Aromatic hydrocarbons (arenes). Aromatic hydrocarbons Aromatic hydrocarbons derivatives chemical properties
Hückel's rule on aromaticity of the (4n+2)-electron system was derived for monocyclic systems. On polycyclic fused (i.e., containing several benzene rings with common vertices) systems, it can be transferred for systems having atoms common to two cycles, for example, for naphthalene, anthracene, phenanthrene, biphenylene shown below: (note 12)
For compounds that have at least one atom in common three cycles (for example, for pyrene), Hückel's rule not applicable.
Bicyclic annulenes - naphthalene or azulene are electronic analogues of -annulenes with ten -electrons (see section ii.2). Both of these compounds have aromatic properties, but naphthalene is colorless, and azulene is colored dark blue, since the bipolar structure, which is a combination of nuclei of cyclopentadienyl anion and tropylium cation, makes a significant contribution to its structure:
The reactivity of condensed aromatic hydrocarbons is somewhat increased compared to monocyclic arenes: they are more easily oxidized and reduced, and enter into addition and substitution reactions. See Section II.5 for reasons for this difference in reactivity.
II.4. Hydrocarbons with isolated benzene rings. Triphenylmethanes.
Of the hydrocarbons with isolated benzene rings, the most interesting are di- and tri-phenylmethanes, as well as biphenyl. (Note 13) The properties of benzene rings in di- and triphenylmethanes are the same as in ordinary alkylbenzenes. Features of their chemical behavior are manifested in properties of the C-H bond of the aliphatic ("methane") part of the molecule. The ease of hetero- or homolytic rupture of this bond depends primarily on the possibility of delocalization of the emerging positive or negative charge (in the case of a heterolytic rupture) or the unpaired electron (in the case of a homolytic rupture). In the di- and especially in the tri-phenylmethane system, the possibility of such a delocalization is extremely high.
Consider first the ability of phenylated methanes to dissociation of C-H bonds with the elimination of a proton( CH-acidity ). The strength of CH-acids, as well as ordinary protic OH-acids, is determined by the stability, and hence the ease of formation, of the corresponding anions (in the case under consideration, carbanions). The stability and ease of formation of anions, in turn, are determined by the possibility of delocalization of the negative charge in them. Each benzene nucleus associated with the benzyl carbon atom can take part in the delocalization of the negative charge arising on it, which can be represented using boundary (resonant) structures:
For diphenylmethane, seven boundary structures can already be depicted:
and for triphenylmethane, ten:
Since the ability to delocalize increases with the number of possible boundary structures, di- and especially triphenylmethyl anions should be especially stable. take part in charge delocalization on the central carbon atom, i.e. rise in a row
CH 4< С 6 Н 5 СН 3 < (С 6 Н 5) 2 СН 2 < (С 6 Н 5) 3 СН
p-values K a of these hydrocarbons, determined by special methods, confirm this assumption. Diphenylmethane (p K a 33) is approximately equal in acidity to ammonia, and triphenylmethane (p K a 31.5) - tert- butanol; triphenylmethane more than 10 10 times acidic than methane (p K a~ 40).(note 15)
Cherry-colored triphenylmethylsodium is usually prepared by reducing triphenylchloromethane with sodium amalgam:
Unlike conventional CH bonds sp 3-hybrid carbon atom, benzyl C-H bond tri- pair- nitrophenylmethane is already heterolytically cleaved with alcohol alkali:
In the latter case, in addition to three benzene nuclei, three nitro groups additionally participate in the delocalization of the negative charge in the anion.
Another type of heterolytic cleavage of the benzyl CH bond is the abstraction of the hydride anion with the formation of the corresponding carbocations benzyl type:
Since benzene nuclei are capable of stabilizing both positive and negative charges, phenylated methanes on hydride mobility hydrogen in the aliphatic part will be the same row as by proton mobility, i.e. CH 4< С 6 Н 5 СН 3 < (С 6 Н 5) 2 СН 2 < (С 6 Н 5) 3 СН.
