Question

Write two differences between $S_{N}1$ and $S_{N}2$ reactions. Out of chlorobenzene and 2,4,6-trinitrochlorobenzene, which one is more reactive towards nucleophilic substitution reactions?

Hint:

Electron-withdrawing groups (like $-NO_2$) strongly activate the benzene ring for nucleophilic substitution.

Answer 1

Step 1: Concept
Nucleophilic substitution reaction mechanisms and substituent effects on reactivity.

Step 2: Meaning
$S_N1$ is unimolecular and stepwise; $S_N2$ is bimolecular and concerted. Reactivity of haloarenes increases with electron-withdrawing groups.

Step 3: Analysis
$S_N1$ forms a carbocation intermediate leading to racemization. $S_N2$ forms a transition state leading to strict inversion. 2,4,6-trinitrochlorobenzene has three strong electron-withdrawing $-NO_2$ groups that drastically stabilize the intermediate carbanion, making it highly reactive compared to chlorobenzene.

Final Answer: Differences: 1. $S_N1$ is a two-step reaction forming a carbocation intermediate, while $S_N2$ is a single-step concerted reaction forming a transition state. 2. $S_N1$ leads to racemization of configuration, whereas $S_N2$ leads to complete inversion of configuration. Reactivity: 2,4,6-trinitrochlorobenzene is far more reactive because the electron-withdrawing $-NO_2$ groups stabilize the intermediate complex.