙(\text"butene-1-yn-3(vinylacetylene)")$
This substance is used to produce chloroprene:
$HC≡C-CH=CH_2+HCl(→)↖(CaCl)H_2C=(CCl-CH)↙(chloroprene)=CH_2$
polymerization of which produces chloroprene rubber:
$nH_2C=CCl-CH=CH_2→(...-H_2C-CCl=CH-CH_2-...)_n$
Alkyne oxidation.
Ethine (acetylene) burns in oxygen with the release of a very large amount of heat:
$2C_2H_2+5O_2→4CO_2+2H_2O+2600kJ$ This reaction is based on the action of an oxy-acetylene torch, the flame of which has a very high temperature (more than $3000°C$), which makes it possible to use it for cutting and welding metals.
In air, acetylene burns with a smoky flame, because. the carbon content in its molecule is higher than in the molecules of ethane and ethene.
Alkynes, like alkenes, decolorize acidified solutions of potassium permanganate; in this case, the destruction of the multiple bond occurs.
Ionic (V.V. Markovnikov's rule) and radical reaction mechanisms in organic chemistry
Types of chemical reactions in organic chemistry
The reactions of organic substances can be formally divided into four main types: substitution, addition, elimination (elimination) and rearrangement (isomerization). Obviously, the whole variety of reactions of organic compounds cannot be reduced to the proposed classification (for example, combustion reactions). However, such a classification will help to establish analogies with the reactions that take place between inorganic substances already familiar to you from the course of inorganic chemistry.
As a rule, the main organic compound participating in the reaction is called the substrate, and the other component of the reaction is conditionally considered as a reagent.
Substitution reactions
Reactions that result in the replacement of one atom or group of atoms in the original molecule (substrate) with other atoms or groups of atoms are called substitution reactions.
Substitution reactions involve saturated and aromatic compounds such as alkanes, cycloalkanes or arenes.
Let us give examples of such reactions.
Under the action of light, hydrogen atoms in a methane molecule can be replaced by halogen atoms, for example, by chlorine atoms:
$CH_4+Cl_2→CH_3Cl+HCl$
Another example of replacing hydrogen with halogen is the conversion of benzene to bromobenzene:
The equation for this reaction can be written differently:
With this form of writing, the reagents, catalyst, reaction conditions are written above the arrow, and the inorganic reaction products are written below it.
Addition reactions
Reactions, as a result of which two or more molecules of reactants combine into one, are called addition reactions.
Unsaturated compounds, such as alkenes or alkynes, enter into addition reactions.
Depending on which molecule acts as a reagent, hydrogenation (or reduction), halogenation, hydrohalogenation, hydration, and other addition reactions are distinguished. Each of them requires certain conditions.
1. hydrogenation- the reaction of the addition of a hydrogen molecule to a multiple bond:
$CH_3(-CH=)↙(\text"propene")CH_2+H_2(→)↖(Pt)CH_3(-CH_2-)↙(\text"propane")-CH_3$
2.Hydrohalogenation— hydrogen halide addition reaction (hydrochlorination):
$(CH_2=)↙(\text"ethene")CH_2+HCl→CH_3(-CH_2-)↙(\text"chloroethane")-Cl$
3.Halogenation- halogen addition reaction:
$(CH_2=)↙(\text"ethene")CH_2+Cl_2→(CH_2Cl-CH_2Cl)↙(\text"1.2-dichloroethane")$
4. Polymerization- a special type of addition reactions, during which molecules of a substance with a small molecular weight are combined with each other to form molecules of a substance with a very high molecular weight - macromolecules.
Polymerization reactions are the processes of combining many molecules of a low molecular weight substance (monomer) into large molecules (macromolecules) of a polymer.
An example of a polymerization reaction is the production of polyethylene from ethylene (ethene) under the action of ultraviolet radiation and a radical polymerization initiator $R:$
$(nCH_2=)↙(\text"ethene")CH_2(→)↖(\text"UV light,R")((...-CH_2-CH_2-...)_n)↙(\text" polyethylene")$
The covalent bond most characteristic of organic compounds is formed when atomic orbitals overlap and the formation of common electron pairs. As a result of this, an orbital common to two atoms is formed, on which a common electron pair is located. When the bond is broken, the fate of these common electrons can be different.
