Hyperconjugative Stability Mechanisms of Tert Butyl Cation and Butene

Hyperconjugation (or no-bond resonance) is a stabilization mechanism that involves the delocalisation of $\sigma$-electrons belonging to a adjacent $\text{C}-\text{H}$ or $\text{C}-\text{C}$ bond into an adjacent empty or partially filled orbital.

1. In the tert-butyl cation $\left[\text{(CH}_3\text{)}_3\text{C}^+\right]$:

   • The positively charged central carbon is $sp^2$ hybridized and possesses a vacant, unhybridized $2p$ orbital ($p$ empty).
   • The adjacent methyl groups possess $\sigma_{\text{C}-\text{H}}$ bonds.
   • Delocalisation occurs via the overlap of the filled $\sigma_{\text{C}-\text{H}}$ orbital electrons into the vacant $p$-orbital of the carbocation center.
   • Thus, the stabilizing interaction mechanism is $\sigma \rightarrow p\text{ (empty)}$ electron delocalisation.

2. In 2-butene $\left[\text{CH}_3-\text{CH}=\text{CH}-\text{CH}_3\right]$:

   • The molecule contains a $C=C$ double bond with a occupied bonding $\pi$-orbital and an empty anti-bonding $\pi^*$-orbital (LUMO).
   • The $\alpha$-hydrogen atoms present on the methyl groups attached to the $sp^2$ carbons provide $\sigma_{\text{C}-\text{H}}$ bond pair electrons.
   • Hyperconjugation involves the overlap of these filled $\sigma_{\text{C}-\text{H}}$ orbitals with the vacant $\pi^*$ anti-bonding orbital of the $\pi$-system.
   • Thus, the stabilizing interaction mechanism is $\sigma \rightarrow \pi^*$ electron delocalisation.

Comparing these mechanisms with the given options, the correct choice is A.

Correct Answer: A