Question

The hyperconjugative stabilities 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. $\sigma \to p$ (filled) and $\sigma \to \pi$ electron delocalisations
• D. $p$ (filled)$\to {\sigma }^{\ast }$ and $\sigma \to {\pi }^{\ast }$ electron delocalisations

Answer 1

The correct option is A $\sigma \to p$ (empty) and $\sigma \to {\pi }^{\ast }$ electron delocalisations

Hyperconjugation is due to the interaction of the electrons in $\sigma$ bond (C-H or C-C) with adjacent empty or partly filled non-bonding p-orbital, antibonding $\pi$ orbital or filled $\pi$ orbital.
This result in extended molecular orbital and increases the stability of the system.

In case of tert-butyl cation, the hyperconjugation is due to the interaction of the electrons in the $\sigma$ bond with adjacent empty p-orbital.

$H_{3}C-\stackrel{C{H}_3}{\stackrel{|}{\underset{C{H}_3}{\underset{|}{C^{\oplus }}}}}$

$\sigma -p\left(empty\right)$

In case of 2-butene, the hyperconjugation is due to the interaction of $\sigma$ orbital with antibonding $\pi$ orbital.

$H_{3}C-CH=CH-C{H}_3$

$\sigma -{\pi }^{\ast }\left(antibonding\right)$