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There are four isomeric forms of butyl:
- with the carbons laid out in a straight chain, and the substituent attached to one of the end carbons (called 1-butyl in the IUPAC nomenclature, or n-butyl in the old naming system);
- with the carbons laid out in a straight chain, and the substituent attached to one of the middle carbons (called 2-butyl in the IUPAC system, or sec-butyl in the old system);
- with three of the carbons and the substituent all attached to the fourth carbon (called tert-butyl in the old system, or 2-methyl-2-propyl in the IUPAC system); and
- with the substituent attached to a carbon which in turn is attached to the middle of a three carbon chain (called isobutyl in the old system, or 2-methyl-1-propyl in IUPAC).
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".
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