Types of reactive particles in organic chemistry
An orbital with an unpaired electron belonging to one atom can overlap with an orbital of another atom that also contains an unpaired electron. This results in the formation of a covalent bond exchange mechanism:
$H + H→H:H,$ or $H-H$
exchange mechanism The formation of a covalent bond is realized if a common electron pair is formed from unpaired electrons belonging to different atoms.
The process opposite to the formation of a covalent bond by the exchange mechanism is bond breaking, in which one electron goes to each atom. As a result, two uncharged particles with unpaired electrons are formed:
Such particles are called free radicals.
free radicals- atoms or groups of atoms that have unpaired electrons.
Reactions that take place under the action and with the participation of free radicals are called free radical reactions.
In the course of inorganic chemistry, these are reactions of interaction of hydrogen with oxygen, halogens, combustion reactions. Please note that reactions of this type are characterized by high speed, release of a large amount of heat.
A covalent bond can also be formed by the donor-acceptor mechanism. One of the orbitals of an atom (or anion), on which an unshared electron pair is located, is overlapped by an unfilled orbital of another atom (or cation), which has an unfilled orbital, and a covalent bond is formed, for example:
$H^(+)+(:O-H^(-))↙(\text"acceptor")→(H-O-H)↙(\text"donor")$
The rupture of a covalent bond leads to the formation of positively and negatively charged particles; since in this case both electrons from the common electron pair remain with one of the atoms, the second atom has an unfilled orbital:
$R:|R=R:^(-)+R^(+)$
Consider the electrolytic dissociation of acids:
$H:|Cl=H^(+)+Cl^(-)$
One can easily guess that a particle having an unshared electron pair $R:^(-)$, i.e. a negatively charged ion, will be attracted to positively charged atoms or to atoms on which there is at least a partial or effective positive charge. Particles with unshared electron pairs are called nucleophilic agents (nucleus- the nucleus, the positively charged part of the atom), that is, the "friends" of the nucleus, the positive charge.
Nucleophiles ($Nu$)- anions or molecules that have a lone pair of electrons that interact with parts of the molecules on which an effective positive charge is concentrated.
Examples of nucleophiles: $Cl^(-)$ (chloride ion), $OH^(-)$ (hydroxide anion), $CH_3O^(-)$ (methoxide anion), $CH_3COO^(-)$ ( acetate anion).
Particles that have an unfilled orbital, on the contrary, will tend to fill it and, therefore, will be attracted to the regions of the molecules that have an increased electron density, a negative charge, and an unshared electron pair. They are electrophiles, "friends" of an electron, a negative charge, or particles with an increased electron density.
electrophiles- cations or molecules that have an unfilled electron orbital, tending to fill it with electrons, as this leads to a more favorable electronic configuration of the atom.
Examples of electrophiles: $NO_2$ (nitro group), -$COOH$ (carboxyl), -$CN$ (nitrile group), -$COH$ (aldehyde group).
Not every particle with an empty orbital is an electrophile. So, for example, alkali metal cations have the configuration of inert gases and do not tend to acquire electrons, since they have a low electron affinity. From this we can conclude that, despite the presence of an unfilled orbital, such particles will not be electrophiles.
Main reaction mechanisms
We have identified three main types of reacting particles - free radicals, electrophiles, nucleophiles - and three types of reaction mechanism corresponding to them:
- free radical;
- electrophilic;
- nucleophilic.
In addition to classifying reactions according to the type of reacting particles, organic chemistry distinguishes four types of reactions according to the principle of changing the composition of molecules: addition, substitution, elimination, or elimination (from lat. elimination- delete, split off) and rearrangement. Since addition and substitution can occur under the action of all three types of reactive species, several main reaction mechanisms can be distinguished.
