How many of the following isomers of C5H11Br are secondary (2°) alkyl bromides?
1-Bromopentane, 2-Bromo-2-methylbutane, 3-Bromopentane, 1-Bromo-3-methylbutane, 2-Bromo-3-methylbutane, 1-Bromo-2,2-dimethylpropane, 2-Bromopentane, 1-Bromo-2-methylbutane
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 3
An alkyl halide is called primary, secondary or tertiary according to the carbon atom that carries the halogen. In a secondary (2°) bromide, the carbon bearing Br is bonded to two other carbon atoms.
Secondary bromides:
3-Bromopentane, \( \ce{CH3CH2CH(Br)CH2CH3} \) (Br carbon bonded to two carbons)
2-Bromo-3-methylbutane, \( \ce{(CH3)2CHCH(Br)CH3} \) (Br carbon bonded to two carbons)
2-Bromopentane, \( \ce{CH3CH2CH2CH(Br)CH3} \) (Br carbon bonded to two carbons)
Not secondary:
1-Bromopentane, \( \ce{CH3CH2CH2CH2CH2Br} \) — Br carbon bonded to one carbon: primary.
2-Bromo-2-methylbutane, \( \ce{(CH3)2CBrCH2CH3} \) — Br carbon bonded to three carbons: tertiary.
1-Bromo-3-methylbutane, \( \ce{(CH3)2CHCH2CH2Br} \) — primary.
1-Bromo-2,2-dimethylpropane, \( \ce{(CH3)3CCH2Br} \) — primary; the CH2 carrying Br is bonded to only one carbon.
1-Bromo-2-methylbutane, \( \ce{CH3CH2CH(CH3)CH2Br} \) — primary.
Secondary bromides: 3 compounds.
NCERT (2026-27 edition):
6.1.2 Compounds Containing sp3 C—X Bond – “…They are further classified as primary, secondary or tertiary according to the nature of carbon to which halogen is attached. If halogen is attached to a primary carbon atom in an alkyl halide, the alkyl halide is called primary alkyl halide or 1° alkyl halide. Similarly, if halogen is attached to secondary or tertiary carbon atom, the alkyl halide is called secondary alkyl halide (2°) and tertiary (3°) alkyl halide, respectively.”