GUEST AUTHOR Leon D. Freedman North Carolina State University
Raleigh
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Teabook Errors, 7 3
The Hofmann Rule
The decomposition of quaternary ammonium hydroxides to yield tertiary amines and alkenes has been investigated by numerous authors since its discovery by Hofmann (1) in 1851. When an unsymmetrical quaternary compound is used, mixtures of alkenes are usually obtained. A generalization, called t,he Hofmann rule, is often made to the effect that the least substituted ethylene is formed preferentially. As an example of this rule, it is stated in several recently published textbooks that the decomposition of tert-butyldimethylethylammonium hydroxide yields predominantly ethylene: CHa
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(CH&C-SJm--CH2CH8 OHe I
1.50'
CHa
This statement and the percentages given, however, are not correct,. Cope and co-workers (8) investigated the thermal decomposition of the above-mentioned quaternary salt and obtained an olefin mixture composed of 7.2 mole % ethylene and 92.8 mole % isoThese results are consistent with a great deal of o her expenmental data. For example, Hughes, Ingold, and Woolf (5) have shown that alkyl substitution a t the a-carbon of a sulfonium salt favors the elimination reaction. Thus the E2 decomposition of the ethoxides of CH&H2S(CH&+, (CH3)2CHS(CH&+, and (CH& CS(CH&+ was found to give rate constants in the ratio of .5.0:114:2930. It seems clear, therefore, that the usual wording of the Hofmann rule represents an
butp
Suggestions of material suitable for this column and guest columns snitable for publication directly should be sent with ss many details as possible, and particularly with references to modern textbooks, to W. H. Eberhardt, School of Chemistry, Georgia Institote of Technology, Atlanta, Gs., 30332. Since the purpose of this column is to prevent the spread and continuation of errors and not the evaluation of individual texts, the sowees of error discussed will not be cited. In order to be presented, an error must occur in at least two independent recent standard books.
662 / Journol of Chemical Education
oversimplification. It is not easy to give a concise and correct statement of the Hofmann rule. I t is certainly trne (as Hofmann first noted) that the ethyl group is eliminated more readily than any other simple primary alkyl group. Tertiary alkyl groups, however, are eliminated much more readily, and even secondary alkyl groups are somewhat more reactive than the ethyl group. Furthermore, if the 6-carbon atom is substituted by a phenyl, vinyl, carhonyl, or similar radical, the substituted primary group may be eliminated more readily than the ethyl group. Thus, in the thermal decomposition of ethyl-p-phenethyl quaternary bases, the rate of formation of styrene is much greater than the rate of formation of ethylene (4). From a purely statistical point of view, one would expect the decomposition of tert-hutyldimethylethylammonium hydroxide to yield an olefin mixture consisting of 75 mole O/o isobutylene and 25 mole % ethylene. As pointed out in the first paragraph, however, the yield of isobutylene is even greater, i.e., 92.8 mole %. One cannot account for this result by assuming (as is often done) that the hydroxide ion preferentially attacks the more acidic 6-hydrogens, since this assumption would lead one to predict ethylene as the major product. I t is possible to rationalize the preferential formation of isobutylene by noting that this olefin is more stable than ethylene and by assuming that the transition state (for the formation of isobutylene) resembles the products more than it does the reactants. A detailed discussion of the Hofmann and related degradations can be found in a recent monograph (5). Literature Cited (1) HOFMANN, A. W., Ann., 78,253 (1851). W. R., (2) COPE,A. C., LEBEL,N. A,, LEE,H. H., AND MOORE, J. Am. Chem. Soe., 79,4720 (1957). (3) HUGKE~, E. D., INGOLD, C. K., AND WOOLF,L. I., J . Chem. Soc., 2084 (1948). (4) COPE,A. C., I N D TROMBULL, E. R., OIQ. Reaclions, 11, 317 (1960). (5) BANTHROPE, D. V., "Elimination Reactions," Elsevier Publishing Co., Amsterdam, 1963.
