Hyperconjugation canonical forms

All of the delocalization discussed so far involves n electrons. Another type, called hyperconjugation, involves <7 electrons. When a carbon attached to at least one hydrogen is attached to an unsaturated atom or one with an unshared orbital, canonical forms such as 110 can be drawn. In such canonical forms, there is no bond at all between the carbon and hydrogen. The effect of 110 on the actual molecule is that the electrons in the C—H bond are closer to the carbon than they would be if 110 did not contribute at all. 

The stability order can be explained by hyperconjugation and by the field effect. In the hyperconjugation explanation, we compare a primary carbocation with a tertiary. It is seen that many more canonical forms are possible for the latter ... 

Scheme2.1. Orbital interaction and canonical forms for hyperconjugation between a bonds.

The high a-reactivity probably results principally from stabilisation of the canonical form 20-11 in the transition state by the electron release from the tin, and the P-reactivity from stabilisation of the resulting P-stannylalkyl radical by hyperconjugation (Section 3.1). 

Hyperconjugation is a stabilizing interaction between a C-H bond and an adjacent a-bond. For example, propene may be regarded as a resonance hybrid of two canonical forms ... 

Figure 2. Canonical forms describing the resonance hybrid of TNT (a) one form of the Kekule structure, (b) some of the potential ionic contributions, and (c) one form describing the hyperconjugative effect of the methyl group.

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