The reaction in which a nucleophile replaces an existing group (the leaving group) in a molecule is called a nucleophilic substitution reaction. Haloalkanes are the substrates in these reactions. In this type of reaction, a nucleophile reacts with a haloalkane (the substrate) having a partial positive charge on the carbon atom bonded to the halogen. A substitution reaction takes place and the halogen atom, called the leaving group, departs as a halide ion. Since the substitution reaction is initiated by a nucleophile, it is called a nucleophilic substitution reaction.
It is one of the most useful classes of organic reactions of alkyl halides in which the halogen is bonded to an sp3 hybridised carbon.
Fluoroethane, chloroethane, bromoethane and iodoethane are each treated with the same nucleophile under the same conditions. Which of them undergoes nucleophilic substitution the fastest?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Iodoethane
In nucleophilic substitution the halogen leaves as a halide ion, the leaving group. With the alkyl group kept the same, the rate depends on how easily the C–X bond breaks.
Down the group the halogen atom becomes larger and the C–X bond becomes longer. Iodine is the largest, so iodide is the best leaving group.
The order is the same for SN1 and SN2: R–I > R–Br > R–Cl >> R–F.
Why the other options are incorrect
Option (2) Chloroethane — Incorrect. The C–Cl bond is shorter than C–Br and C–I, and chloride is a poorer leaving group, so chloroethane comes third.
Option (3) Fluoroethane — Incorrect. Fluorine is the smallest halogen and the C–F bond is the shortest; fluoroethane reacts much the slowest (R–Cl >> R–F).
Option (4) Bromoethane — Incorrect. Bromoethane reacts fast, but bromine is smaller than iodine, so it comes second, after iodoethane.
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 – “As we go down the group in the periodic table, the size of halogen atom increases. Fluorine atom is the smallest and iodine atom is the largest. Consequently the carbon-halogen bond length also increases from C—F to C—I.”