In organic chemistry, an allyl group is a substituent with the structural formula . It consists of a methylene bridge () attached to a vinyl group ().
Allylation is any chemical reaction 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". Allylic C−H bonds are about 15% weaker than the C−H bonds in ordinary sp
3 carbon centers and are thus more reactive.
Benzylic and allylic are related in terms of structure, bond strength, and reactivity. Other reactions that tend to occur with allylic compounds are selenoxide oxidations, , and the Tsuji–Trost reaction. 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 singly allylic. The weakened C−H bonds is reflected in the easy
oxidation of compounds containing 1,4-
pentadiene () linkages. Some polyunsaturated fatty acids feature this pentadiene group:
linoleic acid, α-
linolenic acid, and
arachidonic acid. They are susceptible to a range of reactions with oxygen (O
2), starting with lipid peroxidation. Products include fatty acid
, epoxy-hydroxy polyunsaturated fatty acids,
, divinylether fatty acids, and
. Some of these derivatives are signallng molecules, some are used in plant defense (
), some are precursors to other metabolites that are used by the plant.
One practical consequence of their high reactivity is that polyunsaturated fatty acids have poor shelf life owing to their tendency toward autoxidation, leading, in the case of edibles, to rancidification. Metals accelerate the degradation. These fats tend to polymerize, forming semisolids. This reactivity pattern is fundamental to the film-forming behavior of the "", which are components of and .
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
free radical,
Carbanion, and
Carbocation 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.
- of the allyl anion. The cation is identical, but carries an opposite-sign charge.
In terms of MO theory, the MO diagram has three molecular orbitals: the first one bonding, the second one non-bonding, and the higher energy orbital is antibonding.
Reactions and applications
This heightened reactivity of allylic groups has many practical consequences. The sulfur vulcanization or various rubbers exploits the conversion of allylic groups into crosslinks. Similarly
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 by ammoxidation of propene 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 Halogenation of propylene:
- CH3CH=CH2 + Cl2 -> ClCH2CH=CH2 + HCl
It is the precursor to
allyl alcohol and
epichlorohydrin.
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. Alternatives include carbonyl allylation with allylmetallic reagents, such as allyltrimethylsilane,
or the iridium-catalyzed Krische allylation.
Allylation can be effected also by conjugate addition: the addition of an allyl group to the beta-position of an enone. The Sakurai reaction is a common method for conjugate allylation.
In other cases, compounds undergo retro-allylation, cleaving carbon-carbon bonds.
Oxidation
Allylic C-H bonds are susceptible to oxidation.
One commercial application of
allylic oxidation is the synthesis of
nootkatone, the fragrance of
grapefruit, from
valencene, a more abundantly available
sesquiterpenoid:
In the synthesis of some fine chemicals, selenium dioxide is used to convert alkenes to allylic alcohols:
- R2C=CR'-CHR"2 + O → R2C=CR'-C(OH)R"2
where R, R', R" may be
alkyl or
aryl substituents.
From the industrial perspective, oxidation of benzylic C-H bonds are conducted on a particularly large scale, e.g. production of terephthalic acid, benzoic acid, and cumene hydroperoxide.
Allyl compounds
Many substituents can be attached to the allyl group to give stable compounds. Commercially important allyl compounds include:
-
Crotyl alcohol (CH3CH=CH−CH2OH)
-
Dimethylallyl pyrophosphate, central in the biosynthesis of , a precursor to many natural products, including natural rubber.
-
Transition-metal allyl complexes, such as allylpalladium chloride dimer
See also
