Which of the following is a characteristic of \( S_N2 \) reaction?
(A). Concentration of both reactants determines the reaction rate.
(B). Proceeds with inversion of configuration.
(C). The transition state involves central carbon bonded to five species.
(D). Occurs in polar protic solvent
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (A), (B) and (C) only
In an \( S_N2 \) reaction (substitution, nucleophilic, bimolecular) the nucleophile attacks the carbon from the side opposite to the leaving group, and the new bond forms as the old bond breaks, in a single step with no intermediate. For example:
\( \displaystyle \ce{CH3Cl + OH- -> CH3OH + Cl-} \qquad \text{Rate} = k[\ce{CH3Cl}][\ce{OH-}] \)
(A) Concentration of both reactants determines the reaction rate. — Correct. Both the substrate and the nucleophile take part in the single, rate determining step, so the reaction is second order: first order in each.
(B) Proceeds with inversion of configuration. — Correct. Because of the backside attack, the three other groups flip over like an umbrella turned inside out; an optically active halide gives the product of opposite configuration.
(C) The transition state involves central carbon bonded to five species. — Correct. In the transition state the carbon is partly bonded to the incoming nucleophile and to the leaving group, besides its three other groups: five atoms around carbon, which is why it is unstable and cannot be isolated.
(D) Occurs in polar protic solvent — Incorrect. Polar protic solvents (water, alcohols) surround the nucleophile with hydrogen bonds and weaken it, so they slow down \( S_N2 \) reactions; they favour \( S_N1 \) reactions by solvating the carbocation and the leaving halide ion. \( S_N2 \) reactions go best in polar aprotic solvents such as acetone or DMSO.
NCERT (2026-27 edition):
Substitution Nucleophilic Bimolecular (SN2) – “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.”
“… 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. … As a result configuration is inverted.”