thumb|180px|right|Structure of the allyl group

In organic chemistry, an allyl group is a substituent with the structural formula . It consists of a methylene bridge () attached to a vinyl group (). 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 that adds an allyl group to a substrate.

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 "drying oils", which are components of oil paints and varnishes.

thumb|none|200px|A representative [[triglyceride found in linseed oil features groups with both doubly allylic sites (<span style="color:green;">linoleic acid</span> and <span style="color:red;">alpha-linolenic acid</span>) and a singly allylic site (<span style="color:blue;">oleic acid</span>)]]

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.

thumb|left|540 px|The allylic, homoallylic and doubly allylic sites are highlighted in red

Bonding

The allyl group is widely encountered in organic chemistry. Allylic radicals, anions, and cations are often discussed as intermediates in reactions. All feature three contiguous sp²-hybridized carbon centers and all derive stability from resonance. Each species can be presented by two resonance structures with the charge or unpaired electron distributed at both 1,3 positions.

:thumb|none|320px|[[Resonance (chemistry)|Resonance structure 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.

:thumb|none|200 px|MO diagram for allyl π orbitals. In the radical (shown), the intermediate Ψ<sub>2</sub> orbital is singly occupied; in the cation, unoccupied; and in the anion, full.

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 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 by ammoxidation of propene exploits the easy oxidation of the allylic C−H centers:

:<chem>2CH3-CH=CH2 + 2 NH3 + 3 O2 -> 2CH2=CH-C#N + 6 H2O</chem>

An estimated 800,000 tonnes (1997) of allyl chloride is produced by the chlorination of propylene:

:<chem>CH3CH=CH2 + Cl2 -> ClCH2CH=CH2 + HCl</chem>

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 Hosomi-Sakurai reaction is a common method for conjugate allylation.

center|frameless|349x349px|insert a caption here

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:

thumb|center|322px|The conversion of valencene to nootkatone is an example of allylic oxidation.

In the synthesis of some fine chemicals, selenium dioxide is used to convert alkenes to allylic alcohols:

:R<sub>2</sub>C=CR'-CHR"<sub>2</sub> + [O] → R<sub>2</sub>C=CR'-C(OH)R"<sub>2</sub>

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 (CH<sub>3</sub>CH=CH−CH<sub>2</sub>OH)
  • Dimethylallyl pyrophosphate, central in the biosynthesis of terpenes, a precursor to many natural products, including natural rubber.
  • Transition-metal allyl complexes, such as allylpalladium chloride dimer

See also

  • Propargylic/Homopropargylic
  • Benzylic
  • Vinylic
  • Acetylenic
  • Allylic strain
  • Allylic rearrangement
  • Carroll rearrangement
  • Allylic palladium complex
  • Tsuji–Trost reaction
  • Naloxone

References