In organic chemistry, butyl is a four-carbon alkyl radical or substituent group with general chemical formula , derived from either of the two isomers (n-butane and isobutane) of butane. The isomer n-butane can connect in two ways, giving rise to two "-butyl" groups: If it connects at one of the two terminal carbon atoms, it is normal butyl or n-butyl: (preferred IUPAC name: butyl) If it connects at one of the non-terminal (internal) carbon atoms, it is secondary butyl or sec-butyl: (preferred IUPAC name: butan-2-yl) The second isomer of butane, isobutane, can also connect in two ways, giving rise to two additional groups: If it connects at one of the three terminal carbons, it is isobutyl: (preferred IUPAC name: 2-methylpropyl) If it connects at the central carbon, it is tertiary butyl, tert-butyl or t-butyl: (preferred IUPAC name: tert-butyl) Nomenclature According to IUPAC nomenclature, "isobutyl", "sec-butyl", and "tert-butyl" used to be allowed retained names. The latest guidance changed that: only tert-butyl is kept as preferred prefix, all other butyl-names are removed. In the convention of skeletal formulas, every line ending and line intersection specifies a carbon atom (unless otherwise indicated) saturated with single-linked hydrogen atoms (unless otherwise indicated). The "R" symbol indicates any radical or other non-specific functional group. 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 (or carbons) connected to the first butyl carbon. The prefix "iso" (from "isomer") means "equal" while the prefix 'n-' stands for "normal". Butan-2-yl (sec-butyl) group is chiral. The carbon atom at position 2 is a stereocenter. It has four different groups attached: H-, CH3-, CH3-CH2-, and R-. The names of the two chiral groups are: (2S)-butan-2-yl and (2R)-butan-2-yl. Example The four isomers (ignoring stereoisomers) of "butyl acetate" demonstrate these four isomeric configurations. Here, the acetate radical appears in each of the positions where the "R" symbol is used in the chart above: sec-Butyl acetate is chiral, and has one stereocenter, and two enantiomers. The names of enantiomers are: [(2S)-butan-2-yl] acetate, (+)-sec-Butyl acetate [(2R)-butan-2-yl] acetate, (-)-sec-Butyl acetate Therefore, for butyl acetate, the total number of isomers is five, if stereoisomers are included. Etymology Alkyl radicals are often considered as