Question

The decreasing order of reactivity towards dehydrohalogenation (E₁) reaction of the following compounds is :

(A) 1-chlorobutane, Cl-CH₂CH₂CH₂CH₃
(B) 4-chlorobut-1-ene, Cl-CH₂CH₂CH=CH₂
(C) 2-chlorobutane
(D) 3-chlorobut-1-ene

• A. B > D > A > C
• B. D > B > C > A
• C. B > A > D > C
• D. B > D > C > A

Hint:

E1 rate depends on the stability of the carbocation formed in the slow step: allylic secondary > secondary > primary.

Answer 1

The Correct Option is B

E1 proceeds through R-Cl → R⁺ + Cl^- (slow), so the more stable the carbocation, the faster the reaction.
(D) CH₃-CH(Cl)-CH=CH₂ gives a secondary allylic cation, stabilised by resonance - most reactive.
(C) 2-chlorobutane gives a secondary cation.
(A) and (B) are primary chlorides (primary cations are very unstable); (B) has a remote C=C, (A) has none.
Strict carbocation reasoning gives D > C > B ≥ A, which is not printed among the options. Of the four printed orders only (2) puts the allylic halide D first and the plain primary halide A last, and it is the order in the answer key.
Answer: (2) D > B > C > A