Inorganic chemistry, anallyl group is asubstituent with thestructural formula−CH2−HC=CH2. It consists of amethylene bridge (−CH2−) attached to avinyl group (−CH=CH2).[1][2] The name is derived from the scientific name forgarlic,Allium sativum. In 1844,Theodor Wertheim isolated an allyl derivative fromgarlic oil and named it "Schwefelallyl".[3][4] The term allyl applies to many compounds related toH2C=CH−CH2, some of which are of practical or of everyday importance, for example,allyl chloride.
Allylation is anychemical reaction that adds an allyl group to asubstrate.[1]
A site adjacent to the unsaturated carbon atom is called theallylic position orallylic site. A group attached at this site is sometimes described asallylic. Thus,CH2=CHCH2OH "has an allylichydroxyl group". AllylicC−H bonds are about 15% weaker than the C−H bonds in ordinarysp3 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 areallylic oxidations,ene reactions, and theTsuji–Trost reaction.Benzylic groups are related to allyl groups; both show enhanced reactivity.
ACH2 group connected to two vinyl groups is said to bedoubly allylic. Thebond 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 easyoxidation of compounds containing 1,4-pentadiene (C=C−CH2−C=C) linkages. Somepolyunsaturated fatty acids feature this pentadiene group:linoleic acid, α-linolenic acid, andarachidonic acid. They are susceptible to a range of reactions with oxygen (O2), starting withlipid peroxidation. Products include fatty acidhydroperoxides, epoxy-hydroxy polyunsaturated fatty acids,jasmonates,divinylether fatty acids, andleaf aldehydes. Some of these derivatives are signallng molecules, some are used in plant defense (antifeedants), some are precursors to other metabolites that are used by the plant.[5]
One practical consequence of their high reactivity is that polyunsaturated fatty acids have poor shelf life owing to their tendency towardautoxidation, leading, in the case of edibles, torancidification. 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 ofoil paints andvarnishes.
The termhomoallylic refers to the position on a carbon skeleton next to an allylic position. In but-3-enyl chlorideCH2=CHCH2CH2Cl, the chloride is homoallylic because it is bonded to the homoallylic site.
The allyl group is widely encountered in organic chemistry.[1] Allylicradicals,anions, andcations are often discussed asintermediates in reactions. All feature three contiguous sp²-hybridized carbon centers and all derive stability from resonance.[6] Each species can be presented by tworesonance structures with the charge or unpaired electron distributed at both 1,3 positions.
In terms ofMO theory, theMO diagram has three molecular orbitals: the first one bonding, the second one non-bonding, and the higher energy orbital is antibonding.[2]
This heightened reactivity of allylic groups has many practical consequences. Thesulfur vulcanization or various rubbers exploits the conversion of allylicCH2 groups intoCH−Sx−CH crosslinks. Similarlydrying oils such aslinseed oil crosslink via oxygenation of allylic (or doubly allylic) sites. This crosslinking underpins the properties of paints and the spoilage of foods byrancidification.
The industrial production ofacrylonitrile byammoxidation ofpropene exploits the easy oxidation of the allylic C−H centers:
An estimated 800,000 tonnes (1997) ofallyl chloride is produced by thechlorination ofpropylene:
It is the precursor toallyl alcohol andepichlorohydrin.
Allylation is the attachment of an allyl group to a substrate, usually another organic compound. Classically, allylation involves the reaction of acarbanion with allyl chloride. Alternatives includecarbonyl allylation with allylmetallic reagents, such asallyltrimethylsilane,[9][10][11] or the iridium-catalyzedKrische allylation.
Allylation can be effected also byconjugate addition: the addition of an allyl group to the beta-position of anenone. TheHosomi-Sakurai reaction is a common method for conjugate allylation.[12]
Allylic C-H bonds are susceptible to oxidation.[13] One commercial application ofallylic oxidation is the synthesis ofnootkatone, the fragrance ofgrapefruit, fromvalencene, a more abundantly availablesesquiterpenoid:[14]
In the synthesis of some fine chemicals,selenium dioxide is used to convert alkenes to allylic alcohols:[15]
where R, R', R" may bealkyl oraryl substituents.
From the industrial perspective, oxidation of benzylic C-H bonds are conducted on a particularly large scale, e.g. production ofterephthalic acid,benzoic acid, andcumene hydroperoxide.[16]
Many substituents can be attached to the allyl group to give stable compounds. Commercially important allyl compounds include:
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