Question

Explain the SN1 and SN2 mechanisms with suitable examples and reactivity orders.

Hint:

SN1: two-step, carbocation, $3^\circ$ favored; \quad SN2: one-step, backside attack, methyl and $1^\circ$ favored.

Answer 1

Concept: Nucleophilic substitution reactions involve the replacement of a leaving group by a nucleophile. Based on the reaction mechanism and kinetics, they are classified as SN1 and SN2 reactions.
Step 1: SN1 Mechanism (Unimolecular Nucleophilic Substitution).

• Occurs in two steps:
  • Formation of a carbocation intermediate (slow, rate-determining step)
  • Attack by nucleophile (fast step)
• Rate depends only on substrate concentration: \[ \text{Rate} = k[\text{R–X}] \]
• Favored by tertiary substrates and polar protic solvents

Example: Hydrolysis of tert-butyl bromide: \[ (\text{CH}_3)_3\text{C–Br} \rightarrow (\text{CH}_3)_3\text{C}^+ \rightarrow (\text{CH}_3)_3\text{C–OH} \]
Reactivity order (SN1): \[ 3^\circ>2^\circ>1^\circ>\text{methyl} \]
Step 2: SN2 Mechanism (Bimolecular Nucleophilic Substitution).

• Occurs in a single concerted step
• Nucleophile attacks from the backside while leaving group departs
• Leads to inversion of configuration (Walden inversion)
• Rate depends on both substrate and nucleophile: \[ \text{Rate} = k[\text{R–X}][\text{Nu}^-] \]
• Favored by primary substrates and polar aprotic solvents

Example: Reaction of methyl bromide with hydroxide ion: \[ \text{CH}_3\text{Br} + \text{OH}^- \rightarrow \text{CH}_3\text{OH} + \text{Br}^- \]
Reactivity order (SN2): \[ \text{methyl}>1^\circ>2^\circ>3^\circ \]
Step 3: Key Differences.

• 
  Mechanism: SN1 is two-step; SN2 is one-step.
• 
  Intermediate: SN1 forms carbocation; SN2 has no intermediate.
• 
  Kinetics: SN1 first-order; SN2 second-order.
• 
  Stereochemistry: SN1 gives racemization; SN2 gives inversion.
• 
  Substrate preference: SN1 favors tertiary; SN2 favors primary.

Conclusion: SN1 and SN2 reactions differ fundamentally in mechanism, kinetics, stereochemistry, and substrate preference, making them essential concepts in organic reaction mechanisms.