However, it is usually difficult to experimentally compare the ease of abstraction of the hydride anion, since very active Lewis acids are usually used to carry out such abstraction. Comparative estimates can easily be made by comparing the mobility of a halogen (usually chlorine) under conditions S N 1 reactions, since in this case, as in the case of the elimination of the hydride anion, the stage that determines the rate of transformation is the formation of the corresponding carbocation. Indeed, it turned out that under these conditions, chlorine has the highest mobility in triphenylchloromethane, and the lowest in benzyl chloride:
Ar-CR 2 -Cl ArCR 2 + + Cl - ; R=H or R=Ar
reaction rate: (C 6 H 5) 3 C-Cl > (C 6 H 5) 2 CH-Cl > C 6 H 5 CH 2 -Cl
The reactivity of chlorine in the first of them resembles that in carboxylic acid chlorides, and in the second - in allyl chloride. Below are data on the relative rates of solvolysis of R-Cl chlorides in formic acid at 25 o C:
R-Cl + HCOOH R-O-C(O)H + HCl
Comparative stability of triphenylmethyl ( trityl ) of the cation is also confirmed by many other experimental data. An example is the ease of formation of its salts with non-nucleophilic anions, solutions of which in polar aprotic solvents are electrically conductive (and, therefore, have an ionic structure) and are characteristically colored yellow:
The same is evidenced by the ability of triphenylchloromethane to dissociate into triphenylmethyl cation and chloride anion in a solution of liquid sulfur dioxide:
The stability of the triphenylmethyl cation can be further increased by introducing into benzene rings electron donor groups(for example, amino-, alkyl- and dialkylamino-, hydroxyl, alkoxy). A further increase in the stability of the carbocation leads to a situation where it becomes stable in aqueous solution, that is, the equilibrium of the reaction
shifted to the left. Similar trityl cations not only resistant, but also painted. An example is the intensely purple tri(4-dimethylaminophenyl)methyl cation. Its chloride is used as a dye called " crystal violet ". In crystal violet, the positive charge is dispersed between the three nitrogen atoms and the nine carbon atoms of the benzene nuclei. Participation of one of the three pair-dimethylaminophenyl substituents in the positive charge delocalization can be reflected using the following boundary structures:
All triphenylmethane dyes containing amine or substituted amine groups in the benzene ring acquire a color in an acid medium, which, as shown above with the example of crystal violet, contributes to the formation of a structure with an extended conjugation chain (structure I in the diagram) - the so-called quinoid structure . Below are the formulas for the most common triphenylmethane dyes.
Similar to that considered above for the triphenylmethyl anion and cation, the benzene rings should also have an effect on the stability triphenylmethyl radical . In the latter case, the ease of breaking the bond formed by the central carbon atom with a "non-phenyl" substituent is due, to a certain extent, to other reasons. The fact is that in triphenylmethane, triphenylchloromethane, triphenylcarbinol, etc. the central carbon atom is in sp 3-hybrid state and, accordingly, has a tetrahedral configuration. For this reason, the phenyl nuclei are not located in the same plane and not conjugated. When passing to a triphenylmethyl cation (heterolytic gap) or a radical (homolytic gap), the central carbon atom is in sp 2- hybrid state; as a result of this, the structure is flattened (note 17) and the interaction (conjugation) between the three phenyl nuclei is enhanced. This partially compensates for the energy costs associated with the dissociation under consideration, and thus facilitates it.
Triphenylmethyl radical
can be generated from the corresponding chloride by the action of zinc, copper or silver, which in this case act as electron donors:This radical is quite stable and dimerizes only partially in dilute solutions (in ether, benzene). For a long time, the structure of hexaphenylethylene was attributed to this dimer, but it turned out that, in fact, during dimerization, a bond arises between the central carbon atom of one radical and pair- the position of one of the phenyl nuclei of the other radical:
Apparently, in the case under consideration, one triphenylmethyl radical attacks least spatially obstructed place another, and, naturally, one of those places that participates in the delocalization of the unpaired electron.
The degree of dissociation of such dimers strongly depends on the nature of the aryl radicals. Thus, in a 0.1 M benzene solution at 25°, the triphenylmethyl radical dimerizes by 97%, while the tri-4-nitrophenylmethyl radical does not dimerize at all.
Aromatic hydrocarbons (arenes) are compounds containing an aromatic system, which determines their common features in structure and chemical properties.
Methods for obtaining aromatic hydrocarbons
1. Benzene, toluene, xylenes, naphthalene- isolated from coal tar formed during coal coking.
2. Some oils contain benzene and toluene.
But the main way to obtain arenes from oil is its aromatization: catalytic cyclization and dehydrogenation of alkanes. For example:
3. Obtaining alkylbenzenes (Fradel-Crafts reaction)
4. Obtaining diphenyl
Chemical properties of aromatic hydrocarbons
1. Electrophilic substitution reactions (SE)
Effect of Substituents on the Rate and Direction of ReactionsSE.
