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In organic chemistry, butyl is a four-carbon alkyl substituent with chemical formula -C₄H₉ . It is derived from either of the two isomers of the alkane called butane.

Each of the two isomers of butane give rise to two isomers of the butyl substituent. Thus, n-butane can connect at either the terminal or an internal carbon atoms, giving rise to "n-butyl" and "sec-butyl" substituents.

• n-butyl group; CH₃CH₂CH₂CH₂–, (fully systematic name "butyl")
• sec-butyl group; CH₃CH₂(CH₃)CH–, (fully systematic name "1-methylpropyl")

The second, branched isomer of butane, isobutane can also connect either terminal methyl or internal carbon atoms, giving rise to "isobutyl" and "tertiary butyl" substituents, respectively.

• isobutyl group; (CH₃)₂CHCH₂–, (fully systematic name "2-methylpropyl")
• tert-butyl group; (CH₃)₃C–, (fully systematic name "1,1-dimethylethyl")

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 prefixes iso, sec and tert refer to the number of carbons connected to the primary carbon (also known as R^I ("R prime"), the carbon that is connected to R). Iso means one, sec- means two and tert- means three.

The following are the four isomers of "butyl acetate":

n-butyl acetate	isobutyl acetate	sec-butyl acetate	tert-butyl acetate

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]

• methyl
• ethyl
• propyl
• pentyl / amyl