Chapter - 12
Arenes
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Aliphatic-aromatic hydrocarbons
From our study so far, we know what kind of chemical properties to expect of an aliphatic hydrocarbon, that is, of an alkane, alkene, or alkyne. We know what kind of chemical behavior to expect of the parent aromatic hydrocarbon, benzene. Many important compounds are not just aliphatic or just aromatic, however, but contain both aliphatic and aromatic units; hydrocarbons of this kind are known collectively as arenes. Ethylbenzene, for example, contains a benzene ring and an aliphatic side chain.
What kind of chemical properties might we expect of one of these mixed aliphatic-aromatic hydrocarbons? First, we might expect it to show two sets of chemical properties. The ring of ethylbenzene should undergo the electrophilic.
substitution characteristic of benzene, and the side chain should undergo the free radical substitution characteristic of ethane. Second, the properties of each portion of the molecule should be modified by the presence of the other portion. The ethyl group should modify the aromatic properties of the ring, and the ring should modify the aliphatic properties of the side chain.
Structure and nomenclature
The simplest of the alkylbenzenes, methylbenzene, is given the special name of toluene. Compounds containing longer side chains are named by prefixing the name of the alkyl group to the word -benzene, as, for example, in ethylbenzene, n-propyl benzene, and isobutylene.
The simplest of the Di alkylbenzenes, the dimethylbenzenes, are given the special names of xylenes; we have, then, o-xylene, m-xylene, and p-xylene. Alkylbenzenes containing one methyl group are named as derivatives of toluene, while others are named by prefixing the names of both alkyl groups to the word -benzene. A compound containing a very complicated side chain might be named as a
Physical properties
As compounds of low polarity, the alkylbenzenes possess physical properties that are essentially the same as those of the hydrocarbons we have already studied. They are insoluble in water, but quite soluble in non-polar solvents like ether, carbon tetrachloride, or ligroin. They are almost always less dense than water. As we can see from Table 12.1, boiling points rise with increasing molecular weight, the boiling point increment being the usual 20-3u 3 for each carbon atom.
Since melting points depend not only on molecular weight but also on molecular shape, their relationship to structure is a very complicated one. One important general relationship does exist, however, between melting point and structure of aromatic compounds: among isomeric disubstituted benzenes, the para isomer generally melts considerably higher than the other two. The xylenes, for example, boil within six degrees of one another; yet they differ widely in melting point, the o- and w-isomers melting at -25 and -48, and the/MSomer melting at -f 13. Since dissolution, like melting, involves overcoming the intermolecular forces of the crystal, it is not surprising to find that generally the para isomer is also the least soluble in a given solvent.
The higher melting point and lower solubility of a para isomer is only a special example of the general effect of molecular symmetry on intracrystalline forces. The more symmetrical a compound, the better it fits into a crystal lattice and hence the higher the melting point and the lower the solubility. Para isomers are simply the most symmetrical of disubstituted r d benzenes. We can see (Table 12.1) that
5-tetramethylbenzene melts 85 to 100 higher than the less symmetrical 1,2,3,5- and 1,2,3,4-isomers. A particularly striking example of the effect of symmetry on melting point is that of benzene and toluene. The introduction of a single methyl group into the extremely symmetrical benzene molecule lowers the melting point from 5 to -95.
Industrial source of alkylbenzenes
It would be hard to exaggerate the importance to the chemical industry and to our entire economy of the large-scale production of benzene and the alkylbenzenes. , Just as the alkanes obtained from petroleum are ultimately the source of nearly all our aliphatic compounds, so benzene and the alkylbenzenes are ultimately the source of nearly all our aromatic compounds. When a chemist wishes to make a complicated aromatic compound, whether in the laboratory or in industry, he does not make a benzene ring ; he takes a simpler compound already containing a benzene ring and then adds to it, piece by piece, until he has built the structure he wants.
