Which statement(s) is / are true? A. 3^o alkyl halide undergoes S_N1…

The correct answer is Option (4) → A, B, C, D only

Nucleophilic substitution in alkyl halides goes by two mechanisms. S_N2 is a one-step reaction in which the nucleophile attacks as the halide leaves; its rate depends on both reactants (second order). S_N1 goes in two steps: the C–X bond ionises slowly to a carbocation, which then reacts fast with the nucleophile; its rate depends only on the alkyl halide (first order).

• (A) 3° alkyl halide undergoes S_N1 reaction very fast because of high stability of carbocation — Correct. The slow step forms the carbocation, and a tertiary carbocation is stabilised by three alkyl groups (inductive effect and hyperconjugation).

• (B) S_N1 reactions are generally carried out in polar protic solvents — Correct. Water, alcohols and acetic acid solvate and stabilise the carbocation and the leaving halide ion.

• (C) S_N1 reactions follow first order kinetics — Correct. For \( \ce{(CH3)3CBr} \) + \( \ce{OH^{-}} \), Rate = k[(CH₃)₃CBr].

• (D) S_N2 reactions follow second order kinetics — Correct. For \( \ce{CH3Cl} \) + \( \ce{OH^{-}} \), Rate = k[CH₃Cl][OH⁻].

NCERT (2026-27 edition):
Substitution Nucleophilic Bimolecular (S_N2) – “The reaction between CH₃Cl and hydroxide ion to yield methanol and chloride ion follows a second order kinetics, i.e., the rate depends upon the concentration of both the reactants.”

Substitution Nucleophilic Unimolecular (S_N1) – “S_N1 reactions are generally carried out in polar protic solvents (like water, alcohol, acetic acid, etc.). The reaction between tert-butyl bromide and hydroxide ion yields tert-butyl alcohol and follows the first order kinetics, i.e., the rate of reaction depends upon the concentration of only one reactant, which is tert-butyl bromide. …

… Step I is the slowest and reversible. It involves the C–Br bond breaking for which the energy is obtained through solvation of halide ion with the proton of protic solvent. Since the rate of reaction depends upon the slowest step, the rate of reaction depends only on the concentration of alkyl halide and not on the concentration of hydroxide ion. Further, greater the stability of carbocation, greater will be its ease of formation from alkyl halide and faster will be the rate of reaction. In case of alkyl halides, 3° alkyl halides undergo S_N1 reaction very fast because of the high stability of 3° carbocations.”

Note: The same/similar question has been asked in different years by NTA as per the details given below: