Illustrated Glossary of Organic Chemistry

Common name: A nomenclature system useful for naming simple organic molecules. It often fails for more complex molecules, in which case systematic or (better yet) IUPAC nomenclature is preferable.

The prefix "n-" (or normal) is used when all carbons form a continuous, unbranched (linear) chain. If a functional group (such as an alcohol) is present that functional group is on the end of the chain. Not to be confused with 'nor', which indicates a missing methyl group.

Common name: n-pentane
IUPAC name: pentane
Common name: n-pentanol
IUPAC name: 1-pentanol

The prefix "iso" is used when all carbons except one form a continuous chain. This one carbon is part of an isopropyl group at the end of the chain.

Common name: isopentane
IUPAC name: 2-methylbutane
Common name: isopentyl alcohol
IUPAC name: 3-methyl-1-butanol

"Iso" can also indicate that the molecule is an constitutional isomer of another molecule with a common (or trivial) name.

Common name: phthalic acid
IUPAC name: benzene-1,2-dicarboxylic acid
Common name: isophthalic acid
IUPAC name: benzene-1,3-dicarboxylic acid

The prefix "neo" is used when all but two carbons form a continuous chain, and these two carbons are part of a terminal tert-butyl group.

Common name: neopentane
IUPAC name: 2,2-dimethylpropane
Common name: neopentyl alcohol
IUPAC name: 2,2-dimethyl-1-propanol

The prefix "sec" or "s" is used when the functional group is bonded to a secondary carbon. This prefix is only useful for a four-carbon chain. It is not applicable with a shorter chain, and it is often ambiguous when the chain has five or more carbons.

Common name: sec-butanol (s-butanol)
IUPAC name: 2-butanol

The prefix "tert" or "t" is used when the functional group is bonded to a tertiary carbon.

Common name: tert-butanol (t-butanol)
IUPAC name: 2-methyl-2-propanol