Methyl group hyperconjugation

The importance of a primary steric effect in the nitration of alkyl-benzenes has been mentioned ( 9.1.1). The idea was first introduced by Le Fevre to account for the fact that -alkyltoluenes (alkyl = Et, -Pr,68a t-Bu ) are nitrated mainly adjacent to the methyl group. Without the rate data reported for the alkylbenzenes the effect might equally well have been accounted for by hyperconjugation. 

FIGURE 4.16 Hyperconjugation in ethyl cation. Ethyl cation is stabilized by delocalization of the electrons in the C—H bonds of the methyl group into the vacant 2p orbital of the positively charged carbon. 

In the conformation shown, only the methyl-group C—H that is parallel to the carbocation p orbital can show hyperconjugation. 

For the other alkyl groups, hyperconjugation is diminished because the number of C—H bonds is diminished and in ferf-butyl there are none hence, with respect to this effect, methyl is the strongest electron donor and rert-butyl the weakest. 

As described in Section 3.4.2, hyperconjugative donor-acceptor stabilizations favor conformers in which one of the rotor C—H bonds eclipses an adjacent double bond. (This is equivalent to an ethane-like staggered preference if the double bond is pictured in terms of two bent banana bonds. ) Hence, in the case of a perfectly localized Lewis structure I, the methyl group would be expected to adopt the preferred pseudo-cA conformation la (with in-plane C—H syn to A=C),... 

The tert-butyl cation structure (7) with Cs symmetry is better suited for hyperconjugation than the C3h form and is thus energetically slightly favored.29 The energy surface for methyl-group rotation is however very flat. 

In piperidine the electron lone-pair can occupy either an axial or an equatorial position in 1-methylpiperidine the axial orientation (lb) is favoured by 99 1 over the equatorial (la). PE spectra and ab initio calculations on methylpiperidines indicate that axial 2-methyl substituents lower the amine lone-pair ionization potential by about 0.26 eV, while equatorial 2-methyl substituents as well as methyl groups on carbon atoms 3 and 4 lower the lone-pair IP by less than 0.1 eV63. This establishes the mechanism of stabilization of the amine radical cation as hyperconjugative electron release, which is larger for CC bonds than for CH bonds. The anti-periplanar orientation of the nitrogen lone-pair and the vicinal C—Me bond (lc) is much more favourable for this type of interaction than the synclinal orientation (Id). 

Within the series of methylbenzenes, the effect of the methyl groups is very well reproduced by this relationship. Using the values for the squares of the coefficients and the experimental -values, — 8a/j8 is found to have a value of 0-4. On taking hyperconjugation into account in the case of m-xylene, Sa/)3 is found to have a value of 0-08, which does not explain the difference logiT—log. o = 6 5 (Mackor et al., 1958b). 

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