Where Nu = Nucleophile Find out the correct statement from the options given below for the above 2 reactions.

Step 1: Analyze Reaction (I) - p-Methoxybenzyl chloride

Structure: $\ce{p-CH3O-C6H4-CH2Cl}$

Substituent effect:

• $\ce{-OCH3}$ is a strong electron-donating group (EDG)
• At para position: donates electrons through resonance
• Stabilizes benzylic carbocation

Mechanism:

• Benzylic position with EDG → very stable carbocation
• Favors $S_N1$ mechanism
• Rate = $k[\ce{RCl}]$ (first-order)
• Order: 1

Step 2: Analyze Reaction (II) - p-Nitrobenzyl chloride

Structure: $\ce{p-NO2-C6H4-CH2Cl}$

Substituent effect:

• $\ce{-NO2}$ is a strong electron-withdrawing group (EWG)
• At para position: withdraws electrons through resonance
• Destabilizes benzylic carbocation

Mechanism:

• Benzylic carbocation is destabilized by EWG
• $S_N1$ is disfavored
• Favors $S_N2$ mechanism
• Rate = $k[\ce{RCl}][\ce{Nu}]$ (second-order)
• Order: 2

Step 3: Determine orders

• Reaction (I): 1st order ($S_N1$)
• Reaction (II): 2nd order ($S_N2$)

Answer: (c)

Key Concepts:

Substituent effects on mechanism:

| Substituent | Effect | Carbocation Stability | Favored Mechanism | |-------------|--------|----------------------|-------------------| | EDG (OCH₃, OH, NH₂) | Donate electrons | Stabilized | $S_N1$ (1st order) | | EWG (NO₂, CN, CHO) | Withdraw electrons | Destabilized | $S_N2$ (2nd order) |

Reaction orders:

• $S_N1$: Rate = $k[\ce{RX}]$ → 1st order
• $S_N2$: Rate = $k[\ce{RX}][\ce{Nu}]$ → 2nd order

Benzylic systems:

• Benzylic carbocations are inherently stable (resonance with benzene)
• EDG further stabilizes → strong $S_N1$ preference
• EWG destabilizes → shifts to $S_N2$