Different substituents change the electron density in the benzene ring, and it becomes different on different carbon atoms.
This changes the reaction rate SE and makes it different for different positions of the cycle.
A special position is occupied by halogen substituents:
Due to the +M-effect, they orient the reaction to the ortho- and para-positions (as substituents of the first kind), but their –I-effect exceeds the mesomeric one in absolute value: the total electron density in the cycle decreases and the rate of the SE reaction decreases.
Orientation in disubstituted benzene
1. Consistent orientation:
2. In case of inconsistent orientation, the following are taken into account:
a) the influence of a more strongly activating group:
b) spatial difficulties:
Types of electrophilic substitution reactions
1. Halogenation
2. Nitration
3. Sulfonation
Alkylation and acylation according to Friedel-Crafts
4. Alkylation
5. Acylation
2. Reactions of benzene with the destruction of the aromatic system
1.Oxidation
2. Recovery (hydrogenation)
3. Radical chlorination
3. Side chain reactions of alkylbenzenes
1. Radical substitution
Other alkylbenzenes are chlorinated at the α-position:
2. Oxidation
All monoalkylbenzenes, when oxidized with KMnO4 in an alkaline medium, give benzoic acid.
POLYCYCLIC AROMATIC HYDROCARBONS WITH ISOLATED CYCLES
Aromatic hydrocarbons with multiple benzene rings are divided into:
1. Hydrocarbons with non-condensed cycles. These include biphenyl and di- and triphenylmethanes.
2. Hydrocarbons with condensed cycles. These include naphthalene, anthracene and phenanthrene.
Biphenyl group
Definition: Aromatic compounds in which two (or more) rings (rings) are connected to each other by a single bond are called polycyclic aromatic hydrocarbons with isolated rings.
Biphenyl is considered as an example:
In industry, biphenyl is produced by pyrolysis of benzene:
The laboratory method of preparation is the action of sodium or copper on iodobenzene, or in the presence of electron-withdrawing substituents in the aryl halides, which increase the mobility of the halogen in the nucleus:
Biphenyl is a crystalline substance with T pl. 70 0 C, T b.p. 254 0 C. Thermodynamically stable. It is used in industry as a high-temperature coolant.
Biphenyl participates much more actively than benzene in electrophilic aromatic substitution reactions. Bromination of biphenyl with an equimolar amount of bromine leads to the formation of 4-bromobiphenyl. An excess of bromine leads to the formation of 4,4`-dibromobiphenyl:
Biphenyl nitration reactions, Friedel-Crafts acelation, and other electrophilic aromatic substitution reactions proceed similarly.
Polyphenylmethanes
Definition: Aromatic compounds in which from two to four benzene rings are connected to one carbon atom in the state of sp 3 hybridization.
The founder of the homologous series of polyphenylmethane is toluene, the following compound is diphenylmethane:
Di- and triphenylmethane are produced using benzene by the Friedel-Crafts reaction by two methods:
1. From methylene chloride and chloroform:
2. From benzyl chloride and benzylidene chloride:
Diphenylmethane is a crystalline substance with T pl. 26-27 0 C, has the smell of orange.
When diphenylmethane is oxidized, benzophenone is formed:
The structure of triphenylmethane forms the basis of the so-called dyes of the triphenylmethane series:
1. Malachite green (brilliant green) is obtained by the Friedel-Crafts reaction:
2. Phenolphthalein.
Obtained by the reaction of phenol and phthalic anhydride (phthalic anhydride) in the presence of sulfuric acid:
CONDENSED BENZOID HYDROCARBONS
Hydrocarbons containing two or more benzene rings sharing two carbon atoms are called fused benzenoid hydrocarbons.
Naphthalene
The simplest of the condensed benzoic hydrocarbons is naphthalene:
Positions 1,4,5 and 8 are designated "α", positions 2, 3,6,7 are designated "β".
Ways to get.
The bulk of naphthalene is obtained from coal tar.
In laboratory conditions, naphthalene can be obtained by passing benzene and acetylene vapors over charcoal:
Dehydrocyclization over platinum of benzene homologues with a side chain of four or more carbon atoms:
By the reaction of the diene synthesis of 1,3-butadiene with P-benzoquinone:
Naphthalene is a crystalline substance with T pl. 80 0 C, characterized by high volatility.