Today, petroleum is the chief source of the enormous quantities of benzene, toluene, and the xylenes required for chemicals and fuels. Half of the toluene and xylenes are utilized in high-test gasoline where, in a sense, they replace the aliphatic compounds inferior as fuels from which they were made. (A considerable fraction even of naphthalene, the major component of coal tar distillate, is now being produced from petroleum hydrocarbons.
Preparation of alkylbenzenes
Although a number of the simpler alkylbenzenes are available from industrial sources, the more complicated compounds must be synthesized in one of the ways outlined below.
Friedel-Crafts alkylation is extremely useful since it permits the direct attachment of an alkyl group to the aromatic ring. There are, however, a number of limitations to its use (Sec. 12.8), including the fact that the alkyl group that becomes attached to the ring is not always the same as the alkyl group of the parent halide; this rearrangement of the alkyl group is discussed in Sec.
The most important side-chain conversion involves reduction of ketones either by amalgamated /ink and HC1 (Clementson reduction) or by hydrazine and strong base (Wolff-Kushner reduction). This method is important because the necessary ketones are readily available through a modification of the Friedel-Crafts reaction that involves acid chlorides (see Sec. 19.6). Unlike alkylation by the Friedel Crafts reaction, this method does not involve rearrange.
Friedel-Crafts alkylation
The reaction is carried out by simply mixing together the three components; usually the only problems are those of moderating the reaction by cooling and of trapping the hydrogen halide gas. Since the attachment of an alkyl side chain makes the ring more susceptible to further attack (Sec. 1 1.5), steps must be taken to limit substitution to /?70/70alkylation. As in halogenation of alkanes (Sec. 2.8), this is accomplished by using an excess of the hydrocarbon. In this way an alkyl carbonium ion seeking an aromatic ring is more likely to encounter an unsubstituted ring than a substituted one. Frequently the aromatic compound does double duty, serving as solvent as well as reactant.
From polyhalogenated alkanes it is possible to prepare compounds containing more than one aromatic ring:
Mechanism of Friedel-Crafts alkylation
In Sec. 11.10 we said that two mechanisms are possible for Friedel-Crafts alkylation. Both involve electrophilic aromatic substitution, but they differ as to the nature of the electrophile.
One mechanism for Friedel-Crafts alkylation involves the following steps,
In some of the examples given above, we see that part of the product is made up of unrearranged alkylbenzenes. Must we conclude that part of the reaction does not go by way of carbonium ions? Not necessarily. Attack on an aromatic ring is probably one of the most difficult jobs a carbonium ion is called on to do; that is to say, toward carbonium ions an aromatic ring is a reagent of low reactivity and hence high selectivity. Although there may be present a higher concentration of the more stable, rearranged carbonium ions, the aromatic ring may tend to seek out the scarce unrearranged ions because of their higher reactivity. In some cases, it is quite possible that some of the carbonium ions react with the aromatic ring before they have time to rearrange; the same low stability that makes primary carbonium ions, for example, prone to rearrangement also makes them highly reactive.
On the other hand, there is additional evidence (of a kind we cannot go into here) that makes it very likely that there is a second mechanism for Friedel-Crafts alkylation. In this mechanism, the electrophile is not an alkyl carbonium ion, but an acid-base complex of alkyl halide and Lewis's acid, from which the alkyl group is transferred in one step from halogen to the aromatic ring.
This duality of mechanism does not reflect exceptional behavior, but is usual for electrophilic aromatic substitution. It also fits into the usual pattern for nucleophilic aliphatic substitution (Sec. 14.16), which from the standpoint of the alkyl halide is the kind of reaction taking place. Furthermore, the particular halides (1 and methyl) which appear to react by this second mechanism are just the ones that would have been expected to do so.
Limitations of Friedel-Crafts alkylation
We have encountered three limitations to the use of Friedel-Crafts alkylation: (a) the danger of Poly substitution; (b) the possibility that the alkyl group will rearrange; and (c) the fact that aryl halides cannot take the place of alkyl halides. Besides these, there are several other limitations.
