GUEST AUTHOR H. Goldwhite
Manchester College of Science and Technology Monchester, England
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Textbook Errors,
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The Side-Chain Halogenation of n-Alkyl Benzenes
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nnmber of textbooks1 state that in the halogenation i f f t h e side-chains of normal alkyl benzenes with elemental halogens there is a strong tendency for halogenation to take place preferentially a t the carbon atom next to the aromatic nucleus. Indeed some books imply that halogenation occurs exclusively in this position in the side-chain. However when the literature on this type of reaction is examined it does not bear out this conclusion. In fact past workers have found in general that the reaction leads to mixtures of isomers from which it is hard to isolate pure compounds. There is very little recent work on side-chain halogenation, and much of the early work gave results which were hard to interpret; the following conclusions are those best supported by original work. Chlorination
Ethylbenzene. The chlorination of boiling ethylbenzene gave mainly @-phenylethylchloride ( I , 9). By contrast the chlorination of ethylbenzene in the cold, with illumination of the reaction mixture, gave the isomeric a-phenylethylchloride (3). A more recent investigation of the chlorination of boiling ethylbenzene in diffused daylight showed that a mixture of the aand @-chlorideswas produced plus some m-, @-dichloride, C6H6CHC1CH2C1 (4). The authors of this article suggested that an increase in light intensity gave an increased amount of a-phenylethylchloride, but their experiments were not precise enough to establish this point. They also suggested that the complexity of their reaction mixture arose from subsequent reactions of 6-phenylethylchloride which was first formed. This could lose hydrogen chloride to form styrene which could then combine either with the hydrogen chloride, to give a-phenylethylchloride, or with chlorine, to give the a-, @dichloride. n-Propylbenzene. A careful investigation (5, 6) showed that the main product of chlorination of boiling n-propylbenzene was the @-chloride, C6H5CH2CHC1CHa. Suggestions of material suitable for this column, and gucst columns suitable far publication directly m e eagerly solicited. They should be sent with as many details as possible, and particularly with references to modern textbooks, to Karol J. Mysels, Department of Chemistry, University of Southern California, Los Angeles 7, California. ' Since the purpose of this column is to prevent the spread and continuation of errors and not the evaluation of individual texts.
Bromination
Ethylbenzene. The bromination of ethylbenzene in sunlight a t 0' gave first a-phenylethylbromide and then a,a-dibromoethylbenzene, CsHsCBr2CHa,with no evidence of the formation of any @-bromoderivatives (7). The bromination of boiling ethylbenzene-in complete contrast to its chlorination-gave a mixture of aphenylethylbromide and the a-, pdibromide, CsHr CHBrCHIBr, identical with the product of the addition of bromine to styrene (8, 9). n-Propylbenzene. The bromination of this compound followed the pattern shown by ethylbenzene. Bromination in the cold and in sunlight gave first the cr-bromide and then the apdibromide, CsH6CBr&HzCHa (10). Bromination a t 160°C gave a dibromide which was probably C6HsCHBrCHBrCHs(11). Are Predictions Justified?
These experiments show that the ordinary halogenation-and especially chlorination--of these alkylated benzenes does not favor the a-position of the sidechain over the @-position. It is interesting to speculate about why the assertion that the a-position is favored has crept into textbooks. The reasoning behind the assertion may have run as follows: the halogenation is certainly a radical reaction, and of all the side-chain radicals which could be produced by the abstraction of hydrogen the a-radical (PhCHCH3as it would be from ethylbenzene) is likely to be the most stable; in consequence the halogenation will most probably yield ahalo derivatives. The results show that this is an insufficient argument. The interesting experiments recently carried out on the competitive chlorination and bromination of mixtures of toluene and cyclohexane )% ' 1( show that it is necessary to take into account many factors besides radical stabilization in discussing halogenation. The argument outlined above would lead to the conclusion that toluene (giving a stabilized henzyl radical) would be halogenated more rapidly than cyclohexane, and, in fact, in the competitive bromination toluene reacted about 60 times as fast as cyclohexane. But in competitive chlorination cyclohexane reacted 11 times as rapidly as toluene. This is interpreted as meaning that in the transition state of the chlorination of toluene the methyl C-H bond is not completely broken and that, in consequence, the stabilization of the incipient benzyl radical contributes very little toward making the rupture of the bond easy. The conclusion to he dra17-n from this is that without Volume 37, Number 6, June 1960
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many more experimental results, and without a more detailed knowledge of the mechanism of these radical reactions, it is unwise to predict what the products of any given halogenation will be. Literature Cited (1) F I ~ I GR., , ANDKIESOW, J., Ann., 156,24&51 (1870). R., Ann., 235, 329f (1886). (2) ANSCH~)TZ, J., Monalsh., 8, 102 (1887). (3) SCHRAMM,
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(4) EVANS,E. B.,MABBOTT, E. E., AND TURNER,E. E., J . Chem. Soc., 1927, 1163. G., Gazz. chim. ital., 14, 504 (1884). (5) ERRERA, (6) Ibid., 16, 310-25 (1886), (7) SCHRAMM, J., Bw.,18,350-5(1885). DR., B e . , 6, 4 9 2 4 (1873). (8) RADZISZEWSKI, (9) Ibid., 7, 140-3 (1874). J., Bw.,18, 12741 (1885). (10) SCHRAMM, (11) RADZISZEWSKI, B., Compt. rend., 78,1153f (1874). (12) RUSSELL, G. A,, AND BROWN, H. C.,J. Am. Chem. Soc., 77, 457&82 (1955).
