question_answer

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

A) \[\sigma \to p\](empty) and \[\sigma \to {{\pi }^{*}}\]electron delocalisations

B) \[\sigma \to {{\sigma }^{*}}\] and \[\sigma \to \pi \]electron delocalisations

C) \[\sigma \to p\](filled) and \[\sigma \to \pi \]electron delocalisations

D) p (filled) \[\to {{\sigma }^{*}}\]and \[\sigma \to {{\pi }^{*}}\]electrons delocalisations

Correct Answer: A

Solution :

Spreading out charge by the overlap of an empty p-orbital with an adjacent o-bond is called hyperconjugation. This overlap (the hyperconjugation) delocalises the positive charge on the carbocation, spreading it over a larger volume and this stabilizes the carbocation.

Tertiary butyl carbocation has one vacant p-orbital, hence, it is stabilised by \[\sigma \text{-}p\] (empty) hyperconjugation.

In but-2-ene, stabilisation is due to hyperconjugation between \[\sigma \text{-}{{\pi }^{*}}\]electron delocalisation.