Butyl group

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Isobutyl
)

In

alkyl radical or substituent group with general chemical formula −C4H9, 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: −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

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.

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

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:

butyl acetate sec-butyl acetate isobutyl acetate tert-butyl acetate

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

pentyl, hexyl, heptyl
, etc.

tert-Butyl effect

The tert-butyl

Diels-Alder reaction below. Compared to a hydrogen substituent, the tert-butyl substituent accelerates the reaction rate by a factor of 240.[2]

tert-Butyl effect
tert-Butyl effect

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

Lewis acid.[4][5]

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

  1. ^ Harper, Douglas. "butane. Dictionary.com". Retrieved 9 Mar 2016.[permanent dead link]
  2. ^ .
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