Question

Which isomer of $C_4H_9Br$ is most reactive towards $S_N1$ reaction?

Hint:

$S_N1$ reactivity: $3^\circ \gt 2^\circ \gt 1^\circ$ (follows carbocation stability order).

Answer 1

Step 1: Concept
$S_N1$ reactivity depends on the stability of the carbocation intermediate formed in the rate-determining step.

Step 2: Analysis
The isomers of $C_4H_9Br$ are: 1-bromobutane (primary), 2-bromobutane (secondary), 1-bromo-2-methylpropane (primary), and 2-bromo-2-methylpropane (tertiary). The tertiary carbocation $(CH_3)_3C^+$ is the most stable due to hyperconjugation and inductive effects of three methyl groups.

Step 3: Conclusion
2-Bromo-2-methylpropane (tert-butyl bromide) is most reactive toward $S_N1$ because it forms the most stable tertiary carbocation.

Final Answer: 2-Bromo-2-methylpropane (tert-butyl bromide): $(CH_3)_3CBr$ is most reactive toward $S_N1$.