Naphthalene enters into electrophilic substitution reactions more easily than benzene. In this case, the first substituent almost always becomes in the α-position:
Entry of an electrophilic agent into the β-position is less common. As a rule, this occurs in specific conditions. In particular, the sulfonation of naphthalene at 60 0 C proceeds as a kinetically controlled process with the predominant formation of 1-naphthalenesulfonic acid. Sulfonation of naphthalene at 160 0 C proceeds as a thermodynamically controlled process and leads to the formation of 2-naphthalenesulfonic acid:
When a second substituent is introduced into the naphthalene molecule, the orientation is determined by the nature of the substituent already present in it. Electron donor substituents located in the naphthalene molecule direct the attack to the same ring in the 2nd and 4th positions.
aromatic hydrocarbons- compounds of carbon and hydrogen, in the molecule of which there is a benzene ring. The most important representatives of aromatic hydrocarbons are benzene and its homologues - the products of substitution of one or more hydrogen atoms in the benzene molecule for hydrocarbon residues.
The structure of the benzene molecule
The first aromatic compound, benzene, was discovered in 1825 by M. Faraday. Its molecular formula was established - C 6 H 6. If we compare its composition with the composition of the saturated hydrocarbon containing the same number of carbon atoms - hexane (C 6 H 14), then we can see that benzene contains eight fewer hydrogen atoms. As is known, the appearance of multiple bonds and cycles leads to a decrease in the number of hydrogen atoms in a hydrocarbon molecule. In 1865, F. Kekule proposed its structural formula as cyclohexantriene-1,3,5.
Thus, the molecule corresponding to the Kekule formula contains double bonds, therefore, benzene must have an unsaturated character, i.e., it is easy to enter into addition reactions: hydrogenation, bromination, hydration, etc.
However, numerous experimental data have shown that benzene enters into addition reactions only under harsh conditions(at high temperatures and lighting), resistant to oxidation. The most characteristic of it are the substitution reactions, therefore, benzene is closer in character to saturated hydrocarbons.
Trying to explain these inconsistencies, many scientists have proposed various versions of the structure of benzene. The structure of the benzene molecule was finally confirmed by the reaction of its formation from acetylene. In fact, the carbon-carbon bonds in benzene are equivalent, and their properties are not similar to those of either single or double bonds.
Currently, benzene is denoted either by the Kekule formula, or by a hexagon in which a circle is depicted.
So what is the peculiarity of the structure of benzene?
Based on these studies and calculations, it was concluded that all six carbon atoms are in the state of sp 2 hybridization and lie in the same plane. The unhybridized p-orbitals of carbon atoms that make up double bonds (Kekule's formula) are perpendicular to the plane of the ring and parallel to each other.
They overlap with each other, forming a single π-system. Thus, the system of alternating double bonds depicted in the Kekule formula is a cyclic system of conjugated, overlapping π-bonds. This system consists of two toroidal (donut-like) regions of electron density lying on both sides of the benzene ring. Thus, it is more logical to depict benzene as a regular hexagon with a circle in the center (π-system) than as cyclohexantriene-1,3,5.
The American scientist L. Pauling proposed to represent benzene in the form of two boundary structures that differ in the distribution of electron density and constantly transform into each other:
The measured bond lengths confirm this assumption. It was found that all C-C bonds in benzene have the same length (0.139 nm). They are somewhat shorter than single C-C bonds (0.154 nm) and longer than double ones (0.132 nm).
There are also compounds whose molecules contain several cyclic structures, for example:
Isomerism and nomenclature of aromatic hydrocarbons
For benzene homologues the isomerism of the position of several substituents is characteristic. The simplest homologue of benzene is toluene(methylbenzene) - does not have such isomers; the following homologue is presented as four isomers:
The basis of the name of an aromatic hydrocarbon with small substituents is the word benzene. Atoms in an aromatic ring are numbered starting from senior deputy to junior:
If the substituents are the same, then numbering is carried out according to the shortest path: for example, substance:
called 1,3-dimethylbenzene, not 1,5-dimethylbenzene.
According to the old nomenclature, positions 2 and 6 are called ortho positions, 4 - para-, 3 and 5 - meta positions.
Physical properties of aromatic hydrocarbons
Benzene and its simplest homologues under normal conditions - highly toxic liquids with a characteristic unpleasant odour. They are poorly soluble in water, but well - in organic solvents.
Chemical properties of aromatic hydrocarbons
substitution reactions. Aromatic hydrocarbons enter into substitution reactions.
