Assertion (A): For the same alkyl group, an alkyl iodide is more reactive than the alkyl chloride in both SN1 and SN2 reactions.
Reason (R): In haloalkanes, the carbon atom bonded to the halogen bears a partial positive charge because the halogen is more electronegative than carbon.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Both (A) and (R) are true but (R) is not the correct explanation of (A).
Assertion (A): For the same alkyl group, an alkyl iodide is more reactive than the alkyl chloride in both SN1 and SN2 reactions. — True. For a given alkyl group the reactivity follows the same order in both mechanisms: R–I > R–Br > R–Cl >> R–F.
Reason (R): In haloalkanes, the carbon atom bonded to the halogen bears a partial positive charge because the halogen is more electronegative than carbon. — True. The C–X bond is polarised, carbon δ+ and halogen δ−, and this is why nucleophiles attack that carbon.
Why R does not explain A?
R explains why haloalkanes undergo nucleophilic substitution at all, not why the iodide reacts faster than the chloride; chlorine is in fact more electronegative than iodine.
The correct reason is that iodine, because of its large size, is a better leaving group, so the C–I bond breaks more easily and the iodide is released faster.
NCERT (2026-27 edition):
6.7.1 Reactions of Haloalkanes – “For a given alkyl group, the reactivity of the halide, R-X, follows the same order in both the mechanisms R–I> R–Br>R–Cl>>R–F.” … “As iodine is a better leaving group because of its large size, it will be released at a faster rate in the presence of incoming nucleophile.”
6.3 Nature of C-X Bond – “Halogen atoms are more electronegative than carbon, therefore, carbon-halogen bond of alkyl halide is polarised; the carbon atom bears a partial positive charge whereas the halogen atom bears a partial negative charge.”