Question

Conversion of I to II:

• A. takes place by $S_N1$
• B. takes place by $S_N2$
• C. takes place both by $S_N1$ and $S_N2$
• D. does not take place

Answer 1

Answer: (A)

takes place by $S_N1$

Answer 2

The conversion of 9-bromotriptycene to 9-hydroxytriptycene involves nucleophilic substitution at the bridgehead carbon. Both $\text{S}_{\text{N}}1$ and $\text{S}_{\text{N}}2$ mechanisms are highly unfavorable at the bridgehead position of triptycene due to the rigidity of the structure.

The $\text{S}_{\text{N}}1$ mechanism is disfavored due to the instability of the planar bridgehead carbocation. The $\text{S}_{\text{N}}2$ mechanism is disfavored due to severe steric hindrance to backside attack by the nucleophile.

However, if the reaction does occur, the $\text{S}_{\text{N}}1$ mechanism is generally considered less unfavorable than the $\text{S}_{\text{N}}2$ mechanism at a bridgehead in rigid systems like triptycene, despite the instability of the carbocation. This is because the steric hindrance to $\text{S}_{\text{N}}2$ is extremely high.