[CONTRIBUTION FROM
CHEMICAL LABORATORY OF HOWARD UNIVERSITY ]
THE
THE PRINCIPLE OF VINYLOGY AND THE EFFECT OF ORTHO SUBSTITUENTS ON THE REACTIVITY OF BENZENE DERIVATIVES A. H. BLATT Received January 23, 1996
In a recent article, Oskar Zwecker’ has discussed in considerable detail the occurrence of steric hindrance in ortho-substituted benzene derivatives. His point of view may be illustrated by reference to benzoic acid. The effect on the reactivity of the carboxyl group in benzoic acid of the introduction of any substituent in the ortho position will be comparable with the effect on the reactivity of the carboxyl group in formic acid caused by the replacement of the hydrogen atom in that acid by the same substituent. Taking a methyl group as the substituent the following pairs are to be compared :
I n other terms, any monosubstituted benzene derivative is comparable with the hydrogen compound of the substituent and the effect of introducing a substituent in the position ortho to the first substituent is similar to the effect of introducing this same substituent into the hydrogen compound of the first substituent. It is not the purpose of this paper to challenge the validity of Zwecker’s point of view but rather to show that this point of view is simply a special case of a much wider generalization -the principle of vinylogy as formulated by Fuson.2 More specifically Zwecker’s comparisons are valid because they form a special case of an extension of the principle of vinylogy. As stated by Fuson, the principle of vinylogy deals with the effect of an unsaturated group on substituents attached to that group and shows that this effect 1
OSKARZWECKER,Ber., 68, 1289 (1935).
* FUSON,Chem. Rev., 16, 1
(1935). Zwecker’s analogies may also be considered as following from Angeli’s rule. See ANQELI,Ahren’s Sammlung, 28, 1 (1926).
154
155
REACTIVITY O F BENZENE DERIVATIVES
m a y be transmitted through intervening vinylene groups. Thus the hydrogen atoms of the methyl group in ethyl acetate are activated sufficiently to condense with esters. If a vinylene group is interposed, as in eth,yl crotonate, the hydrogen atoms in the terminal methyl group are still sufficiently active to undergo ester condensations.
CH, C=O
I
OCzHs
CH, CH=CHC=O
I
OCzHs
We are dealing with the effects of saturated substituents on the reactivity of a functional groupj usually unsaturated, and we shall show that when a given effect on the reactivity of a functional group i s produced by a saturated TABLE I* ESTERIFICATION WITH METHANOL (T = 15°C.) SUBSTANCE
I
HC02H. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CH,CO*H.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CzHsCOzH. ....................................................
k
1224 104 91
TABLE I1 ESTERIFICATION WITH METHANOL (2' = 15OC.) SUBSTANCE
CHZ=CH COiH.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CH:
