Inorganic chemistry,butyl is a four-carbonalkylradical orsubstituent group with generalchemical formula−C₄H₉, derived from either of the twoisomers (n-butane and isobutane) ofbutane.

The isomern-butane can connect in two ways, giving rise to two "-butyl" groups:

• If it connects at one of the two terminal carbonatoms, it isnormal butyl orn-butyl:−CH₂−CH₂−CH₂−CH₃ (preferred IUPAC name:butyl)
• If it connects at one of the non-terminal (internal) carbon atoms, it issecondary butyl orsec-butyl:−CH(CH₃)−CH₂−CH₃ (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 isisobutyl:−CH₂−CH(CH₃)₂ (preferred IUPAC name: 2-methylpropyl)
• If it connects at the central carbon, it istertiary butyl,tert-butyl ort-butyl:−C(CH₃)₃ (preferred IUPAC name:tert-butyl)

According toIUPAC nomenclature, "isobutyl", "sec-butyl", and "tert-butyl" used to be allowed retained names. The latest guidance changed that: onlytert-butyl is kept as preferred prefix, all other butyl-names are removed. In the convention ofskeletal formulas, every line ending and line intersection specifies a carbon atom (unless otherwise indicated) saturated with single-linkedhydrogen atoms (unless otherwise indicated). The "R" symbol indicates anyradical or other non-specific functional group.

Butyl is the largest substituent for whichtrivial 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 R^I or R-prime carbon. The prefixessec (from "secondary") andtert (from "tertiary") refer to the number of additionalside chains (or carbons) connected to the first butyl carbon. The prefix "iso" or "iso" means "isolated" while the prefix 'n-' stands for "normal".

Butan-2-yl (sec-butyl) group ischiral. The carbon atom at position 2 is astereocenter. It has four different groups attached:−H,−CH₃,−CH₂−CH₃, and−R (the R group is not equal to those three groups). The names of the two chiral groups are: (2S)-butan-2-yl and (2R)-butan-2-yl.

The four isomers (ignoringstereoisomers) 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 twoenantiomers. 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.

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 with preferred IUPAC name derived from its history. The word "butyl" is derived frombutyric acid, a four-carboncarboxylic acid found inrancidbutter. The name "butyric acid" comes fromLatinbutyrum,butter. Subsequent preferred IUPAC names for alkyl radicals in the series are simply named from theGreek number that indicates the number of carbon atoms in the group:pentyl,hexyl,heptyl, etc.

tert-Butyl group is special because it is bulky and it lacks alpha-H atoms. One way to assess the bulkiness oftert-Butyl uses the concept ofligand cone angle.^[1]

The bulkiness of thetert-butylsubstituent is used in chemistry forkinetic stabilization. The effect of thetert-butyl group oncyclization reactions is called theThorpe–Ingold effect.^[2]

Thetert-butyl effect is an example ofsteric hindrance.

tert-Butyl groups lack alpha-hydrogen atoms. This aspect stabilizesdi-tert-butyl chromate. Otherwise alpha-H's are abstracted by Cr(VI).^[3][4]

Atert-butyl (tBu) ether is an acid-labileprotecting group for alcohols.^[5]

A traditional way to introduce thetBu group to a hydroxyl group is by treating the compound withisobutylene in the presence of aBrønsted acid orLewis acid.^[6][7]

Various acids can be used to cleave thetBu group, including both Brønsted acids such astrifluoroacetic acid and Lewis acids such astitanium tetrachloride.^[5]