Cycloalkanes: nomenclature, isomerism, preparation
Cycloalkanes are saturated hydrocarbons whose carbon atoms form a ring; the general formula is CₙH₂ₙ (n ≥ 3). They differ from the corresponding alkane by two fewer hydrogen atoms: propane C₃H₈ → cyclopropane C₃H₆, butane → cyclobutane C₄H₈, pentane → cyclopentane C₅H₁₀, hexane → cyclohexane C₆H₁₂. The name is the alkane name preceded by “cyclo”; the ring is the parent chain and carbons are numbered to give the substituents the lowest numbers: 1,2-dimethylcyclobutane, 1-ethyl-3-methylcyclopentane. Isomerism begins at cyclobutane: C₄H₈ fits both cyclobutane and methylcyclopropane. In the laboratory cycloalkanes are made by acting with zinc on compounds with halogens at the two ends of a carbon chain: BrCH₂–CH₂–CH₂Br + Zn → cyclopropane + ZnBr₂. In industry benzene and its homologues are hydrogenated over a catalyst to give cyclohexane and its derivatives: C₆H₆ + 3H₂ → C₆H₁₂.
Using plasticine balls and sticks, build ring models of cyclopropane, cyclobutane and cyclopentane. Notice which ring feels “strained” and which is freer (angles of 60°, 90°, 108°). Under adult supervision.