Choose the correct statements for Markovnikov's rule.
(A) An alkene is converted to corresponding alkyl halide by reaction with HCl, HBr and HI.
(B) But-1-ene on reaction with HCl gives 2-chlorobutane as major product.
(C) But-1-ene on reaction with HCl gives 1-chlorobutane as major product.
(D) Addition of hydrogen takes place to the doubly bonded carbon atom which is more substituted.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A) and (B) only
Note: None of the given options was correct in the NTA question paper. To provide additional question practice to the students, the question has been slightly modified so that the question has one clear answer.
Alkenes react with hydrogen halides (HCl, HBr, HI) by electrophilic addition to give alkyl halides. When the alkene is unsymmetrical, two products are possible, and the major one is given by Markovnikov’s rule: the negative part of the adding molecule (X) goes to the double-bond carbon that has fewer hydrogen atoms, so the hydrogen goes to the carbon that already has more hydrogen atoms (the less substituted carbon).
The reason is the stability of the carbocation formed in the first step. \( \ce{H^{+}} \) adds to the \( \ce{CH2} \) end so that the positive charge sits on the more substituted carbon, giving the more stable secondary carbocation:
\( \displaystyle \ce{CH3CH2CH=CH2 + HCl -> CH3CH2CH(Cl)CH3} \)
(A) An alkene is converted to corresponding alkyl halide by reaction with HCl, HBr and HI. — Correct. Hydrogen halides add across the C=C bond; the reactivity order is HI > HBr > HCl.
(B) But-1-ene on reaction with HCl gives 2-chlorobutane as major product. — Correct. Hydrogen adds to C-1 (the \( \ce{CH2} \) end) and chlorine to C-2, as the rule predicts.
(C) But-1-ene on reaction with HCl gives 1-chlorobutane as major product. — Incorrect. 1-Chlorobutane would need the less stable primary carbocation, so it is only the minor product.
(D) Addition of hydrogen takes place to the doubly bonded carbon atom which is more substituted. — Incorrect. Hydrogen adds to the carbon with more hydrogen atoms, which is the less substituted carbon.
NCERT (2026-27 edition):
From Alkenes – “(i) Addition of hydrogen halides: An alkene is converted to corresponding alkyl halide by reaction with hydrogen chloride, hydrogen bromide or hydrogen iodide. Propene yields two products, however only one predominates as per Markovnikov’s rule.”