1. Bromination. When reacting with bromine in the presence of a catalyst, iron (III) bromide, one of the hydrogen atoms in the benzene ring can be replaced by a bromine atom:
2. Nitration of benzene and its homologues. When an aromatic hydrocarbon interacts with nitric acid in the presence of sulfuric acid (a mixture of sulfuric and nitric acids is called a nitrating mixture), a hydrogen atom is replaced by a nitro group - NO 2:
Reduction of nitrobenzene is obtained aniline- a substance that is used to obtain aniline dyes:
This reaction is named after the Russian chemist Zinin.
Addition reactions. Aromatic compounds can also enter into addition reactions to the benzene ring. In this case, cyclohexane and its derivatives are formed.
1. Hydrogenation. The catalytic hydrogenation of benzene proceeds at a higher temperature than the hydrogenation of alkenes:
2. Chlorination. The reaction proceeds under illumination with ultraviolet light and is a free radical:
Chemical properties of aromatic hydrocarbons - compendium
Benzene homologues
The composition of their molecules corresponds to the formula CnH2n-6. The closest homologues of benzene are:
All benzene homologues following toluene have isomers. Isomerism can be associated both with the number and structure of the substituent (1, 2), and with the position of the substituent in the benzene ring (2, 3, 4). Compounds of the general formula C 8 H 10 :
According to the old nomenclature used to indicate the relative position of two identical or different substituents in the benzene ring, prefixes are used ortho-(abbreviated as o-) - substituents are located at neighboring carbon atoms, meta-(m-) - through one carbon atom and pair-(n-) - substituents against each other.
The first members of the homologous series of benzene are liquids with a specific odor. They are lighter than water. They are good solvents. Benzene homologues enter into substitution reactions:
bromination:
nitration:
Toluene is oxidized by permanganate when heated:
Reference material for passing the test:
periodic table
Solubility table
Classification, nomenclature, isomerism
There are three main types of condensed systems: 1) linearly condensed (naphthalene, anthracene); 2) angularly condensed (phenanthrene); 4) peri-condensed (pyrene).
Naphthalene has 4 identical a - and 4 identical b - positions; there are two monosubstituted naphthalene - a - and b -. To indicate the position of substituents, the numbering of atoms in cycles is also used.
Anthracene has three sets of identical positions: (1-,4-,5-,8-); (2-,3-,6-,7-); (9-,10-). Thus, there are three monosubstituted anthracenes (1-, 2- and 9-).
Phenantrene contains 5 pairs of equivalent positions: 1 and 8, 2 and 7, 3 and 6, 4 and 5, 9 and 10. There are 5 isomers for monosubstituted phenanthrenes.
Acquisition Methods
The main source of condensed aromatic hydrocarbons is coal tar, which contains 8-12% naphthalene, 4-5% phenanthrene, 1-1.8% anthracene. Naphthalene is also isolated from oil refining products. The oil obtained during the catalytic reforming of gasoline is enriched with alkylnaphthalenes, which are converted into naphthalene by hydrodealkylation in the presence of a mixture of Co and Mo oxides.
Physical properties and structure
Naphthalene, anthracene and phenanthrene are colorless crystalline substances. Phenanatrene has a lower melting point and better solubility than anthracene.
Molecules of naphthalene, anthracene and phenanthrene have a flat structure, but the lengths of the C-C bonds in them are different. In naphthalene and anthracene, the C(1)-C(2) bond has the shortest length and the highest multiplicity; in phenanthrene, the C(9)-C(10) bond.
Hückel's closed aromaticity rule p -electron system is valid for monocyclic systems. It can be transferred to polycyclic condensed systems, provided that the bonds common to two cycles do not introduce serious perturbations into p -electronic system in comparison with the corresponding annules, but only provide the necessary coplanarity. Hückel's rule holds for polycyclic systems having atoms common to two cycles. Naphthalene (contains 10 p -electrons), as well as anthracene and phenanthrene (contain 14 p -electrons) are aromatic hydrocarbons. Aromatic properties are possessed by an electronic analogue and isomer of naphthalene - azulene, containing condensed seven- and five-membered rings. A significant contribution to its structure is made by the bipolar structure, which is a combination of the nuclei of the cyclopentadienyl anion and the tropylium cation:
For compounds having atoms common to three rings, Hückel's rule does not apply. For example, pyrene is an aromatic hydrocarbon, although its p-system contains 16 electrons, that is, it does not obey the formula (4n + 2).
