Butyl

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 in two ways, giving rise to two "-butyl" groups:
The second, branched isomer of butane, isobutane, can also connect in two ways, giving rise to more two groups: