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In
organic chemistry Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, ...
, butyl is a four-
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...
alkyl In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl is derived from a cycloa ...
radical or substituent group with general
chemical formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbol ...
, 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
atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, ...
s, 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 A chemical nomenclature is a set of rules to generate systematic names for chemical compounds. The nomenclature used most frequently worldwide is the one created and developed by the International Union of Pure and Applied Chemistry (IUPAC). T ...
, "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 formula The skeletal formula, or line-angle formula or shorthand formula, of an organic compound is a type of molecular structural formula that serves as a shorthand representation of a molecule's bonding and some details of its molecular geometry. A ...
s, every line ending and line intersection specifies a carbon atom (unless otherwise indicated) saturated with single-linked
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
atoms (unless otherwise indicated). The "R" symbol indicates any radical or other non-specific functional group. Butyl is the largest substituent for which
trivial name In chemistry, a trivial name is a non systematic name for a chemical substance. That is, the name is not recognized according to the rules of any formal system of chemical nomenclature such as IUPAC inorganic or IUPAC organic nomenclature. A ...
s 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 chain In organic chemistry and biochemistry, a side chain is a chemical group that is attached to a core part of the molecule called the "main chain" or backbone. The side chain is a hydrocarbon branching element of a molecule that is attached to a ...
s (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 Chirality is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is distinguishable from i ...
. The carbon atom at position 2 is a
stereocenter In stereochemistry, a stereocenter of a molecule is an atom (center), axis or plane that is the focus of stereoisomerism; that is, when having at least three different groups bound to the stereocenter, interchanging any two different groups c ...
. It has four different groups attached: H-, CH3-, CH3-CH2-, and R-. The names of the two chiral groups are: (2''S'')-butan-2-yl and (2''R'')-butan-2-yl.


Example

The four isomers (ignoring
stereoisomer In stereochemistry, stereoisomerism, or spatial isomerism, is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in ...
s) of "
butyl acetate ''n''-Butyl acetate is an organic compound with the formula . A colorless, flammable liquid, it is the ester derived from n-butanol and acetic acid. It is found in many types of fruit, where it imparts characteristic flavors and has a sweet smel ...
" 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: * 2''S'')-butan-2-ylacetate, (+)-''sec''-Butyl acetate * 2''R'')-butan-2-ylacetate, (-)-''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 butyric acid, a four-carbon carboxylic acid found in rancid
butter Butter is a dairy product made from the fat and protein components of churned cream. It is a semi-solid emulsion at room temperature, consisting of approximately 80% butterfat. It is used at room temperature as a spread, melted as a condimen ...
. The name "butyric acid" comes from
Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...
''butyrum'', butter. Subsequent preferred IUPAC names for alkyl radicals in the series are simply named from the
Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
number that indicates the number of carbon atoms in the group:
pentyl Pentyl is a five-carbon alkyl group or substituent with chemical formula -C5H11. It is the substituent form of the alkane pentane. In older literature, the common non-systematic name amyl was often used for the pentyl group. Conversely, the name ...
,
hexyl In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl is derived from a cycloalk ...
,
heptyl In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl is derived from a cycloalk ...
, etc.


''tert''-Butyl effect

The ''tert''-butyl substituent is very bulky and is used in chemistry for kinetic stabilization, as are other bulky groups such as the related
trimethylsilyl A trimethylsilyl group (abbreviated TMS) is a functional group in organic chemistry. This group consists of three methyl groups bonded to a silicon atom minus;Si(CH3)3 which is in turn bonded to the rest of a molecule. This structural group is ch ...
group. The effect of the ''tert''-butyl group on the progress of a chemical reaction is called the
Thorpe–Ingold effect The Thorpe–Ingold effect, gem-dimethyl effect, or angle compression is an effect observed in chemistry where increasing steric hindrance favours ring closure and intramolecular reactions. The effect was first reported by Beesley, Thorpe, and In ...
illustrated in the Diels-Alder reaction below. Compared to a hydrogen substituent, the ''tert''-butyl substituent accelerates the reaction rate by a factor of 240. The ''tert''-butyl effect is an example of
steric hindrance Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape ( conformation) and reactivity of ions ...
.


Tert-butyl as Protecting group

Tert-butyl (''t''Bu) ether is an acid-labile protecting group for alcohols.


Protection & formation

A traditional way to introduce ''t''Bu group to hydroxyl group is by treating the compound with
isobutylene Isobutylene (or 2-methylpropene) is a hydrocarbon with the chemical formula . It is a four-carbon branched alkene (olefin), one of the four isomers of butylene. It is a colorless flammable gas, and is of considerable industrial value. Producti ...
in the presence of Bronsted acid or Lewis acid.


Deprotection & Cleavage


References

{{Functional group Alkyl groups