Question

Question: Which benzylic halide reacts faster in an S_N1 reaction? Explain

Short answer

Answer

Compound A

Step-by-step solution

SN1 mechanism 

TheS_N1 reaction forms a stable carbocation.

The more stable the carbocation, the faster is the reactivity of S_N1 mechanism.

Benzylic halide

Benzylic halide is the most stable carbocation. The electron-withdrawing groups decrease the stability of the carbocation, whereas the electron-donating groups increase it.

Explanation

Benzylic halide A reacts faster in theS_N1 reaction. In compound A, the para position is occupied by the methoxy group, which is an electron donor for ortho-para positions through the delocalization of electrons.

In compound B, the para position is occupied by the CHO group which is electron-withdrawing at ortho-para positions.