The number of structurally isomeric alkyl halides possible for C5H11C…

The correct answer is Option (4) → 8

• Structural isomers have the same molecular formula but a different arrangement of atoms: a different carbon chain, or the halogen on a different carbon.

• To count the isomers of \( \ce{C5H11Cl} \), take each possible carbon skeleton of \( \ce{C5H12} \) and put the chlorine on each different kind of carbon in turn. Positions that are equivalent by symmetry give the same compound and are counted once.

\( \ce{C5H12} \) has three skeletons:

• Pentane, \( \ce{CH3CH2CH2CH2CH3} \): three different positions (C1 = C5, C2 = C4, C3).
  1-Chloropentane, \( \ce{CH3CH2CH2CH2CH2Cl} \)
  2-Chloropentane, \( \ce{CH3CH2CH2CH(Cl)CH3} \)
  3-Chloropentane, \( \ce{CH3CH2CH(Cl)CH2CH3} \)

• 2-Methylbutane, \( \ce{(CH3)2CHCH2CH3} \): four different positions (C1 and the methyl on C2, which are equivalent; C2; C3; C4).
  1-Chloro-2-methylbutane, \( \ce{ClCH2CH(CH3)CH2CH3} \)
  2-Chloro-2-methylbutane, \( \ce{(CH3)2C(Cl)CH2CH3} \)
  2-Chloro-3-methylbutane, \( \ce{(CH3)2CHCH(Cl)CH3} \)
  1-Chloro-3-methylbutane, \( \ce{(CH3)2CHCH2CH2Cl} \)

• 2,2-Dimethylpropane, \( \ce{C(CH3)4} \): all four methyl groups are equivalent, so only one position.
  1-Chloro-2,2-dimethylpropane, \( \ce{(CH3)3CCH2Cl} \)

Total: 3 + 4 + 1 = 8 structural isomers. Stereoisomers are not counted here; 2-chloropentane, 1-chloro-2-methylbutane and 2-chloro-3-methylbutane each also exist as a pair of enantiomers.

NCERT (2026-27 edition):
Example 6.1 – “Draw the structures of all the eight structural isomers that have the molecular formula C₅H₁₁Br. Name each isomer according to IUPAC system and classify them as primary, secondary or tertiary bromide.
CH₃CH₂CH₂CH₂CH₂Br 1-Bromopentane (1°)
CH₃CH₂CH₂CH(Br)CH₃ 2-Bromopentane (2°)
CH₃CH₂CH(Br)CH₂CH₃ 3-Bromopentane (2°)
(CH₃)₂CHCH₂CH₂Br 1-Bromo-3-methylbutane (1°)
(CH₃)₂CHCHBrCH₃ 2-Bromo-3-methylbutane (2°)
(CH₃)₂CBrCH₂CH₃ 2-Bromo-2-methylbutane (3°)
CH₃CH₂CH(CH₃)CH₂Br 1-Bromo-2-methylbutane (1°)
(CH₃)₃CCH₂Br 1-Bromo-2,2-dimethylpropane (1°)”