1.Free radical substitution:
$(CH_4)↙(\text"methane")+Br_2(→)↖(\text"UV light")(CH_3Br)↙(\text"bromomethane")+HBr$
2. Free radical addition:
$nCH_2=CH_2(→)↖(\text"UV light,R")(...-CH_2-CH_2-...)_n$
3. Electrophilic substitution:
4. Electrophilic connection:
$CH_3-(CH=)↙(\text"propene")CH_2+HBr(→)↖(\text"solution")(CH_3-CHBr-CH_3)↙(\text"2-bromopropane")$
$CH_3(-C≡)↙(\text"propyne")CH+Cl_2(→)↖(\text"solution")(CH_3-CCl=CHCl)↙(\text"1,2-dichloropropene")$
5. Nucleophilic addition:
In addition, we will consider the cleavage or elimination reactions that take place under the influence of nucleophilic particles - bases.
6. Elimination:
$CH_3-CHBr-CH_3+NaOH(→)↖(\text"alcohol solution")CH_3-CH=CH_2+NaBr+H_2O$
Rule of V. V. Markovnikov
A distinctive feature of alkenes (unsaturated hydrocarbons) is the ability to enter into addition reactions. Most of these reactions proceed by the mechanism electrophilic addition.
Hydrohalogenation (addition of hydrogen halide):
$CH_3(-CH-)↙(\text"propene")CH_2+HBr→CH_3(-CHBr-CH_3)↙(\text"2-bromopropane")$
This reaction is subject to V. V. Markovnikov's rule: when a hydrogen halide is added to an alkene, hydrogen is added to a more hydrogenated carbon atom, i.e. the atom at which there are more hydrogen atoms, and the halogen - to the less hydrogenated one.
; in this case, one p-bond is broken and one or two s-bonds are formed. To denote the addition of reactions, the symbol Ad is used (from the English. addition - accession); for p-tions of cycloaddition, such a symbol is not used.
Depending on the nature of the substrate, addition reactions are distinguished by isolated or conjugated multiple bonds, for example: C = C, C = C, C = C-C = C, C = O, C = N, C = N. Distinguish p-tion homolytic (Ad R) and heterolytic. accessions. The latter, depending on the charge of the attacking reagent, are subdivided into districts of electroph. (Ad E) and nucleoph. (AdN)additions. The behavior of the reagent depends on the type of substrate and the conditions for conducting the p-tion (solvent, the presence of a catalyst, the effect of UV irradiation, etc.). Mn. reagents in different conditions can show dec. types of reactions. abilities, eg. halogens can act as radical, electrophobic. and even the nucleoph. agents.
Naib. the addition reactions over multiple carbon-carbon bonds have been studied. These processes proceed according to a stepwise (staged) or synchronous (coordinated) mechanism. With a stepwise mechanism, the first stage is the attack of the nucleophile, electrophile, or free. radical, the second is the recombination of the resulting intermediate with positive., Negative. or a neutral particle, for example:
Electrof. or nucleoph. the particles need not be ions; they can represent an electron-withdrawing or electron-donating part (group) of a molecule. R-tions of Ad N are possible only with C=C bonds activated by electron-withdrawing substituents; the implementation of Ad E requires either unsubstituted C=C bonds or those activated by electron-donating substituents. For p-tion Ad R, the nature of the substituent in the C=C bond does not matter much.
Stereochem. the result of the step addition depends on the p-tion mechanism and the nature of the reacting compounds. Yes, electro. addition to olefins can proceed as son-addition - particles Y and W attack the molecule from one side of the plane of the double bond or as anti-attachment - particles attack from different sides of the plane; in some cases, districts go non-stereospecifically. Nucleof. addition involving carbanions proceeds, as a rule, non-stereospecifically. In triple bond addition reactions, syn addition leads to the cis isomer, and anti addition to the trans isomer.
In the case of a synchronous mechanism, the attack on both C atoms is carried out simultaneously and the p-tion proceeds as a dipolar addition (see Cycloaddition), while the addition of the reaction at a double or triple bond proceeds as a son-addition (see, for example, Reppe reactions).
