Question

Explain why ${\left(C{H}_3\right)}_3{C}^{\oplus }$ is more stable than $C{H}_3\stackrel{\oplus }{C}{H}_2$ and $\stackrel{\oplus }{C}{H}_3$ is less stable.

• A. Hyperconjugation
• B. Resonance
• C. Tautomerism
• D. Inductive effect

Answer 1

Answer: (A), (D)

Hyperconjugation
Inductive effect

Hyperconjugation interaction in ${\left(C{H}_3\right)}_3{C}^{\oplus }$ (I) is greater than that of $C$ in $C{H}_3{\stackrel{\oplus }{CH}}_2$ (II), as (I) has nine $(C-H)$ bonds and (II) has three $(C-H)$ bonds.<br><br>In ${\stackrel{\oplus }{CH}}_3$, the vacant $p$ orbital is perpendicular to the plane in which $(C-H)$ bond lies; hence cannot overlap with it. $<br><br>$Thus, ${\stackrel{\oplus }{CH}}_3$ lacks hyperconjugative stability.
Also, as the number of alkyl groups increases, the +I effect increases and the electron density on positively charged carbon increases.
Hence, ${\left(C{H}_3\right)}_3{C}^{\oplus }$ is more stable than $C{H}_3\stackrel{\oplus }{C}{H}_2$ and $\stackrel{\oplus }{C}{H}_3$ is less stable.