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There are four isomers of the butyl substituent. Butyl is the largest substituent for which trivial names are commonly used for all isomers. The four groups are:
- n-butyl : with the carbons laid out in a straight chain, and the substituent attached to one of the end carbons; IUPAC name butyl
- isobutyl : with the substituent attached to a carbon which in turn is attached to the middle of a three carbon chain; IUPAC name 2-methylpropyl
- sec-butyl : with the carbons laid out in a straight chain, and the substituent attached to one of the middle carbons; IUPAC name 1-methylpropyl
- tert-butyl : with three of the carbons and the substituent all attached to the fourth carbon; IUPAC name 1,1-dimethylethyl
As the number of carbons in an alkyl chain increases, butyl is the last to be named historically instead of through Greek numbers. The name is derived from butyric acid, a four carbon carboxylic acid found in rancid butter. The name of butyric acid, in turn, comes from Latin butyrum, "butter".