Hyperconjugation - Hyperconjugation

This refers to conjugation between the σ electrons of a single bond and the π electrons of a double or triple bond. This effect is seen when an alkyl group is bonded to an unsaturated carbon atom, and is particularly noticeable in methyl groups. For example, the fact that propylene is more stable than ethylene (the heat of hydrogenation of the double bond is smaller) is explained by the delocalization of the methyl group electrons due to hyperconjugation.

Hyperconjugation is also the reason why the length of the ^C2 - ^C3 bond in propylene is shorter (1.46 Å) than the length of a simple single bond (1.54 Å). It also affects various physical properties such as the dipole moment, ionization potential, and absorption spectrum. When the hydrogen in the methyl group of propylene is replaced by another methyl group, the effect of hyperconjugation decreases in the order of ethyl and isopropyl, and is not seen in the tert -butyl group.

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

単結合のσ電子と，二重結合または三重結合のπ電子との間の共役をいう．アルキル基が不飽和炭素原子に結合しているような場合にみられる効果で，とくにメチル基においていちじるしい．たとえば，エチレンに比べてプロピレンのほうが安定(二重結合の水素化熱が小さい)なのは，超共役によるメチル基電子の非局在化によって説明される．

また，プロピレンのC²-C³結合の長さが，単純な単結合の長さ(1.54 Å)よりも短い(1.46 Å)のも超共役のためである．そのほか，双極子モーメント，イオン化電位，吸収スペクトルなど物理的諸性質にも影響を与える．プロピレンのメチル基の水素をメチル基で置換していくと，エチル，イソプロピルの順に超共役の効果は減じ，tert-ブチル基ではみられなくなる．

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