Question

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

• A. $\sigma \to p$ (empty) and $\sigma \to {\pi }^{\ast }$ electron delocalisations
• B. $\sigma \to {\sigma }^{\ast }$ and $\sigma \to \pi$ electron delocalisations
• C. $\sigma \to \pi$ (filled) and $\sigma \to \pi$ electron delocalisations
• D. $\pi$ (filled) $\to {\sigma }^{\ast }$ and $\sigma \to {\pi }^{\ast }$ 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 $\sigma -{\pi }^{\ast }$ electron delocalisation.