General formula and homologous series of alkanes. Rational nomenclature
Alkanes are open-chain saturated hydrocarbons whose carbon atoms are joined only by σ-bonds; the general formula is CₙH₂ₙ₊₂. The first members are methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀; from pentane on the name is a numeral for the number of carbons (mostly Greek: penta-, hexa-, hepta-, octa-, deca-; nona- is Latin) plus “-ane”. Substances with similar structure and close properties whose compositions differ by a –CH₂– group are homologues, and their set is a homologous series. Removing one hydrogen atom from an alkane gives a radical of formula CₙH₂ₙ₊₁–, named by changing “-ane” to “-yl”: methyl CH₃–, ethyl C₂H₅–, propyl C₃H₇–, isopropyl (CH₃)₂CH–, isobutyl (CH₃)₂CH–CH₂–. Compounds are named in three nomenclatures: trivial (historical), rational and systematic. In rational nomenclature an alkane is regarded as a derivative of methane: the most branched carbon is the “methane centre”, the radicals attached to it are listed from the simplest to the most complex, and the name ends in “methane”: (CH₃)₂CH₂ is dimethylmethane (propane), (C₂H₅)₂CH₂ is diethylmethane (pentane).
Alkane card game: write formulas from methane to decane on 10 sheets, shuffle them, and let a partner sort them in order; say that neighbouring cards differ by –CH₂–.