Organic Chemistry Basics 2 Question 52
52. The hyperconjugative stabilities of tert-butyl cation and 2-butene, respectively, are due to
(2013 Adv.)
(a) $\sigma \rightarrow p$ (empty) and $\sigma \rightarrow \pi^{*}$ electron delocalisations
(b) $\sigma \rightarrow \sigma^{*}$ and $\sigma \rightarrow \pi$ electron delocalisations
(c) $\sigma \rightarrow p$ (filled) and $\sigma \rightarrow \pi$ electron delocalisations
(d) $p$ (filled) $\rightarrow \sigma^{}$ and $\sigma \rightarrow \pi^{}$ electrons delocalisations
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Solution:
- PLAN 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.