(d) An aromatic ring less reactive than that of the halobenzene's does not undergo the Friedel-Crafts reaction; evidently the carbonium ion, R+, is a less powerful nucleophile than NO2 * and the other electron-deficient reagents that bring about electrophilic aromatic substitution.
Despite these numerous limitations, the Friedel-Crafts reaction, in its various modifications (for example, acylation, Sec. 19.6), is an extremely useful synthetic tool.
Reactions of alkylbenzenes
The most important reactions of the alkylbenzenes are outlined below, with toluene ana ethylbenzene as specific examples; essentially the same behavior is shown by compounds bearing other side chains. Except for hydrogenation and oxidation, these reactions involve either electrophilic substitution in the aromatic ring or free-radical substitution in the aliphatic side chain.
In following sections, we shall be mostly concerned with (a) how experimental conditions determine which portion of the molecule aromatic or aliphatic is attacked, and (b) how each portion of the molecule modifies tph'- reactions of the other portion.
Oxidation of alkylbenzenes
Identification of alkylbenzenes. The number and relative positions of side chains can frequently be determined by oxidation to the corresponding acids. Suppose, for example, that we are trying to identify an unknown liquid of formula C8 H10 and boiling point 137-139 that we have shown .in other ways to be an alkylbenzene (Sec. 12.22). Looking in Table 12.1 (p. 375), we find that it could be any one of four compounds: o-9 m~, or p-xylene, or ethylbenzene. As shown below, oxidation of each of these possible hydrocarbons yields a different acid, and these acids can readily be distinguished from each other by their melting points or the melting points of derivatives.
Electrophilic aromatic substitution in alkylbenzenes
Because of its electron-releasing effect, an alkyl group activates a benzene ring to which it is attached and directs ortho and para.
Treatment with methyl chloride and AIC13 at converts toluene chiefly into <? and p-xylenes; at 80, however, the chief product is w-xylene. Furthermore, either o- or p-xylene is readily converted into m-xylene by treatment with Ally and HC1 at.
How do your account for this effect of temperature on orientation? Suggest a role for the HC1.
Why is Poly substitution a complicating factor in Friedel-Crafts alkylation but not in aromatic nitration, sulfonation, or halogenation.
Halogenation of alkylbenzenes: ring vs. side chain
Alkylbenzenes clearly offer two main areas to attack by halogens: the ring and the side chain. We can control the position of attack simply by choosing the proper reaction conditions.
Halogenation of alkanes requires conditions under which halogen atoms are formed, that is, high temperature or light. Halogenation of benzene, on the other hand, involves transfer of positive halogen, which is promoted by acid catalysts like ferric chloride.
We might expect, then, that the position of attack in, say, toluene would be governed by which attacking particle is involved, and therefore by the conditions employed. This is so: if chlorine is bubbled into boiling toluene that is exposed to
ultraviolet light, substitution occurs almost exclusively in the side chain; in the absence of light and in the presence of ferric chloride, substitution occurs mostly in the ring. (Compare the foregoing with the problem of substitution vs. addition in the halogenation of alkenes (Sec. 6.21), where atoms bring about substitution and ions or, more accurately, molecules that can transfer ions bring about addition.)
Like nitration and sulfonation, ring halogenation yields chiefly the o- an
Side-chain halogenation of alkylbenzenes
Chlorination and bromination of side chains differ from one another in orientation and reactivity in one very significant way. Let us look first at brominating, and then at chlorination.
An alkylbenzene with a side chain more complicated than methyl offers more than one position for attack, and so we must consider the likelihood of obtaining a mixture of isomers. Bromination of ethylbenzene, for example, could theoretically yield two products: 1-bromo-l-phenylethane and 2-bromo-l-phenylethane. Despite
a probability factor that favors 2-bromo-l -phenylethane by 3:2, the fund is 1-bromo-l-phenylethane. Evidently abstraction of the hydrogens aliened to the carbon next to the aromatic ring is greatly preferred.