P addition reactions at conjugated double bonds proceeding by a stepwise mechanism lead to the formation of 1,2- and 1,4-addition products:
Synchronous 1,4-addition to dienes proceeds as follows. way:
A special type of addition reactions is conjugated addition. The flow of such p-tions is accompanied by the binding of the p-solvent (or a specially added reagent) at the final stage of the process. For example, conjugated elektrof. the addition of halogens to alkenes in CH 3 COOH leads, along with 1,2-dihalides, to b-acetoxyalkyl halides:
Examples of conjugated nucleoph. accession - Michael reaction and interaction. activated alkenes with cyanide anion in protic p-solvents SH:
In the case of addition reactions on multiple carbon-hetero-atom bonds, in which put. the charge is localized on the C atom (C=O, C=N, C=N and C=S bonds), nucleophiles always attach to the C atom, and electrophiles to the heteroatom. In naib. nucleophilic addition reactions at the carbonyl group have been studied to a degree:
P connection reaction at the C atom can be one of the stages of p-tion substitution in aromatic. in a row, for example:
Chemical properties of alkanes
Alkanes (paraffins) are non-cyclic hydrocarbons, in the molecules of which all carbon atoms are connected only by single bonds. In other words, there are no multiple, double or triple bonds in the molecules of alkanes. In fact, alkanes are hydrocarbons containing the maximum possible number of hydrogen atoms, and therefore they are called limiting (saturated).
Due to saturation, alkanes cannot enter into addition reactions.
Since carbon and hydrogen atoms have fairly close electronegativity, this leads to the fact that the CH bonds in their molecules are extremely low polarity. In this regard, for alkanes, reactions proceeding according to the mechanism of radical substitution, denoted by the symbol S R, are more characteristic.
1. Substitution reactions
In reactions of this type, carbon-hydrogen bonds are broken.
RH + XY → RX + HY
Halogenation
Alkanes react with halogens (chlorine and bromine) under the action of ultraviolet light or with strong heat. In this case, a mixture of halogen derivatives with different degrees of substitution of hydrogen atoms is formed - mono-, di-tri-, etc. halogen-substituted alkanes.
On the example of methane, it looks like this:
By changing the halogen/methane ratio in the reaction mixture, it is possible to ensure that any particular methane halogen derivative predominates in the composition of the products.
reaction mechanism
Let us analyze the mechanism of the free radical substitution reaction using the example of the interaction of methane and chlorine. It consists of three stages:
- initiation (or chain initiation) - the process of formation of free radicals under the action of energy from the outside - irradiation with UV light or heating. At this stage, the chlorine molecule undergoes a homolytic cleavage of the Cl-Cl bond with the formation of free radicals:
Free radicals, as can be seen from the figure above, are called atoms or groups of atoms with one or more unpaired electrons (Cl, H, CH 3 , CH 2, etc.);
2. Chain development
This stage consists in the interaction of active free radicals with inactive molecules. In this case, new radicals are formed. In particular, when chlorine radicals act on alkane molecules, an alkyl radical and hydrogen chloride are formed. In turn, the alkyl radical, colliding with chlorine molecules, forms a chlorine derivative and a new chlorine radical:
3) Break (death) of the chain:
Occurs as a result of the recombination of two radicals with each other into inactive molecules:
2. Oxidation reactions
Under normal conditions, alkanes are inert with respect to such strong oxidizing agents as concentrated sulfuric and nitric acids, permanganate and potassium dichromate (KMnO 4, K 2 Cr 2 O 7).
Combustion in oxygen
A) complete combustion with an excess of oxygen. Leads to the formation of carbon dioxide and water:
CH 4 + 2O 2 \u003d CO 2 + 2H 2 O
B) incomplete combustion with a lack of oxygen:
2CH 4 + 3O 2 \u003d 2CO + 4H 2 O
CH 4 + O 2 \u003d C + 2H 2 O
Catalytic oxidation with oxygen
As a result of heating alkanes with oxygen (~200 o C) in the presence of catalysts, a wide variety of organic products can be obtained from them: aldehydes, ketones, alcohols, carboxylic acids.
