Identify the correct decreasing order of reactivity of the following compounds towards the S _ N 1 reaction: (I) Ph - CH ₂- Br (II) Ph - Br (III)

Step-by-step solution

The reactivity of a compound towards the S _ N 1 reaction depends on the stability of the intermediate carbocation formed after the departure of the leaving group ( Br ⁻ ). Let us analyze the carbocations formed from each compound: 1. Compound (I) forms the benzyl carbocation ( Ph - CH ₂⁺ ). This carbocation is stabilized by resonance with the -electrons of the benzene ring. 2. Compound (II) forms the phenyl carbocation ( Ph ⁺ ). This carbocation is highly unstable because the positive charge resides on an sp^2 hyb