Condensed aromatic hydrocarbons are less stabilized than benzene. The delocalization energy of naphthalene, determined from the heats of hydrogenation, is 255 kJ/mol, which is less than for two isolated benzene nuclei (150 kJ/mol x 2 = 300 kJ/mol). The stabilization energy of anthracene is 350, and that of phenanthrene is 385 kJ/mol, which is less than three times the stabilization energy of benzene.
Chemical properties
1) Electrophilic substitution reactions
Naphthalene, anthracene and phenanthrene enter into electrophilic substitution reactions more easily than benzene. This is due to the lower loss of stabilization energy at the stage of formation of the s-complex. The loss of stabilization energy as a result of disruption of the aromatic system during the formation of the s-complex in benzene is 150 kJ/mol. A similar value for naphthalene, in which, after the destruction of the aromatic system of one ring, the aromatic system of benzene remains, will be 255-150 = 105 kJ/mol. As a result of violation of the aromaticity of the central rings in anthracene and phenanthrene, each of them will contain two isolated benzene rings and the loss of stabilization energy will be 350 - 2x150 = 50 kJ/mol for anthracene and 385 - 2x150 = 85 kJ/mol for phenanthrene. If the aromaticity of the peripheral nuclei is disturbed, the aromatic system of naphthalene remains in anthracene and phenanthrene, and the loss of stabilization energy will be 350 – 255 = 95 kJ/mol for anthracene and 385 – 255 = 130 kJ/mol for phenanthrene.
From the data presented, it can be concluded that the central nuclei in anthracene and phenanthrene will be more reactive than the peripheral ones. Electrophilic substitution in these systems will in most cases go to the 9,10-position.
Electrophilic substitution in naphthalene occurs predominantly in the a-position. The direction of attack of the electrophile is determined by the relative stability of the s-complexes leading to substitution products at the a- and b-positions. For the arenonium ion formed upon attack at the a-position, two energetically favorable resonance structures can be depicted, in which the aromatic system of the second ring is not affected, while only one is affected upon attack at the b-position.
Energetically less favorable resonance structures, in which the aromaticity of both rings is violated, cannot be completely ruled out, but their contribution to the resonance stabilization is small.
Naphthalene nitrates under milder conditions than benzene, with the formation of a-nitronaphthalene as the main product.
The halogenation of naphthalene is also much easier than the halogenation of benzene. The latter can be used as a solvent in these reactions. Bromine reacts more selectively than chlorine.
The composition of naphthalene acylation products depends on the nature of the solvent.
Possibly, such selectivity of naphthalene acylation is related to the large volume of the CH 3 COCl complex. AlCl 3 . PhNO 2 versus CH 3 COCl complex. AlCl 3 . CS2.
Sulfonation of naphthalene is a classic example of the thermodynamic control of the composition of the reaction products. Under very mild conditions, only a-naphthalenesulfonic acid is formed. This condition is met by the sulfonation of naphthalene with chlorosulfonic acid at low temperatures. The ratio of isomers during sulfonation with 96% sulfuric acid depends on temperature: under mild conditions, the product of kinetic control, a-naphthalene sulfonic acid, predominates, under more severe conditions, the thermodynamically more stable b-naphthalene sulfonic acid predominates.
Anthracene
and phenanthrene.
Electrophilic substitution in these condensed systems can proceed both by the classical S E Ar mechanism with the formation of arenonium ions, and by the addition-elimination mechanism.
It has been proven that the halogenation and nitration of anthracene under mild conditions proceed through the intermediate formation of 9,10-addition products, which are easily converted into 9-anthracene derivatives.
The given examples demonstrate the "diene" nature of anthracene and its tendency to 1,4-addition reactions, which are characteristic of conjugated dienes.
At the same time, the acylation of anthracene is carried out under conditions typical for S E (Ar) processes.
In phenanthrene, the 9-10 carbon-carbon bond exhibits double bond properties in alkenes. Thus, bromination of phenanthrene at low temperature in a CCl 4 solution leads to the predominant formation of the 9,10-addition product.
Under more severe conditions or in the presence of a Lewis acid, only 9-bromophenanthrene is formed.
