Decreasing order towards SN1 reaction for the following compounds is:

Step 1: Understand $S_N1$ mechanism for aryl halides

Problem: Aryl halides (halogen directly on benzene) generally do not undergo $S_N1$ because:

• Aryl carbocations are extremely unstable (sp² hybridized)
• No resonance stabilization
• Very high energy

But if the question asks about $S_N1$ reactivity, we consider relative stability if forced to react.

Step 2: Analyze substituent effects

For $S_N1$, we need to stabilize the carbocation (if it forms).

Electron-donating groups (EDG): Stabilize carbocation (increase reactivity) Electron-withdrawing groups (EWG): Destabilize carbocation (decrease reactivity)

Step 3: Analyze each compound

(a) p-Nitrochlorobenzene:

• $\ce{NO2}$ at para: Strong EWG
• Destabilizes any positive charge
• Least reactive for $S_N1$ ✗

(b) p-Methoxychlorobenzene:

• $\ce{-OCH3}$ at para: Strong EDG (resonance)
• Stabilizes positive charge through resonance
• Most reactive for $S_N1$ ✓✓

(c) m-Dichlorobenzene:

• Cl at meta: Weak EWG (inductive only, no resonance)
• Slightly destabilizes
• Low reactivity ✗

(d) Chlorobenzene:

• No additional substituent
• Baseline reactivity
• Between (b) and (c)

Step 4: Order reactivity

Decreasing order for $S_N1$:

(b) [p-OCH₃] > (d) [plain] > (c) [m-Cl] > (a) [p-NO₂]

EDG (stabilizes) > neutral > weak EWG > strong EWG (destabilizes)

Answer: (c) (b) > (d) > (c) > (a)

Key Points:

• $S_N1$: Carbocation stability is key
• EDG: Stabilize carbocation → faster $S_N1$
• EWG: Destabilize carbocation → slower $S_N1$
• Resonance effects: Stronger than inductive effects
• Para/ortho positions: Allow resonance with substituent
• Meta position: Only inductive effect (no resonance)