Alkyl substituents polar effect

In the El cb mechanism, the direction of elimination is governed by the kinetic acidity of the individual p protons, which, in turn, is determined by the polar and resonance effects of nearby substituents and by the degree of steric hindrance to approach of base to the proton. Alkyl substituents will tend to retard proton abstraction both electronically and sterically. Preferential proton abstraction from less substituted positions leads to the formation of the less substituted alkene. This regiochemistry is opposite to that of the El reaction. 

Polar effects of alkyl groups 243 Steric effects of alkyl groups 246 Kinetic effects of aryl substituents 252 Selectivity relationships and transition-state shifts in arylolefin bromination 256... 

While p for the polar effects of alkyl groups does not vary with the solvent, pn for the polar effects of aromatic substituents is solvent-dependent. For example, there is a fairly linear log/log relationship between the bromination rates of styrenes in acetic acid and methanol (36) with a slope higher than unity, to be compared with 0.99 in (24). The p+-value for styrenes... 

However, although the accuracy of these predictions is impressive one must remember that they apply to a very closely defined series. We are still unable to predict the specific effects of substituents and have no answer to the intriguing question of why does the introduction of the OCH2 unit into the phenylethanolamines give such good (3-blockers, or why the addition of a polar group (amide or ether) to a para-alkyl substituent to some extent relieves the steric crowding (cf ICI 66082 (15) and metoprolol (id)) table 2. 

Special Reference to the Polar Effects of Substituents. Part XI. The Aldehydecyanohydrin Reaction Evidence for the Mesomeric Effects of Alkyl Groups. J. chem. Soc. (London) 1942, 191. 

All of these studies have used alkyl groups as the substituents on the C=C bond, which, however, differ only slightly in their polar effects. In order to find out the extent of electronic contribution to the overall reactivity, a broader range of substituents is necessary. The literature yields earlier data of this type (5) for the hydrogenation of imsaturated compounds CH2=CH X (where X = —CH NHj, —CHjCOOH, —CHjCN, —CHjOH,... 

When no important 7r-electron effect is possible, as with alkyl groups, the orientation effect of a substituent is controlled by its polar effect and the degree to which it polarizes the bonding electrons of the ring. Alkyl groups... 

Similarly the introduction of alkyl substituents into the radical has only a small effect. Table 2 illustrates the effect that alkyl substituents attached to the trivalent carbon atom have on the rate of addition to ethylene. The variation in rate is very small and in fact the apparently regular decrease in rate of the more branched radicals is always within possible experimental error (the data for CH3CH2CH2- gives a relative rate of 0.25). In general both Tables 1 and 2 show that the introduction of methyl substituents into either the alkene or the alkyl radical reduces the rate of addition. This could be accounted for either by polar forces or by steric hindrance. 

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