Decreasing order towards SN 1 reaction for the following compounds is:

Solution

Chemistry ma’am · 3 steps

SN1 is all about the carbocation: ask what each para group does to the benzyl cation's positive charge.

1. Rate-deciding step. In SN1 the C-Cl bond breaks first to give a carbocation, and that slow step sets the rate. All four compounds give a benzyl cation, so the para substituent decides how stable that cation is.
2. Para effects. The positive charge of a benzyl cation spreads onto the ortho and para ring carbons. A para group that pushes electrons in stabilises it. A group that pulls electrons out destabilises it.

   Compound	Para group	Effect on the cation	SN1 rank
   b	-OCH₃	strong +R pushes a lone pair right onto the cationic carbon, most stable	1
   d	-H	plain benzylic resonance only, the reference	2
   c	-Cl	−I outweighs its weak +R, slightly destabilising	3
   a	-NO₂	strong −I and −R pull electrons out, least stable	4

3. So. Cation stability, and so the SN1 reactivity, falls in the order b > d > c > a, which is option (d).

The idea to keep. For SN1, rank the carbocations, not the halides. Electron donors at ortho or para speed SN1 up, electron withdrawers slow it down, and halogens withdraw overall because −I beats +R.

Answer from the NTA final answer key. Solution by PrepWiser.