The figure shows the reaction between chloromethane and hydroxide ion, which follows second order kinetics.
Which of the following correctly describes the species shown within the square brackets?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → It is a transition state, in which the carbon atom is bonded to five atoms at the same time.
A second order reaction of an alkyl halide goes by the SN2 mechanism: the C–O bond forms while the C–Cl bond breaks, in a single step, and no intermediate is formed.
In the bracketed species the dotted lines show the HO group and the Cl atom both partly attached to the same carbon, which also holds three H atoms lying in one plane.
The carbon is therefore bonded to five atoms at the same time, and the species is the transition state.
Why the other options are incorrect
Option (1) It is a carbocation intermediate, in which the carbon atom is bonded to three atoms. — Incorrect. A carbocation forms only in the SN1 mechanism; the bracketed carbon still has both HO and Cl partly attached.
Option (3) It is a stable intermediate that can be isolated, in which the carbon atom is bonded to five atoms. — Incorrect. A carbon with partial bonds to five atoms is unstable and cannot be isolated; the SN2 reaction forms no intermediate.
Option (4) It is a transition state, in which the carbon atom is bonded to four atoms. — Incorrect. Three H atoms, the O of the incoming OH and the outgoing Cl are all attached: five atoms, not four.
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.”
“…These two processes take place simultaneously in a single step and no intermediate is formed. … In transition state all the three C-H bonds are in the same plane and the attacking and leaving nucleophiles are partially attached to the carbon. … Such structures are unstable and cannot be isolated. Thus, in the transition state, carbon is simultaneously bonded to five atoms.”
Fig. 6.2: Red ball represents the incoming hydroxide ion and green ball represents the outgoing halide ion