Consider the following statements about the mechanisms of nucleophilic substitution in haloalkanes.
(A) The reaction between CH3Cl and hydroxide ion to give methanol follows second order kinetics.
(B) In the transition state of an SN2 reaction, the carbon atom is simultaneously bonded to five atoms.
(C) In an SN1 reaction, the step in which the C–Br bond breaks to give a carbocation is the fastest step.
(D) The rate of the SN1 reaction between tert-butyl bromide and hydroxide ion depends on the concentrations of both tert-butyl bromide and hydroxide ion.
Which of the statements given above are incorrect? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (C) and (D) only
An SN2 reaction takes place in one step: the nucleophile attacks as the leaving group departs, so the rate depends on both reactants. An SN1 reaction takes place in two steps: a slow ionisation to a carbocation, then a fast attack by the nucleophile, so the rate depends on the alkyl halide alone.
(A) The reaction between CH3Cl and hydroxide ion to give methanol follows second order kinetics. — Correct statement. This is the SN2 example: the rate depends on the concentration of both CH3Cl and OH−.
(B) In the transition state of an SN2 reaction, the carbon atom is simultaneously bonded to five atoms. — Correct statement. In the transition state the carbon holds three hydrogens plus the partly bonded incoming and leaving groups, five in all. Such a structure is unstable and cannot be isolated.
(C) In an SN1 reaction, the step in which the C–Br bond breaks to give a carbocation is the fastest step. — Incorrect statement. This first step is the slowest step (and it is reversible). The energy to break the C–Br bond comes from solvation of the halide ion by the protic solvent.
(D) The rate of the SN1 reaction between tert-butyl bromide and hydroxide ion depends on the concentrations of both tert-butyl bromide and hydroxide ion. — Incorrect statement. The reaction follows first order kinetics. The slow step involves only tert-butyl bromide, so the rate depends on its concentration alone, not on that of hydroxide ion.
NCERT (2026-27 edition):
6.7.1 Reactions of Haloalkanes – “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.” … “Thus, in the transition state, carbon is simultaneously bonded to five atoms.”
“…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.”