For example, methane, depending on the nature of the catalyst, can be oxidized to methyl alcohol, formaldehyde, or formic acid:
3. Thermal transformations of alkanes
Cracking
Cracking (from the English to crack - to tear) is a chemical process occurring at high temperature, as a result of which the carbon skeleton of alkane molecules breaks with the formation of alkene and alkane molecules with lower molecular weights compared to the original alkanes. For example:
CH 3 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 3 → CH 3 -CH 2 -CH 2 -CH 3 + CH 3 -CH \u003d CH 2
Cracking can be thermal or catalytic. For the implementation of catalytic cracking, due to the use of catalysts, significantly lower temperatures are used compared to thermal cracking.
Dehydrogenation
The elimination of hydrogen occurs as a result of breaking the C-H bonds; carried out in the presence of catalysts at elevated temperatures. Dehydrogenation of methane produces acetylene:
2CH 4 → C 2 H 2 + 3H 2
Heating methane to 1200 ° C leads to its decomposition into simple substances:
CH 4 → C + 2H 2
Dehydrogenation of other alkanes gives alkenes:
C 2 H 6 → C 2 H 4 + H 2
When dehydrogenating n-butane, butene-1 and butene-2 are formed (the latter in the form cis- and trance-isomers):
Dehydrocyclization
Isomerization
Chemical properties of cycloalkanes
The chemical properties of cycloalkanes with more than four carbon atoms in the cycles are generally almost identical to those of alkanes. For cyclopropane and cyclobutane, oddly enough, addition reactions are characteristic. This is due to the high tension within the cycle, which leads to the fact that these cycles tend to break. So cyclopropane and cyclobutane easily add bromine, hydrogen or hydrogen chloride:
Chemical properties of alkenes
1. Addition reactions
Since the double bond in alkene molecules consists of one strong sigma bond and one weak pi bond, they are quite active compounds that easily enter into addition reactions. Alkenes often enter into such reactions even under mild conditions - in the cold, in aqueous solutions and organic solvents.
Hydrogenation of alkenes
Alkenes are able to add hydrogen in the presence of catalysts (platinum, palladium, nickel):
CH 3 -CH \u003d CH 2 + H 2 → CH 3 -CH 2 -CH 3
Hydrogenation of alkenes proceeds easily even at normal pressure and slight heating. An interesting fact is that the same catalysts can be used for the dehydrogenation of alkanes to alkenes, only the dehydrogenation process proceeds at a higher temperature and lower pressure.
Halogenation
Alkenes easily enter into an addition reaction with bromine both in aqueous solution and in organic solvents. As a result of the interaction, initially yellow solutions of bromine lose their color, i.e. discolor.
CH 2 \u003d CH 2 + Br 2 → CH 2 Br-CH 2 Br
Hydrohalogenation
It is easy to see that the addition of a hydrogen halide to an unsymmetrical alkene molecule should theoretically lead to a mixture of two isomers. For example, when hydrogen bromide is added to propene, the following products should be obtained:
Nevertheless, in the absence of specific conditions (for example, the presence of peroxides in the reaction mixture), the addition of a hydrogen halide molecule will occur strictly selectively in accordance with the Markovnikov rule:
The addition of a hydrogen halide to an alkene occurs in such a way that hydrogen is attached to a carbon atom with a larger number of hydrogen atoms (more hydrogenated), and a halogen is attached to a carbon atom with a smaller number of hydrogen atoms (less hydrogenated).
Hydration
This reaction leads to the formation of alcohols, and also proceeds in accordance with the Markovnikov rule:
As you might guess, due to the fact that the addition of water to the alkene molecule occurs according to the Markovnikov rule, the formation of primary alcohol is possible only in the case of ethylene hydration:
CH 2 \u003d CH 2 + H 2 O → CH 3 -CH 2 -OH
It is by this reaction that the main amount of ethyl alcohol is carried out in the large-capacity industry.
Polymerization
A specific case of the addition reaction is the polymerization reaction, which, unlike halogenation, hydrohalogenation and hydration, proceeds through a free radical mechanism:
Oxidation reactions
Like all other hydrocarbons, alkenes burn easily in oxygen to form carbon dioxide and water. The equation for the combustion of alkenes in excess oxygen has the form:
C n H 2n + (3/2)nO 2 → nCO 2 + nH 2 O
Unlike alkanes, alkenes are easily oxidized. Under the action of an aqueous solution of KMnO 4 on alkenes, discoloration, which is a qualitative reaction to double and triple CC bonds in molecules of organic substances.
