Q4(b). Two C₇H₁₄ cycloalkanes from their base peaks

Introduction. A cycloalkane is CₙH₂ₙ; 14n = 98 gives n = 7: both are C₇H₁₄ (one degree of unsaturation, the ring). In alkyl-substituted cycloalkanes the commonest fragmentation is loss of the side chain as a radical, leaving a cycloalkyl cation; the base peak tells which alkyl group was lost.

Scheme: Left: ethylcyclopentane. Right: methylcyclohexane.

1. Base peak 69: 98 - 69 = 29, loss of an ethyl radical. The ion C₅H₉⁺ (m/z 69) is the cyclopentyl cation. Compound: ethylcyclopentane.
2. Base peak 83: 98 - 83 = 15, loss of a methyl radical. The ion C₆H₁₁⁺ (m/z 83) is the cyclohexyl cation. Compound: methylcyclohexane.
3. The ring–substituent bond breaks most easily because it gives a secondary cation and the most stable radical; cleavage of the ring itself needs two bonds to break.
4. Cycloheptane has no side chain to lose (its spectrum is dominated by ring-opening ions); a methyl or ethyl group on a smaller ring is needed for a base peak at M-15 or M-29. The textbook pair is ethylcyclopentane and methylcyclohexane.

Conclusion. Base peak 69: ethylcyclopentane (loss of C₂H₅, 29). Base peak 83: methylcyclohexane (loss of CH₃, 15).

Application. Side-chain loss to the ring cation is used to identify naphthenes (cycloalkanes) in petroleum fractions by GC–MS.