Methyl hyperconjugation

However, 1-phenylpropene undergoes hydroboration at the C-1 position. This is not what one would expect based on the steric effect of a phenyl group. This is because of the combined effects of phenyl conjugation (-K) and methyl hyperconjugation (-tK), which act to decrease the amount of electron density at the C-2 position and to increase it at the C-1 position (Fig. 4.1) [6]. Evidently, these mesomeric effects are strong enough to override the steric effects of the phenyl group. 

Fig. 4.1 Diagram to show combined effects of phenyl conjugation (-K) and methyl hyperconjugation (+K) in 1-phenylpropene [6]...

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. 

The introduction of a methyl substituent into the empirical calculations may be performed according to two main different models the pseudoheteroatomic model and the hyperconjugated model (161-166). Both approximations have been used in rr-electron methods (HMO, w, PPP). On the other hand, in the all-valence-electrons... 

The 2-methyl-2-butyl cation provides the opportunity to explore the effect of C—C hyperconjugation. At the 6-31G /MP4 level of calculation, little energy difference is found between structures C and D which differ in alignment of CH3 or H with the empty p orbital. ... 

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. 

Kinetic studies of mercuration have also been used as a test for hyperconjugation. Toluene and toluene-aaa-mercuric acetate (0.5 M) in acetic acid containing water (0.25 M) and perchloric acid (0.050 M) and an isotope effect, ArH/ArD = 1.00 0.03, obtained. This insignificant effect was considered as evidence against the participation of hyperconjugation in electron supply by a methyl group449. 

The concept of hyperconjugation arose from the discovery of apparently anomalous electron-release patterns for alkyl groups. By the field effect alone, the order of electron release for simple alkyl groups connected to an unsaturated system is fert-butyl > isopropyl > ethyl > methyl, and this order is observed in many phenomena. Thus, the dipole moments in the gas phase of PhCHa, PhC2Hs, PhCH(CHa)2, and PhC(CHa)a are, respectively, 0.37, 0.58, 0.65 and 0.700. ... 

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. 

Cieplak apphed this effect to the explanation of the shift of selectivity in the reactions of the pentamethylcyclopentadienes 21,19, and 20 having substiments of formyl, vinyl, and (hydroxyimino)methyl moiety at the 5-positions [14]. He pointed out that the observed result is consistent with the notion that in the case of formyl moiety the hyperconjugative effect is enhanced by lone pair back-donation due to ... 

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