Butyl group
In
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: −CH2−CH2−CH2−CH3 (preferred IUPAC name: butyl)
- If it connects at one of the non-terminal (internal) carbon atoms, it is secondary butyl or sec-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 is isobutyl: −CH2−CH(CH3)2 (preferred IUPAC name: 2-methylpropyl)
- If it connects at the central carbon, it is tertiary butyl, tert-butyl or t-butyl: −C(CH3)3 (preferred IUPAC name: tert-butyl)
Nomenclature
According to
Skeletal formula of butyl (here connected to an R group) |
Common name | Preferred IUPAC name
|
Alternate notation | Fully systematic name | Symbol |
---|---|---|---|---|---|
n-butyl | butyl | butyl | butan-1-yl | Bu, n-Bu, nBu, nBu | |
sec-butyl | butan-2-yl | 1-methylpropyl | butan-2-yl | s-Bu, sBu, sBu | |
isobutyl, iso-butyl | 2-methylpropyl | 2-methylpropyl | 2-methylpropan-1-yl | i-Bu, iBu, iBu | |
tert-butyl | tert-butyl | 1,1-dimethylethyl | 2-methylpropan-2-yl | t-Bu, tBu, tBu |
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[citation needed]. 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" or "iso" means "isolated" 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, −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.
Example
The four isomers (ignoring
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 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 from
, etc.tert-Butyl effect
The tert-butyl
The tert-butyl effect is an example of steric hindrance.
tert-Butyl as protecting group
A tert-butyl (tBu) ether is an acid-labile protecting group for alcohols.[3]
Protection
A traditional way to introduce the tBu group to a hydroxyl group is by treating the compound with
Deprotection
Various acids can be used to cleave the tBu group, including both Brønsted acids such as trifluoroacetic acid and Lewis acids such as titanium tetrachloride.[3]
References
- ^ Harper, Douglas. "butane. Dictionary.com". Retrieved 9 Mar 2016.[permanent dead link]
- ^ ISBN 9780470053485.
- ISSN 0022-3263.
- ISSN 0369-8718.