Experimental data show that it is not always possible to predict in advance the outcome of a particular electrophilic substitution reaction in condensed systems. For example, the acylation of phenanthrene does not lead to the formation of 9-acetylphenanthrene, but proceeds as follows:
2) Oxidation
Oxidation of condensed aromatic hydrocarbons results in different products depending on the reagent used and the reaction conditions. Reagents based on chromium (VI) in an acid medium oxidize naphthalene and alkylnaphthalenes to naphthoquinones, while sodium dichromate in an aqueous solution oxidizes only alkyl groups. The oxidation of naphthalene with potassium permanganate in an alkaline medium is accompanied by the destruction of one aromatic ring with the formation of monocyclic dicarboxylic acids:
Anthracene is smoothly oxidized with sodium bichromate in sulfuric acid or chromium (VI) oxide in acetic acid to anthraquinone:
3) Hydrogenation
Condensed aromatic hydrocarbons are more easily hydrogenated than benzene. During the catalytic hydrogenation of naphthalene, the sequential reduction of aromatic rings occurs.
Anthracene and phenanthrene are hydrogenated to 9,10-dihydro derivatives.
Of the hydrocarbons with isolated benzene rings, di- and triphenylmethanes, as well as biphenyl, are of the greatest interest.
Electrophilic substitution reactions
Experimental data show that biphenyl is more active in electrophilic substitution reactions than benzene. Electrophilic reagents attack ortho- and pair-positions of phenyl rings, and predominantly pair-position ( ortho-hydrogen atoms of one ring spatially shield ortho- the position of the other ring, which makes it difficult to attack them with an electrophile).
The structure of the s-complex formed after the attack of a biphenyl molecule by an electrophile can be represented as the following set of boundary structures:
The formation of resonant structures (IY), (Y), and (YI) should be difficult for the following reasons: 1) both rings in them should be coplanar, which will lead to a fairly strong mutual repulsion of ortho hydrogen atoms; 2) the aromatic system of the second benzene ring is disturbed, which is energetically unfavorable. On the other hand, the resonance structure (II) suggests a certain participation of the second ring in the delocalization of the positive charge in the s-complex. It is most likely that in this case a positive inductive rather than mesomeric (the condition for the formation of resonant structures IY, Y, and YI) effect of the second benzene ring manifests itself.
Biphenyl is easily halogenated, sulfonated, nitrated.
On passing from biphenyl to fluorene, in which both benzene rings are strictly coplanar and their mutual influence is more pronounced, the rate of electrophilic substitution reactions increases sharply. In this case, as a rule, 2-substituted fluorenes are formed.
In di- and triphenylmethanes, benzene rings are completely autonomous and in electrophilic substitution reactions they behave like monosubstituted benzenes containing bulky alkyl substituents.
Reactions of methylene and methine groups in di- and triarylmethanes
Features of the chemical behavior of di- and triphenylmethanes are manifested in the properties of the CH bond of the aliphatic ("methane") part of the molecule. The ease of hetero- or homolytic rupture of this bond depends primarily on the possibility of delocalization of the resulting positive or negative charge (in the case of a heterolytic rupture) or an unpaired electron (in the case of a homolytic rupture). In the di- and especially in the triphenylmethane system, the possibility of such a delocalization is extremely high.
Let us consider the ability of phenylated methanes to dissociate C-H bonds with proton elimination ( CH-acidity). The strength of CH-acids, as well as ordinary protonic acids, is determined by the stability, and hence the ease of formation, of the corresponding anions (in this case, carbanions). The stability and ease of formation of anions, in turn, are determined by the possibility of delocalization of the negative charge in them. Each benzene nucleus associated with the benzyl carbon atom can take part in the delocalization of the negative charge arising on it, which can be represented using resonance structures:
For diphenylmethane, seven boundary structures can already be depicted:
and for triphenylmethane, ten. Since the capacity for delocalization increases with the number of possible boundary structures, di- and especially triphenylmethyl anions should be particularly stable. In this regard, it can be expected that the CH-acidity of methanes will increase with an increase in the number of phenyl rings, which can take part in charge delocalization on the central carbon atom, i.e. rise in the following order:
CH 4< С 6 Н 5 СН 3 < (С 6 Н 5) 2 СН 2 < (С 6 Н 5) 3 СН
p-values K a of these hydrocarbons, determined by special methods, confirm this assumption. Diphenylmethane (p K a 33) is approximately equal in acidity to ammonia, and triphenylmethane (p K a 31.5) - tert- butanol and is more than 10 10 times more acidic than methane (p K a~ 40).
Cherry-colored triphenylmethylsodium is usually prepared by reducing triphenylchloromethane with sodium amalgam.
Unlike conventional CH bonds sp 3-hybrid carbon atom, benzyl C-H bond tri-( pair- nitrophenyl)methane is already heterolytically cleaved with alcoholic alkali.
