The hyperconjugative stabilities of tert-butyl cation and 2-butene, respectively, are due to

Options

A. `sigma-p` (empty) and `sigma-pi^(**)` electron delocalisations
B. `sigma-sigma^(**)` and `sigma-pi^(**)` electron delocalisations
C. `sigma-p` (filled) and `sigma-pi` electron delocalisations
D. p (filled) `rarr sigma^(**)` and `sigma-pi^(**)` electron delocalisations

Answer

A

Explanation

In tert-butyl cation, the carbon bearing positive charge has an empty p-orbital and hence hyperconjugation here involves `sigma`-p (empty) electron delocalisation.

In 2-butene, `pi`-bond contains two electrons, one electron from each of the two carbon atoms, i.e., `pi`-MO contains a pair of electrons, but `pi^(**)` MO is empty and hence electrons of the `sigma` C-H bond can move only into the `pi^(**)` orbital. Thus, option (a) is correct.