Question

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

• A. $\sigma \rightarrow p $ (empty) and $\sigma \, \rightarrow \pi ^* $ electron delocalisations
• B. $\sigma \rightarrow \sigma^* \,$ and $\, \sigma \rightarrow \pi $ electron delocalisations
• C. $\sigma \rightarrow \pi$ (filled) and $\sigma \rightarrow \pi $ electron delocalisations
• D. $\pi$ (filled) $\rightarrow \sigma ^* $ and $ \sigma \rightarrow \pi ^* $ electrons delocalisations

Answer 1

Answer: (A)

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

tertiary butyl carbocation has one vacant $p$-orbital, hence, it is stabilised by $\sigma-p$ (empty) hyperconjugation.

In $2$-butene, stabilisation is due to hyperconjugation between $\sigma-\pi^{*}$ electron delocalisation.