In organic chemistry, butyl is a four-carbon alkyl radical or substituent group with general chemical formula -C4H9, derived from either of the two isomers of butane.
The isomer n-butane can connect either at one of the two terminal carbon atoms or at one of the two internal carbon atoms, giving rise to two "-butyl" groups:
The second, branched isomer of butane, isobutyl, can connect either at one of the three terminal carbons or at the central carbon, giving rise to another two groups:
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According to IUPAC nomenclature, "isobutyl", "sec-butyl", and "tert-butyl" are all retained trivial names.
| Skeletal formula | Common name | IUPAC name | Systematic name | Alternate notation |
|---|---|---|---|---|
| n-butyl | butyl | butyl | butan-1-yl | |
| isobutyl | isobutyl | 2-methylpropyl | 2-methylpropan-1-yl | |
| sec-butyl | sec-butyl | 1-methylpropyl | butan-2-yl | |
| tert-butyl | tert-butyl | 1,1-dimethylethyl | 2-methylpropan-2-yl |
Butyl is the largest substituent for which trivial names are commonly used for all isomers.
The butyl group's carbon that is connected to the rest (R) of the molecule is called the RI or R-prime carbon . The prefixes sec (from "secondary") and tert (from "tertiary") refer to the number of additional side chains connected to the first butyl carbon. The prefix "iso" (from "isomer") means "equal" while the prefix 'n-' stands for "normal".
The following are the four isomers of "butyl acetate":
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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.
The tert-butyl substituent is very bulky and used in chemistry for kinetic stabilisation together with other bulky groups such as the related trimethylsilyl group. The effect that the t-butyl group exerts on the progress of a chemical reaction is called the tert-butyl effect.
This effect is illustrated in the Diels-Alder reaction below, where the tert-butyl substituent causes a reaction rate acceleration by a factor of 240 compared to hydrogen as the substituent.[1]