Just why benzylic hydrogens are less reactive toward chlorine atoms than even secondary hydrogens is not understood. It has been attributed to polar factors (Sec. 32.4), but this hypothesis has been questioned.
Resonance stabilization of the benzyl radical
How are we to account for the stability of the benzyl radical? Bond dissociation energies indicate that 19 kcal/mole less energy (104 - 85) is needed to form the benzyl radical from toluene than to form the methyl radical from methane.
Contribution from the three structures, V-VII, stabilizes the radical in a way that is not possible for the molecule. Resonance thus lowers the energy content of the benzyl radical more than it lowers the energy content of toluene. This extra stabilization of the radical evidently amounts to 19 kcal/mole
Molecular structure and rate of reaction. Resonance stabilized ben/yd radical formed faster than methyl radical. (Plots aligned with each other for easy comparison.)
We say, then, that the benzyl radical is stabilized by resonance. When we use this expression, we must always bear in mind that we actually mean that the benzyl radical is stabilized by resonance to a greater extent than the hydrocarbon from which it is formed.
In terms of orbitals, delocalization results from overlap of the p orbital occupied by the odd electron with the TT cloud of the ring.
Triphenylmethyl: a stable free radical
We have said that benzyl and allyl free radicals are stabilized by resonance; but we must realize, of course, that they are stable only in comparison with simple alkyl radicals like methyl or ethyl. Benzyl and allyl free radicals are extremely reactive, unstable particles, whose fleeting existence (a few thousandths of a second) has been proposed simply because it is the best way to account for certain experimental observations. We do not find bottles on the laboratory shelf labeled "benzyl radicals" or "allyl radicals." Jes there, then, any direct evidence for the existence of free radicals?
In 1900 a remarkable paper appeared in the Journal of the American Chemical Society and in the Brichta der Deutscher Chemische Gesellschaft\ its author was the young Russian-born chemist Moses Gomberg, who was at that time an instructor at the University of Michigan. Gomberg was interested in completely prenylated alkanes. He had prepared tetraphenyl methane (a synthesis a number of eminent chemists had previously attempted, but unsuccessfully), and he had now set himself the task of synthesizing hex phenylethane. Having available triphenyl chloromethane (Sec. 12.6), he went about the job in just the way we might today: he tried to couple together two triphenylmethyl groups by use of a metal (Sec. 9.4). Since sodium did not work very well, he used instead finely divided silver, mercury, or, best of all, zinc dust. He allowed a benzene solution of triphenyl chloromethane to stand over one of these metals, and then filtered the solution free of the metal halide. When the benzene was evaporated, there was left behind a white crystalline solid which after recrystallization melted at 185; this he thought was hex phenylethane.
Second, crowding among the large aromatic rings tends to stretch and weaken the carbon-carbon bond joining the triphenylmethyl groups in the dimer. Once the radicals are formed, the bulky groups make it difficult for the carbon atoms to approach each other closely enough for bond formation: so difficult, in fact, that hex phenylethane is not formed at all, but instead dimer I even with the sacrifice of aromaticity of one ring. Even so, there is crowding in the dimer, and the total effect is to lower the dissociation energy to only 1 1 kcal/mole, as compared with a dissociation energy of 80-90 kcal for most carbon-carbon single bonds.
It would be hard to overestimate the importance of Gomberg's contribution to the field of free radicals and to organic chemistry as a whole. Although triphenylmethyl was isolable only because it was not a typical free radical, its chemical properties showed what kind of behavior to expect of free radicals in general; most important of all, it proved that such things as free radicals could exist.
Preparation of alkenyl benzenes. Conjugation with ring
An aromatic hydrocarbon with a side chain containing a double bond can be prepared by essentially the same methods as simple alkenes (Sees. 5.12 and 5.19). In general, these methods involve elimination of atoms or groups from two adjacent carbons. The presence of the aromatic ring in the molecule may affect the orientation of elimination and the ease with which it takes place.
