Which of the following structural isomers of C5H11Br is a tertiary (3…

The correct answer is Option (3) → 2-Bromo-2-methylbutane

An alkyl halide is primary, secondary or tertiary according to the carbon that carries the halogen: a tertiary carbon is bonded to three other carbon atoms. Only the carbon bearing Br counts, not other carbons in the chain.

Option (1) 1-Bromo-2,2-dimethylpropane, (CH₃)₃CCH₂Br — Incorrect. The molecule has a carbon bonded to four carbons, but Br is on the CH₂ group, which is bonded to only one carbon. It is a primary bromide.

Option (2) 2-Bromo-3-methylbutane, (CH₃)₂CHCH(Br)CH₃ — Incorrect. The carbon bearing Br is bonded to two carbons, so it is a secondary bromide; the branch is on the next carbon.

Option (3) 2-Bromo-2-methylbutane, (CH₃)₂CBrCH₂CH₃ — Correct. C-2 carries the Br and is bonded to three carbon atoms (two CH₃ groups and the CH₂CH₃ group), so this is the tertiary bromide.

Option (4) 1-Bromo-3-methylbutane, (CH₃)₂CHCH₂CH₂Br — Incorrect. Br is on the end CH₂, bonded to one carbon, so it is a primary bromide.

NCERT (2026-27 edition):
6.1.2 Compounds Containing sp³ C—X Bond (X= F, Cl, Br, I) – “…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.”

6.2 Nomenclature (Example 6.1, the eight isomers of C₅H₁₁Br) – “…(CH₃)₂CBrCH₂CH₃ 2-Bromo-2-methylbutane (3°)”