Isobutyl
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 two additional groups:
According to IUPAC nomenclature, "isobutyl", "sec-butyl", and "tert-butyl" are all retained names. 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".
The four isomers 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:
Alkyl radicals are often considered as a series, a progression sequenced by the number of carbon atoms involved. In that progression, Butyl (containing 4 carbon atoms) is the fourth, and the last to be named for its history. The word "butyl" is derived from butyric acid, a four-carbon carboxylic acid found in rancid butter. The name "butyric acid" comes from Latin butyrum, butter. Subsequent alkyl radicals in the series are simply named from the Greek number that indicates the number of carbon atoms in the group: pentyl, hexyl, heptyl, etc.
...
Wikipedia