Question

Which of the following alkyl halides is most reactive towards substitution by \( S_N1 \) mechanism?

• A. \( (CH_3)_3C-Br \)
• B. \( (CH_3)_3C-I \)
• C. \( (CH_3)_3C-F \)
• D. \( (CH_3)_3C-Cl \)

Hint:

In SN1 reactions, look for the best combination of a stable carbocation and a good leaving group—tertiary iodides are ideal.

Answer 1

The Correct Option is B

The \( S_N1 \) reaction mechanism involves a **carbocation intermediate**, so: - **Stability of the carbocation** is the key factor. - **Tertiary (3°) alkyl halides** form highly stable carbocations due to hyperconjugation and inductive effects. - Among halides, **iodide (I⁻)** is the best leaving group due to its large size and weak bond with carbon. So, \( (CH_3)_3C-I \) is a tertiary alkyl iodide, combining: - A stable tertiary carbocation, - An excellent leaving group (I⁻). Thus, it is **most reactive** in SN1. Other choices: - Br⁻ and Cl⁻ are poorer leaving groups than I⁻. - F⁻ is the worst due to strong C–F bond.