Chemistry / Unit 12 Chem 2438 worked example, 60 to 90 seconds
IUPAC Alkane Naming
The idea
Naming an alkane means identifying its longest continuous carbon chain for the base name, numbering that chain from the end nearest any substituent, and listing substituents alphabetically with position numbers chosen to keep those numbers as low as possible; when two numbering directions give the same lowest locant set, the direction assigning the lower number to whichever substituent is cited first alphabetically is chosen instead.
In practice
A four-carbon chain with a bromine on the second carbon from one end and a chlorine on the first is named by numbering from the chlorine end, giving 2-bromo-1-chlorobutane rather than the higher alternative. For a seven-carbon chain carrying a bromo substituent and a methyl substituent positioned so each numbering direction gives the identical locant set {2,6}, the tie is broken by assigning the lower number to whichever substituent is cited first alphabetically — bromo before methyl — yielding 2-bromo-6-methylheptane rather than 6-bromo-2-methylheptane.
Where people slip
Numbering from whichever end is drawn first, rather than checking which direction gives the lowest substituent numbers, produces a technically wrong name even when every substituent is correctly identified. Likewise, breaking a genuine locant-set tie by chain-drawing order instead of alphabetical priority assigns the wrong low number to the wrong substituent, even though both substituents and both locants are individually correct.
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