Oxidation of alkenes with potassium permanganate in a neutral or slightly alkaline solution leads to the formation of diols (dihydric alcohols):
C 2 H 4 + 2KMnO 4 + 2H 2 O → CH 2 OH–CH 2 OH + 2MnO 2 + 2KOH (cooling)
In an acidic environment, a complete cleavage of the double bond occurs with the transformation of the carbon atoms that formed the double bond into carboxyl groups:
5CH 3 CH=CHCH 2 CH 3 + 8KMnO 4 + 12H 2 SO 4 → 5CH 3 COOH + 5C 2 H 5 COOH + 8MnSO 4 + 4K 2 SO 4 + 17H 2 O (heating)
If the double C=C bond is at the end of the alkene molecule, then carbon dioxide is formed as the oxidation product of the extreme carbon atom at the double bond. This is due to the fact that the intermediate oxidation product, formic acid, is easily oxidized by itself in an excess of an oxidizing agent:
5CH 3 CH=CH 2 + 10KMnO 4 + 15H 2 SO 4 → 5CH 3 COOH + 5CO 2 + 10MnSO 4 + 5K 2 SO 4 + 20H 2 O (heating)
In the oxidation of alkenes, in which the C atom at the double bond contains two hydrocarbon substituents, a ketone is formed. For example, the oxidation of 2-methylbutene-2 produces acetone and acetic acid.
The oxidation of alkenes, which breaks the carbon skeleton at the double bond, is used to establish their structure.
Chemical properties of alkadienes
Addition reactions
For example, the addition of halogens:
Bromine water becomes colorless.
Under normal conditions, the addition of halogen atoms occurs at the ends of the butadiene-1,3 molecule, while π bonds are broken, bromine atoms are attached to the extreme carbon atoms, and free valences form a new π bond. Thus, as if there is a "movement" of the double bond. With an excess of bromine, one more bromine molecule can be added at the site of the formed double bond.
polymerization reactions
Chemical properties of alkynes
Alkynes are unsaturated (unsaturated) hydrocarbons and therefore are capable of entering into addition reactions. Among the addition reactions for alkynes, electrophilic addition is the most common.
Halogenation
Since the triple bond of alkyne molecules consists of one stronger sigma bond and two weaker pi bonds, they are able to attach either one or two halogen molecules. The addition of two halogen molecules by one alkyne molecule proceeds by the electrophilic mechanism sequentially in two stages:
Hydrohalogenation
The addition of hydrogen halide molecules also proceeds by the electrophilic mechanism and in two stages. In both stages, the addition proceeds in accordance with the Markovnikov rule:
Hydration
The addition of water to alkynes occurs in the presence of ruthium salts in an acidic medium and is called the Kucherov reaction.
As a result of the hydration of the addition of water to acetylene, acetaldehyde (acetic aldehyde) is formed:
For acetylene homologues, the addition of water leads to the formation of ketones:
Alkyne hydrogenation
Alkynes react with hydrogen in two steps. Metals such as platinum, palladium, nickel are used as catalysts:
Alkyne trimerization
When acetylene is passed over activated carbon at high temperature, a mixture of various products is formed from it, the main of which is benzene, a product of acetylene trimerization:
Dimerization of alkynes
Acetylene also enters into a dimerization reaction. The process proceeds in the presence of copper salts as catalysts:
Alkyne oxidation
Alkynes burn in oxygen:
C n H 2n-2 + (3n-1) / 2 O 2 → nCO 2 + (n-1) H 2 O
The interaction of alkynes with bases
Alkynes with a triple C≡C at the end of the molecule, unlike other alkynes, are able to enter into reactions in which the hydrogen atom in the triple bond is replaced by a metal. For example, acetylene reacts with sodium amide in liquid ammonia:
HC≡CH + 2NaNH 2 → NaC≡CNa + 2NH 3,
and also with an ammonia solution of silver oxide, forming insoluble salt-like substances called acetylenides:
Thanks to this reaction, it is possible to recognize alkynes with a terminal triple bond, as well as to isolate such an alkyne from a mixture with other alkynes.
