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, J ...
, an allyl group is a
substituent
A substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. (In organic chemistry and biochemistry, the terms ''substituent'' and ''functional group'', as well as '' side ...
with the
structural formula
The structural formula of a chemical compound is a graphic representation of the molecular structure (determined by structural chemistry methods), showing how the atoms are possibly arranged in the real three-dimensional space. The chemical bond ...
, where R is the rest of the molecule. It consists of a
methylene bridge () attached to a
vinyl group ().
The name is derived from the scientific name for
garlic
Garlic (''Allium sativum'') is a species of bulbous flowering plant in the genus '' Allium''. Its close relatives include the onion, shallot, leek, chive, Welsh onion and Chinese onion. It is native to South Asia, Central Asia and northeas ...
, . In 1844,
Theodor Wertheim isolated an allyl derivative from
garlic oil and named it "".
The term allyl applies to many compounds related to , some of which are of practical or of everyday importance, for example,
allyl chloride.
Allylation is any
chemical reaction
A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking ...
that adds an allyl group to a
substrate.
Nomenclature
A site adjacent to the unsaturated carbon atom is called the allylic position or allylic site. A group attached at this site is sometimes described as allylic. Thus, "has an allylic
hydroxyl group
In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...
". Allylic
C−H bonds are about 15% weaker than the C−H bonds in ordinary
sp3 carbon centers and are thus more reactive.
Benzylic
In organic chemistry, benzyl is the substituent or molecular fragment possessing the structure . Benzyl features a benzene ring () attached to a methylene group () group.
Nomenclature
In IUPAC nomenclature, the prefix benzyl refers to a substi ...
and allylic are related in terms of structure, bond strength, and reactivity. Other reactions that tend to occur with allylic compounds are
allylic oxidation
Organoselenium compounds (or seleno-organic) are chemical compounds containing carbon-to- selenium chemical bonds. Organoselenium chemistry is the corresponding science exploring their properties and reactivity. Selenium belongs with oxygen and su ...
s,
ene reactions, and the
Tsuji–Trost reaction
The Tsuji–Trost reaction (also called the Trost allylic alkylation or allylic alkylation) is a palladium-catalysed substitution reaction involving a substrate that contains a leaving group in an allylic position. The palladium catalyst first co ...
.
Benzylic groups are related to allyl groups; both show enhanced reactivity.
Pentadienyl group
A group connected to two vinyl groups is said to be doubly allylic. The
bond dissociation energy of C−H bonds on a doubly allylic centre is about 10% less than the bond dissociation energy of a C−H bond that is allylic. The weakened C−H bonds reflect the high stability of the resulting
pentadienyl radicals. Compounds containing the linkages, e.g.
linoleic acid
Linoleic acid (LA) is an organic compound with the formula COOH(CH2)7CH=CHCH2CH=CH(CH2)4CH3. Both alkene groups are ''cis''. It is a fatty acid sometimes denoted 18:2 (n-6) or 18:2 ''cis''-9,12. A linoleate is a salt or ester of this acid.
...
derivatives, are prone to autoxidation, which can lead to polymerization or form semisolids. This reactivity pattern is fundamental to the film-forming behavior of the "
drying oils", which are components of
oil paint
Oil paint is a type of slow-drying paint that consists of particles of pigment suspended in a drying oil, commonly linseed oil. The viscosity of the paint may be modified by the addition of a solvent such as turpentine or white spirit, and ...
s and
varnishes.
Homoallylic
The term homoallylic refers to the position on a carbon skeleton next to an allylic position. In but-3-enyl chloride , the chloride is homoallylic because it is bonded to the homoallylic site.
Bonding
The allyl group is widely encountered in organic chemistry.
[Jerry March, "Advanced Organic Chemistry" 4th Ed. J. Wiley and Sons, 1992: New York. .] Allylic
radicals
Radical may refer to:
Politics and ideology Politics
*Radical politics, the political intent of fundamental societal change
*Radicalism (historical), the Radical Movement that began in late 18th century Britain and spread to continental Europe and ...
,
anions
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by con ...
, and
cations are often discussed as
intermediates in reactions. All feature three contiguous sp²-hybridized carbon centers and all derive stability from resonance.
[Organic Chemistry John McMurry 2nd ed. 1988] Each species can be presented by two
resonance structures with the charge or unpaired electron distributed at both 1,3 positions.
:
In terms of
MO theory
In chemistry, molecular orbital theory (MO theory or MOT) is a method for describing the electronic structure of molecules using quantum mechanics. It was proposed early in the 20th century.
In molecular orbital theory, electrons in a molecule ...
, the
MO diagram
Mo or MO may refer to:
Arts and entertainment Fictional characters
* Mo, a girl in the '' Horrible Histories'' TV series
* Mo, also known as Mortimer, in the novel ''Inkheart'' by Cornelia Funke
* Mo, in the webcomic ''Jesus and Mo''
* Mo, the ...
has three molecular orbitals: the first one bonding, the second one non-bonding, and the higher energy orbital is antibonding.
[Organic Chemistry 4th Ed. Morisson & Boyd 1988.]
