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: $S_N1$ mechanism and carbocation stability.
The $S_N1$ rate-determining step forms a carbocation intermediate. The more stable the carbocation, the faster the $S_N1$ reaction.
Step 2: Isomers of $C_4H_9Br$.
(i) 1-Bromobutane: primary carbocation (unstable). (ii) 2-Bromobutane: secondary carbocation (moderate stability). (iii) 1-Bromo-2-methylpropane: primary carbocation (unstable). (iv) 2-Bromo-2-methylpropane (tert-butyl bromide): tertiary carbocation $(CH_3)_3C^+$ (most stable, stabilised by hyperconjugation from three methyl groups).
Step 3: Conclusion.
2-Bromo-2-methylpropane (tert-butyl bromide, $(CH_3)_3CBr$) is most reactive toward $S_N1$ because it generates the most stable tertiary carbocation.