SINCE the review by C. L. Deasy the concept of hyperconjugation has become well established, and, furthermore, has been extended and widely applied ; consequently, the present more extensive review was undertaken.Historical Introduction and Definition of the Term.-If electron displacement in alkyl groups resulted solely from the operation of the inductive effect, then the order But > Pri > E t > Me should always be obtained for those reactions which require accession of electrons to the reaction region.Experimental evidence that this order is not universally applicable was first provided by J. W. Baker and W. S. Nathan,2 who studied the rate of reaction of various alkyl-substituted benzyl bromides with pyridine. It was found that all alkyl substituents increased the reaction rate, while with a single p-alkyl substituent the rate decreased in the orderSince the reaction studied is one facilitated by electron accession towards the side chain, it follows that the accelerating effects of alkyl groups must be related to their capacity for electron release, and, furthermore, that the relative magnitude of this electron release diminishes in the order indicated in (a), which order is exactly the reverse of that expected on the basis of the inductive effect of alkyl groups. The methyl group, it would seem, is therefore capable of permitting additional electron release, by some mechanism which is either greatly reduced or inoperative in the more highly alkylated groups. Since an anomaly of this kind is not found in every type of system containing alkyl groups, Baker and Nathan pointed out that the additional mechanism of electron release by the methyl group in p-methylbenzyl bromide is to be associated with the presence of the attached conjugated system of the aromatic nucleus. It was therefore suggested that when a methyl group is attached to a conjugated system, the pair of electrons forming the C-H bond in the methyl group, are appreciably less localised than are those in a similarly situated carbon-carbon bond. Hence a methyl group attached to the requisite system permits electron release by what is essentially a type of electromeric effect :- ( a ) This new effect acts in addition to, and in the same direction as, the inductive effect, but, since it depends on the number of C-H bonds, its magnitude clearly will diminish in the order Me > E t > Pri >
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