It should be noted that all silver and copper acetylenides are explosive substances.
Acetylides are able to react with halogen derivatives, which is used in the synthesis of more complex organic compounds with a triple bond:
CH 3 -C≡CH + NaNH 2 → CH 3 -C≡CNa + NH 3
CH 3 -C≡CNa + CH 3 Br → CH 3 -C≡C-CH 3 + NaBr
Chemical properties of aromatic hydrocarbons
The aromatic nature of the bond affects the chemical properties of benzenes and other aromatic hydrocarbons.
A single 6pi electron system is much more stable than conventional pi bonds. Therefore, for aromatic hydrocarbons, substitution reactions are more characteristic than addition reactions. Arenes enter into substitution reactions by an electrophilic mechanism.
Substitution reactions
Halogenation
Nitration
The nitration reaction proceeds best under the action of not pure nitric acid, but its mixture with concentrated sulfuric acid, the so-called nitrating mixture:
Alkylation
The reaction in which one of the hydrogen atoms at the aromatic nucleus is replaced by a hydrocarbon radical:
Alkenes can also be used instead of halogenated alkanes. Aluminum halides, ferric iron halides or inorganic acids can be used as catalysts.<
Addition reactions
hydrogenation
Accession of chlorine
It proceeds by a radical mechanism under intense irradiation with ultraviolet light:
Similarly, the reaction can proceed only with chlorine.
Oxidation reactions
Combustion
2C 6 H 6 + 15O 2 \u003d 12CO 2 + 6H 2 O + Q
incomplete oxidation
The benzene ring is resistant to oxidizing agents such as KMnO 4 and K 2 Cr 2 O 7 . The reaction does not go.
Division of substituents in the benzene ring into two types:
Consider the chemical properties of benzene homologues using toluene as an example.
Chemical properties of toluene
Halogenation
The toluene molecule can be considered as consisting of fragments of benzene and methane molecules. Therefore, it is logical to assume that the chemical properties of toluene should to some extent combine the chemical properties of these two substances taken separately. In particular, this is precisely what is observed during its halogenation. We already know that benzene enters into a substitution reaction with chlorine by an electrophilic mechanism, and catalysts (aluminum or ferric halides) must be used to carry out this reaction. At the same time, methane is also capable of reacting with chlorine, but by a free radical mechanism, which requires irradiation of the initial reaction mixture with UV light. Toluene, depending on the conditions under which it undergoes chlorination, is able to give either the products of substitution of hydrogen atoms in the benzene ring - for this you need to use the same conditions as in the chlorination of benzene, or the products of substitution of hydrogen atoms in the methyl radical, if on it, how to act on methane with chlorine when irradiated with ultraviolet radiation:
As you can see, the chlorination of toluene in the presence of aluminum chloride led to two different products - ortho- and para-chlorotoluene. This is due to the fact that the methyl radical is a substituent of the first kind.
If the chlorination of toluene in the presence of AlCl 3 is carried out in excess of chlorine, the formation of trichlorine-substituted toluene is possible:
Similarly, when toluene is chlorinated in the light at a higher chlorine / toluene ratio, dichloromethylbenzene or trichloromethylbenzene can be obtained:
Nitration
The substitution of hydrogen atoms for nitrogroup, during the nitration of toluene with a mixture of concentrated nitric and sulfuric acids, leads to substitution products in the aromatic nucleus, and not in the methyl radical:
Alkylation
As already mentioned, the methyl radical is an orientant of the first kind, therefore, its Friedel-Crafts alkylation leads to substitution products in the ortho and para positions:
Addition reactions
Toluene can be hydrogenated to methylcyclohexane using metal catalysts (Pt, Pd, Ni):
C 6 H 5 CH 3 + 9O 2 → 7CO 2 + 4H 2 O
incomplete oxidation
Under the action of such an oxidizing agent as an aqueous solution of potassium permanganate, the side chain undergoes oxidation. The aromatic nucleus cannot be oxidized under such conditions. In this case, depending on the pH of the solution, either a carboxylic acid or its salt will be formed.