On an industrial scale, the elimination generally involves dehydrogenation. For example, styrene, the most important of these compounds and perhaps the most important synthetic aromatic compound can be prepared by simply heating ethylbenzene to about 600 in the presence of a catalyst. The ethylbenzene, in
turn, is prepared by a Friedel-Crafts reaction between two simple hydrocarbons, benzene and ethylene.
Dehydrohalogenation of l-phenyl-2-chloropropane, or dehydration of l-phenyl-2-propanol, could yield two products: 1-phenylpropene or 3-phenylpropene. Actually, only the first of these products is obtained. We saw earlier (Sees. 5.14 and 5.23) that where isomeric alkenes can be formed by elimination, the preferred product is the more stable alkene. This seems to be the case here, too. That 1-phenylpropene is much more stable than its isomer is shown by the fact that 3-phenylpropene is rapidly converted into 1-phenylpropene by treatment with hot alkali.
Reactions of alkenyl benzenes
As we might expect, alkenyl benzenes undergo two sets of reactions: substitution in the ring, and addition to the double bond in the side chain. Since both ring and double bond are good sources of electrons, there may be competition between the two sites for certain electrophilic reagents; it is not surprising that, in general, the double bond shows higher reactivity than the resonance-stabilized benzene ring. Our main interest in these reactions will be the way in which the aromatic ring affects the reactions of the double.
Addition to conjugated alkenyl (benzenes: orientation. Stability of the benzyl cation
Addition of an unsymmetrical reagent to a double bond may in general yield two different products. In our discussion of alkenes (Sees. 6.11 and 6.17), we found that usually one of the products predominates, and that we can predict which it will be in a fairly simple way: in either electrophilic or free-radical addition, the first step takes place in the way that yields the more stable particle, carbonium ion in one kind of reaction, free radical in the other kind. Does this rule apply to reactions of alkylbenzenes.
The orbital picture of the benzyl cation is similar to that of the benzyl free radical (Sec. 12.14) except that the p orbital that overlaps the *r cloud is an empty one. Thep orbital contributes no electrons, but permits further delocalization of the n electrons to include the carbon nucleus of the side
How do your account for the following facts? (a) Triphenyl chloromethane is completely ionized in certain solvents (e.g., liquid SO2); (b) triphenylcarbinol, (C6H5) 3 COH, dissolves in concentrated H2 SO4 to give a solution that has the same intense >callow color as triphenyl chloromethane solutions. (Note: This yellow color is different from that of solutions of phenylmethyl.)
Addition to conjugated alkenyl benzenes: reactivity
On the basis of the stability of the particle being formed, we might expect addition to a conjugated alkylbenzene, which yields a stable benzyl cation or free radical, to occur faster than addition to a simple alkene
The fact is that conjugated alkenyl benzenes are much more reactive than simple alkenes toward both ionic and free-radical addition. Here again as in most cases of this sort resonance stabilization of the transition state leading to a carbonium ion or free radical is more important than resonance stabilization of the reactant. We must realize, however, that this is not always true.
Alkylbenzenes
The preparations and properties of the alkylbenzenes are just what we might expect from our knowledge of benzene and the alkynes.
Analysis of alkylbenzenes
Aromatic hydrocarbons with saturated side chains are distinguished from alkenes by their failure to decolorize bromine in carbon tetrachloride (without evolution of hydrogen bromide) and by their failure to decolorize cold, dilute, neutral permanganate solutions. (Oxidation of the side chains requires more vigorous conditions; see Sec
This test is given by any aromatic compound that can undergo the Friedel-Crafts reaction, with the particular color produced being characteristic of the aromatic system involved: orange to red from halobenzene, blue from naphthalene, purple from phenanthrene, green from anthracene.
Analysis of alkenyl- and alkynyl benzenes
Aromatic hydrocarbons with unsaturated side chains undergo the reactions characteristic of aromatic rings and of the carbon-carbon double or triple bond. (Their analysis by spectroscopic methods is discussed in Sees.
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