In the latter case, in addition to three benzene nuclei, three nitro groups additionally participate in the delocalization of the negative charge in the anion.
Another type of heterolytic cleavage of the benzyl CH-bond is the abstraction of the hydride anion with the formation of the corresponding benzyl-type carbocations:
Since benzene nuclei are capable of stabilizing both positive and negative charges, phenylated methanes in terms of the hydride mobility of hydrogen in the aliphatic part will make up the same series as in terms of proton mobility:
CH 4< С 6 Н 5 СН 3 < (С 6 Н 5) 2 СН 2 <(С 6 Н 5) 3 СН.
However, as a rule, it is difficult to compare experimentally the ease of abstraction of the hydride anion, since very active Lewis acids are usually used to carry out such abstraction. Comparative estimates can easily be made by comparing the mobility of a halogen (usually chlorine) under conditions S N 1 reactions, since in this case, as in the case of the elimination of the hydride anion, the stage that determines the rate of transformation is the formation of the corresponding carbocation.
Ar-CR 2 -Cl® ArCR 2 + + Cl - ; (R = H, Ar)
Indeed, it turned out that under these conditions, chlorine has the highest mobility in triphenylchloromethane, and the lowest in benzyl chloride.
(C 6 H 5) 3 C-Cl> (C 6 H 5) 2 CH-Cl> C 6 H 5 CH 2 -Cl
The reactivity of chlorine in triphenylchloromethane resembles that in carboxylic acid chlorides, and in diphenylmethane - in allyl chloride. Below are data on the relative rates of solvolysis of R-Cl chlorides in formic acid at 25 o C:
R-Cl + HCOOH ® R-O-C(O)H + HCl
|
CH 2 \u003d CH-CH 2 |
C 6 H 5 -CH 2 |
(CH 3) 3 C |
(C 6 H 5) 2CH |
(C 6 H 5) 3 C |
|
|
Relative speeds |
0.04 |
0.08 |
3 . 10 6 |
Comparative stability of triphenylmethyl ( trityl) of the cation is also confirmed by many other experimental data. An example is the ease of formation of its salts with non-nucleophilic anions, solutions of which in polar aprotic solvents are electrically conductive (and, therefore, have an ionic structure) and are characteristically colored yellow:
The same is evidenced by the ability of triphenylchloromethane to dissociate into triphenylmethyl cation and chloride anion in a solution of liquid sulfur dioxide:
The stability of the triphenylmethyl cation increases with the introduction of electron-donating groups (for example, amino-, alkyl- and dialkylamino-, hydroxyl, alkoxyl) into the benzene rings. A further increase in the stability of the carbocation leads to a situation where it becomes stable in an aqueous solution, that is, the equilibrium of the reaction
shifted to the left.
Similar trityl cations are colored. An example is the intensely purple tri(4-dimethylaminophenyl)methyl cation. Its chloride is used as a dye called "crystal violet". In crystal violet, the positive charge is dispersed between the three nitrogen atoms and the nine carbon atoms of the benzene nuclei. Participation of one of the three pair-dimethylaminophenyl substituents in the positive charge delocalization can be reflected using the following boundary structures:
All triphenylmethane dyes containing amino groups in the benzene ring take on a color in an acidic environment, which contributes to the formation of quinoid structure with an extended chain of conjugation. Below are the formulas for the most common triphenylmethane dyes.
|
(P-R 2 N-C 6 H 4) 2 C + (C 6 H 5) Cl - |
R = CH 3 malachite green |
|
R = C 2 H 5 brilliant green |
|
|
R=H Debner violet |
|
|
(P-R 2 N-C 6 H 4) 3 C + Cl - |
R=H parafuxin |
|
R= CH 3 crystal violet |
The benzene rings should have a similar effect on the stability of the triphenylmethyl radical. The triphenylmethyl radical can be generated from the corresponding chloride by the action of zinc, copper, or silver, which in this case act as electron donors.
The triphenylmethyl radical is quite stable and dimerizes only partially in dilute solutions (in ether, benzene). During dimerization, a bond arises between the central carbon atom of one radical and pair- the position of one of the phenyl nuclei of the other radical.
Apparently, one triphenylmethyl radical attacks the least spatially hindered site of the other. The degree of dissociation of such dimers strongly depends on the nature of the aryl radicals. Thus, in a 0.1 M solution in benzene at 25°C, the triphenylmethyl radical dimerizes by 97%, while the tri-(4-nitrophenyl)methyl radical does not dimerize at all.
