Q.13 The type of nucleophilic substitution and the possible products for each of the reactions 𝑷 and 𝑸 are:

Nucleophilic Substitution Mechanisms in Reactions P and Q

Reaction P involves a primary benzyl bromide with NaCN in DMF, favoring SN2 due to the strong nucleophile CN⁻, polar aprotic solvent, and unhindered primary substrate with benzylic activation. Reaction Q features a secondary benzyl bromide in H₂O/MeOH, promoting SN1 because of the polar protic solvents stabilizing the carbocation intermediate and secondary benzylic position.​

Reaction P Analysis

Primary benzyl bromide undergoes clean backside attack by CN⁻ in DMF, which enhances nucleophile strength without solvation, yielding the substitution product PhCH₂CN. No carbocation forms, preventing rearrangements or elimination, as SN1 is disfavored for primary substrates despite benzylic stability.​

Reaction Q Analysis

Secondary benzyl bromide ionizes first in protic H₂O/MeOH to form a resonance-stabilized benzylic carbocation, followed by solvent attack (MeOH or H₂O) yielding PhCH(OMe)OH or PhCH(OH)₂, with possible racemization.​

Option Evaluation

• (A) P: SN₂, PhCH₂CN; Q: SN₁, PhCH(OH)OMe: Correct for P (SN2 product). For Q, SN1 yields solvolysis products like PhCH(OH)OMe (mixed hemiacetal from MeOH/H₂O), matching option notation.​

• (B) P: SN₁, PhCH(OMe)CN; Q: SN₂, PhCH(OH)OMe: Incorrect; P cannot be SN1 (primary, aprotic), no OMe source.​

• (C) P: SN₂, PhCH₂CN + CN⁻; Q: SN₁, PhCH(OMe)OH: Partially correct for P but adds erroneous free CN⁻; Q product fits SN1 solvolysis.​

• (D) P: SN₁, PhCH(OMe)CN + CN⁻; Q: SN₂, PhCH₂OMe: Wrong; P not SN1, wrong products; Q not primary-like SN2.​

Correct choice: (A).

Primary benzyl bromide with NaCN in DMF exemplifies classic SN2 nucleophilic substitution, while secondary benzyl bromide in H2O-MeOH demonstrates SN1 via carbocation. These reactions highlight solvent and substrate effects crucial for CSIR NET Life Sciences preparation.​

Key Mechanism Factors

• Substrate: Primary favors SN2; secondary benzylic enables SN1 carbocation stability.​

• Nucleophile/Base: Strong CN⁻ drives SN2; weak solvents (MeOH/H₂O) support SN1 solvolysis.​

• Solvent: Polar aprotic DMF accelerates SN2; protic H₂O-MeOH stabilizes ions for SN1.​

Products and Stereochemistry

SN2 in P inverts configuration at benzyl carbon, yielding pure PhCH₂CN. SN1 in Q racemizes, producing mixture of PhCH(OMe)OH or hydration products from solvent nucleophiles.​