rs is given and a multiplier sed to indicate the total number of identical substituents; i.e., dimethyL. trimethyl. tetraethy L., etc 2, 3-dimethy Butane 2, 3, 4-trime thy pentane Numbering of the carbons in the parent chain is always done in the direction that gives the lowest number to the subs tituent whic h is encountered first, or, the lowest number at the first point of difference. If there are different substituents at equivalent positions on the cha in, the substituent of lower alphabeticalorder is given the lowest num ber. 3-ethy Heptane not 5-ethy Heptane 3-ethy1-5-methy Heptane not 5-ethy1-3-methy Thept In constructing the name, substituents are arranged in alphabetical order, without regard for ultipl 2, 2, 5-trimethy Hexane 2, 3, 5-trimethy Hexane t255-trime thy The 2,4 5-trimethy Hexane For complex molecules, the IUPAC system of nomenclature can generate somewhat bewildering, and equally complex nomenclature. This is further complicated by the fact that many common molecules are routinely referred to using simple names which are descriptive of the molecule, or have arisen historically. The most common examples of these are the substituent names for side-chains containing three to five carbons, where the prefixes iso.,sec. are commonly used. The structures corresponding to these substituents are shown below Common Substituent names CH,CH, CH ropy l CHCH(CH3 )2 butyl CH, CH2 CH isobutyl CH2CHCH(CH3)chain where the substituent occurs is given and a multiplier is used to indicate the total number of identical substituents; i.e., dimethyl... trimethyl... tetraethyl..., etc. • Numbering of the carbons in the parent chain is always done in the direction that gives the lowest number to the substituent which is encountered first, or, the lowest number at the first point of difference. If there are different substituents at equivalent positions on the chain, the substituent of lower alphabetical order is given the lowest number. • In constructing the name, substituents are arranged in alphabetical order, without regard for multipliers. For complex molecules, the IUPAC system of nomenclature can generate somewhat bewildering, and equally complex nomenclature. This is further complicated by the fact that many common molecules are routinely referred to using simple names which are descriptive of the molecule, or have arisen historically. The most common examples of these are the substituent names for side-chains containing three to five carbons, where the prefixes iso..., sec-..., tert-..., and neo are commonly used. The structures corresponding to these substituents are shown below: Common Substituent Names: propyl -CH2CH2CH3 isopropyl -CHCH(CH3)2 butyl -CH2CH2CH3 isobutyl -CH2CHCH(CH3)2