R, although Alk is sometimes used to specifically symbolize an alkyl group (as opposed to an alkenyl group or aryl group). Structure and classification Ordinarily the C-C single bond distance is . Saturated hydrocarbons can be linear, branched, or cyclic. The third group is sometimes called cycloalkanes. Very complicated structures are possible by combining linear, branch, cyclic alkanes. Isomerism Alkanes with more than three carbon atoms can be arranged in various ways, forming structural isomers. The simplest isomer of an alkane is the one in which the carbon atoms are arranged in a single chain with no branches. This isomer is sometimes called the n-isomer (n for "normal", although it is not necessarily the most common). However, the chain of carbon atoms may also be branched at one or more points. The number of possible isomers increases rapidly with the number of carbon atoms. For example, for acyclic alkanes: C1: methane only C2: ethane only C3: propane only C4: 2 isomers: butane and isobutane C5: 3 isomers: pentane, isopentane, and neopentane C6: 5 isomers: hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, and 2,3-dimethylbutane C7: 9 isomers: heptane, methylhexane (2 isomers), dimethylpentane (4 isomers), 3-ethylpentane, 2,2,3-trimethylbutane C8: 18 isomers: octane, 2-methylheptane, 3-methylheptane, 2,3-dimethylhexane, 3,4-dimethylhexane, 2,3,4-trimethylpentane, 3,3-dimethylhexane, 2,2-trimethylpentane, 2,4-dimethylhexane, 2,2,4-trimethylpentane, 2,3,3-Trimethylpentane, 3,3,4-trimethyl-pentane, 3,4,4-trimethylpentane, 2,4,4-trimethylpentane, (5 isomers) C9: 35 isomers C10: 75 isomers C12: 355 isomers C32: 27,711,253,769 isomers C60: 22,158,734,535,770,411,074,184 isomers, many of which are not stable Branched alkanes can be chiral. For example, 3-methylhexane and its higher homologues are chiral due to their stereogenic center at carbon atom number 3. The above list only includes differences of connectivity, not stereochemistry. In addition to the alkane isomers, the chain of carbon atoms may form one or more rings. Such compounds are called cycloalkanes, and are also excluded from the above list because changing the number of rings changes the molecular formula. For example, cyclobutane and methylcyclopropane are isomers of each other (C4H8), but are not isomers of butane (C4H10). Nomenclature The IUPAC nomenclature (systematic way of naming compounds) for alkanes is based on identifying hydrocarbon chains. Unbranched, saturated hydrocarbon chains are named systematically with a Greek numerical prefix denoting the number of carbons and the suffix "-ane". In 1866, August Wilhelm von Hofmann suggested systematizing nomenclature by using the whole sequence of vowels a, e, i, o and u to create suffixes -ane, -ene, -ine (or -yne), -one, -une, for the hydrocarbons CnH2n+2, CnH2n, CnH2n−2,