:
Reactions and applications
This heightened reactivity of allylic groups has many practical consequences. The
sulfur vulcanization
Sulfur vulcanization is a chemical process for converting natural rubber or related polymers into materials of varying hardness, elasticity, and mechanical durability by heating them with sulfur or sulfur-containing compounds. Sulfur forms cr ...
or various rubbers exploits the conversion of allylic groups into crosslinks. Similarly
drying oils such as
linseed oil crosslink via oxygenation of allylic (or doubly allylic) sites. This crosslinking underpins the properties of paints and the spoilage of foods by
rancidification.
The industrial production of
acrylonitrile
Acrylonitrile is an organic compound with the formula and the structure . It is a colorless, volatile liquid although commercial samples can be yellow due to impurities. It has a pungent odor of garlic or onions. In terms of its molecula ...
by
ammoxidation of
propene
Propylene, also known as propene, is an unsaturated organic compound with the chemical formula CH3CH=CH2. It has one double bond, and is the second simplest member of the alkene class of hydrocarbons. It is a colorless gas with a faint petrol ...
exploits the easy oxidation of the allylic C−H centers:
:
2CH3-CH=CH2 + 2 NH3 + 3 O2 -> 2CH2=CH-C#N + 6 H2O
An estimated 800,000 tonnes (1997) of
allyl chloride is produced by the
chlorination of
propylene
Propylene, also known as propene, is an unsaturated organic compound with the chemical formula CH3CH=CH2. It has one double bond, and is the second simplest member of the alkene class of hydrocarbons. It is a colorless gas with a faint petro ...
:
:
CH3CH=CH2 + Cl2 -> ClCH2CH=CH2 + HCl
It is the precursor to
allyl alcohol and
epichlorohydrin
Epichlorohydrin (abbreviated ECH) is an organochlorine compound and an epoxide. Despite its name, it is not a halohydrin. It is a colorless liquid with a pungent, garlic-like odor, moderately soluble in water, but miscible with most polar organ ...
.
Allylation
Allylation is the attachment of an allyl group to a substrate, usually another organic compound. Classically, allylation involves the reaction of a
carbanion with allyl chloride. Another well-developed process involves addition of
allyltrimethylsilane
Allyltrimethylsilane is the organosilicon compound with the formula (CH3)3SiCH2CH=CH2. The molecule consists of the trimethylsilyl group attached to allyl group. This colorless liquid is used in organic synthesis
Organic synthesis is a spec ...
to carbonyls, i.e.
carbonyl allylation In organic chemistry, carbonyl allylation describes methods for adding an allyl anion to an aldehyde or ketone to produce a homoallylic alcohol. The carbonyl allylation was first reported in 1876 by Alexander Zaitsev and employed an allylzinc reage ...
.
Allylation can be effected also by
conjugate addition: the addition of an allyl group to the beta-position of an
enone. The
Hosomi-Sakurai reaction is a common method for conjugate allylation.
Allyl compounds
Many substituents can be attached to the allyl group to give stable compounds. Commercially important allyl compounds include:
*
Crotyl alcohol (CH
3CH=CH−CH
2OH)
*
Dimethylallyl pyrophosphate
Dimethylallyl pyrophosphate (DMAPP; or alternatively, dimethylallyl diphosphate (DMADP); also isoprenyl pyrophosphate) is an isoprenoid precursor. It is a product of both the mevalonate pathway and the MEP pathway of isoprenoid precursor biosynt ...
, central in the biosynthesis of
terpene
Terpenes () are a class of natural products consisting of compounds with the formula (C5H8)n for n > 1. Comprising more than 30,000 compounds, these unsaturated hydrocarbons are produced predominantly by plants, particularly conifers. Terpenes ...
s, a precursor to many natural products, including
natural rubber.
*
Transition-metal allyl complexes, such as
allylpalladium chloride dimer
See also
*
Allylic strain
*
Carroll rearrangement The Carroll rearrangement is a rearrangement reaction in organic chemistry and involves the transformation of a β- keto allyl ester into a α-allyl-β-ketocarboxylic acid. This organic reaction is accompanied by decarboxylation and the final produc ...
*
Allylic palladium complex
*
Tsuji–Trost reaction
The Tsuji–Trost reaction (also called the Trost allylic alkylation or allylic alkylation) is a palladium-catalysed substitution reaction involving a substrate that contains a leaving group in an allylic position. The palladium catalyst first co ...
*
Propargylic/Homopropargylic
*
Benzylic
In organic chemistry, benzyl is the substituent or molecular fragment possessing the structure . Benzyl features a benzene ring () attached to a methylene group () group.
Nomenclature
In IUPAC nomenclature, the prefix benzyl refers to a substi ...
*
Vinylic
*
Acetylenic
*
Naloxone
Naloxone, sold under the brand names Narcan (4 mg) and Kloxxado (8 mg) among others, is a medication used to reverse or reduce the effects of opioids. It is commonly used to counter decreased breathing in opioid overdose. Effects begin withi ...
*
Allylic rearrangement An allylic rearrangement or allylic shift is an organic reaction in which the double bond in an allyl chemical compound shifts to the next carbon atom. It is encountered in nucleophilic substitution.
In reaction conditions that favor a SN1 react ...
References
{{Authority control
Alkenyl groups
Allyl compounds