Consider the statements for Substitution Nucleophilic Bimolecular (SN2) reaction.
(A) The nucleophile approaches the carbon bearing the leaving group from the back side.
(B) Nucleophilic displacement of the leaving group causes retention of configuration.
(C) Nucleophilic displacement of the leaving group causes inversion of configuration.
(D) In case of alkyl halides, 3° alkyl halides undergo SN2 reaction faster.
Choose the incorrect statements from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (B) and (D) only
In an SN2 reaction, the nucleophile attacks and the leaving group departs in one step, so the rate depends on both the alkyl halide and the nucleophile:
\( \displaystyle \text{Rate}=k[\ce{R-X}][\ce{Nu^-}] \)
The nucleophile attacks from the side opposite to the leaving group, and the configuration of the carbon is inverted. Bulky groups block this approach, so the reactivity is CH3X > 1° > 2° > 3°.
(A) The nucleophile approaches the carbon bearing the leaving group from the back side. — Correct. This is the backside attack of the SN2 mechanism.
(B) Nucleophilic displacement of the leaving group causes retention of configuration. — Incorrect. Backside attack gives inversion, not retention.
(C) Nucleophilic displacement of the leaving group causes inversion of configuration. — Correct. The product has the opposite configuration to the starting halide.
(D) In case of alkyl halides, 3° alkyl halides undergo SN2 reaction faster. — Incorrect. Tertiary halides are the slowest, because three alkyl groups hinder the backside attack.
NCERT (2026-27 edition):
Substitution Nucleophilic Bimolecular (SN2) – “… It depicts a bimolecular nucleophilic substitution (SN2) reaction; the incoming nucleophile interacts with alkyl halide causing the carbon-halide bond to break and a new bond is formed between carbon and attacking nucleophile. … These two processes take place simultaneously in a single step and no intermediate is formed. … As a result configuration is inverted, the configuration of carbon atom under attack inverts in much the same way as an umbrella is turned inside out when caught in a strong wind. This process is called as inversion of configuration.”
“Since this reaction requires the approach of the nucleophile to the carbon bearing the leaving group, the presence of bulky substituents on or near the carbon atom have a dramatic inhibiting effect. Of the simple alkyl halides, methyl halides react most rapidly in SN2 reactions because there are only three small hydrogen atoms. Tertiary halides are the least reactive because bulky groups hinder the approaching nucleophiles. Thus the order of reactivity followed is: Primary halide > Secondary halide > Tertiary halide.”