Neutral, negative, and positive hyperconjugation

The effect of alkyl substiments on the stabilities of carbenium ions provides the electronic basis of the textbook Markovnikov s rule. The stabilizing effect of positive hyperconjugation increases for stronger o-donors. For example, the stabihzing effect of a silyl substituent in p-silylethyl cation is calculated to be ca. 38kcal/mol stronger than a C-H donor of the ethyl cation in the gas phase (see Section 6.3). The effects of Ge, Sn, and Hg are also substantial. For example, hyperconjugative activation by a Sn-C bond can accelerate a reaction by a factor of 10 .  

Classification of hyperconjugation and conjugation as positive or negative is useful when either referring to an individual interaction or to an imbalanced situation, as is the case of an interaction with a very strong donor or acceptor orbital with the rest of the molecule dominating over other delocalization effects. This imbalance often occurs when either a lone pair acts as a donor or when an empty p-orbital acts as an acceptor. Use of these terms in other situations can be uninformative or even misleading. 

In this type of hyperconjugation, donor and acceptor interactions are balanced, and often there is no dominating effect. As a result, the importance of sacrificial hyperconjugation in neutral hydrocarbons has been initially controversial. However, now the effects of hyperconjugation on X-ray geometries of nentral molecules are well-documented. ° 

Neglect of neutral hyperconjugation can lead to fundamental misconceptions. For example, omission of hyperconjugative effects led to the apparent disappearance of conjugation between two triple bonds in butadiyne. We will further illustrate the importance of this ubiquitous phenomenon in Chapter 6. 

In summary, this chapter has provided three possible ways to classify delocalizing interactions 1) by the type of orbitals, 2) by the number of electrons, 3) by directionality (or lack of thereof) of the overall electron density transfer. We will come back to the first of these classifications and discuss its quantitative aspects in mnch more detail in Chapter 5, after giving a brief outline of practical approaches to the quantitative analysis of delocalization in Chapter 4. 

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