Alkanes

The first three alkanes -- methane (CH4), ethane (C2H6) and propane (C3H8) -- have only one possible structure each. From the fourth member (C4H10, butane) onward, more than one structure becomes possible for the same molecular formula: these constitutional (structural) isomers differ in how their atoms are connected. C4H10 has two such isomers -- n-butane (an unbranched, 'normal' chain of four carbons) and iso-butane / 2-methylpropane (a branched chain) -- an example of chain isomerism, since the two structures differ only in how the carbon skeleton is arranged. Pentane (C5H12) similarly has three chain isomers, built by first drawing the straight five-carbon backbone and then rearranging the backbone into the other possible skeletons before filling in hydrogens so every carbon has four bonds. IUPAC names for branched alkanes are assigned using the general nomenclature rules from Unit 11 -- identify the longest chain, number it to give the lowest locants to substituents, and cite each alkyl-group substituent (methyl, ethyl, propyl, etc.) with its locant, in alphabetical order, using multiplying prefixes (di, tri...) when a substituent repeats. Worked examples include 2-methylpentane, 2,4-dimethylpentane, 3,3-dimethylpentane, 3-ethyl-2-methylpentane, 3-ethyl-4,5-dipropyloctane, 2,3-dimethylpentane and 4-ethyl-2,7-dimethyloctane. The reverse process -- drawing a structural formula from a given IUPAC name -- is illustrated …

The strongly exothermic nature of alkane combustion explains their extensive use as fuels: methane in natural gas is used for home heating; a propane-butane mixture (LPG) is used for domestic cooking; and petrol/gasoline, a complex mixture of many hydrocarbons, fuels internal-combustion engines. Carbon black (from incomplete combustion) is used in making p …