A Guidebook to Mechanism in Organic Chemistry

Factors affecting Electron-availability in Bonds

H—C-r-H H—C H®

(XXV)

This effect has been called hypercmjugation and has been used suc­
cessfully to explain a number of otherwise unconnected phenomena.
It should be emphasised that it is not suggested that a proton actually
becomes free in (XXIV) or (XXV)^for if it moved from its original
position one of the conditions necessary for delocalisation to occur
would be controverted (p. 12)., *
The reason for the reversal oi electron-donating ability in going
Me->Et->isoPr->t-Bu is that hyperconjugatioH depends for its
operation on hydrogen attached to carbon atoms a- to the unijgu-
rated system. This is clearly at a maximum with Me (XXIV) and non­
existent with t-Bu (XXV^I),"provided it is assumed that no similar
efjf2Ct of comparable magnitude occurs in C—C bonds,

H H Me

I I I

H—C—CH=CH, Me—C—CH CH 2 x Me—C—CH=CH 2

I ' - I I

H H H

(XXIV) (XXVI) (XXVII)

Me

Me—C—CH=CHS

I

Me

(XXVIII)

hence the increased electron-donating ability of methyl groups under

these conditions. This is believed to be the reason for the increased

stabilisation of defines in which the double bond is not terminal

H H®

H-C^CH=^CH 8 ~ H-C=CH-CH 8

i H

(XXIV)