Arrange the following compounds in the increasing order of the rate of reaction towards nucleophilic substitution.
Choose the correct answer from the following options:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → E < B < D < C < A
Haloarenes undergo nucleophilic substitution through a negatively charged intermediate (carbanion). In this intermediate the negative charge sits on the carbon atoms ortho and para to the carbon carrying the halogen. Groups that withdraw electrons stabilise the carbanion and increase the rate; the –\( \ce{CH3} \) group releases electrons (+I effect and hyperconjugation), so it destabilises the carbanion and slows down the reaction. It does so most strongly from an ortho or para position, where the negative charge develops; from the meta position its effect is small.
E (2,4-dimethylchlorobenzene) — two \( \ce{CH3} \) groups, at the ortho and para positions: the slowest, first in the order.
B (2-chlorotoluene) — one \( \ce{CH3} \) group at the ortho position, where the charge develops; it is also the nearest to the carbon under attack and crowds the approach of the nucleophile: second.
D (4-chlorotoluene) — one \( \ce{CH3} \) group at the para position, where the charge develops, but far from the carbon under attack: third.
C (3-chlorotoluene) — the \( \ce{CH3} \) group is at the meta position, where the negative charge never sits, so it slows the reaction only a little: fourth.
A (chlorobenzene) — no electron-releasing group: the fastest, last in the order.