The mechanism of SN1 reaction is given as: A student writes general characteristics based on the given mechanism as:…

Step 1: Evaluate each statement

(A) "Reaction favoured by weak nucleophiles"

Analysis:

• $S_N1$ rate = $k[\ce{RX}]$ (independent of nucleophile)
• Weak nucleophiles don't compete with $S_N2$
• $S_N1$ proceeds even with weak nucleophiles (e.g., water, alcohols)
• TRUE ✓

(B) "R⁺ easily formed if substituents are bulky"

Analysis:

• Bulky substituents provide steric hindrance
• Hinder $S_N2$ (backside attack blocked)
• Don't necessarily stabilize carbocation
• Actually, alkyl groups stabilize carbocation through hyperconjugation
• Bulky = more alkyl groups = more stable carbocation
• TRUE ✓

(C) "Reaction accompanied by racemization"

Analysis:

• Carbocation is planar (sp² hybridized)
• Nucleophile can attack from either face
• 50% retention + 50% inversion = racemic mixture
• If starting with pure enantiomer → get racemic product
• TRUE ✓

(D) "Reaction favoured by non-polar solvents"

Analysis:

• $S_N1$ requires carbocation formation
• Carbocation needs stabilization by solvent
• Polar protic solvents (water, alcohols) stabilize ions
• Non-polar solvents cannot stabilize carbocation
• $S_N1$ is NOT favored by non-polar solvents
• FALSE ✗

Answer: (b) (A), (B) and (C) are correct

Key Concepts:

$S_N1$ Characteristics:

| Feature | $S_N1$ | $S_N2$ | |---------|--------|--------| | Rate law | $k[\ce{RX}]$ | $k[\ce{RX}][\ce{Nu}]$ | | Mechanism | Two-step | One-step | | Intermediate | Carbocation | Transition state | | Stereochemistry | Racemization | Inversion | | Substrate | 3° > 2° | 1° > 2° | | Nucleophile | Weak OK | Strong needed | | Solvent | Polar protic | Polar aprotic | | Leaving group | Good needed | Good needed |

Solvent effects:

• Polar protic (H₂O, ROH): Stabilize ions → favor $S_N1$
• Polar aprotic (DMSO, DMF): Don't solvate anions → favor $S_N2$
• Non-polar: Don't stabilize ions → disfavor both