Editing Allyl group (section)

==Nomenclature==
[[File:Lipid peroxidation.svg|thumb|right|The [[Radical (chemistry)|free radical]] pathway for the first phase of the oxidative rancidification of fats]]

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, {{chem2|CH2\dCHCH2OH}} "has an allylic [[hydroxyl group]]".  Allylic [[Carbon–hydrogen bond|C−H bonds]] are about 15% weaker than the C−H bonds in ordinary [[Orbital hybridisation|sp<sup>3</sup>]] 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 [[allylic oxidation#Selenoxide oxidations|allylic oxidation]]s, [[ene reaction]]s, and the [[Tsuji–Trost reaction]].  [[Benzyl]]ic groups are related to allyl groups; both show enhanced reactivity.

===Pentadienyl group===
{{main|Pentadienyl group}}
A {{chem2|CH2}} 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]] ({{chem2|1=C=C\sCH2\sC\dC}}) linkages.  Some [[polyunsaturated fatty acid]]s feature this pentadiene group: [[linoleic acid]], α-[[linolenic acid]], and [[arachidonic acid]]. They are susceptible to a range of reactions with oxygen (O<sub>2</sub>), starting with  [[lipid peroxidation]]. Products include fatty acid [[hydroperoxide]]s, epoxy-hydroxy polyunsaturated fatty acids, [[jasmonate]]s, [[divinylether fatty acids]], and [[leaf aldehyde]]s.  Some of these derivatives are signallng molecules, some are used in plant defense ([[antifeedant]]s), some are precursors to other metabolites that are used by the plant.<ref name=ARPB>{{cite journal |doi=10.1146/annurev.arplant.53.100301.135248 |title=The Lipoxygenase Pathway |date=2002 |last1=Feussner |first1=Ivo |last2=Wasternack |first2=Claus |journal=Annual Review of Plant Biology |volume=53 |pages=275–297 |pmid=12221977 }}</ref>

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 oil]]s", which are components of [[oil paint]]s and [[varnish]]es.
[[File:Triglyceride unsaturated Structural Formulae V.1.png|thumb|none|200px|A representative [[triglyceride]] found in [[linseed oil]] features groups with both doubly allylic {{chem2|CH2}} sites ([[linoleic acid|<span style="color:green;">linoleic acid</span>]] and <span style="color:red;">alpha-linolenic acid</span>) and a singly allylic site ([[oleic acid|<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 {{chem2|CH2\dCHCH2CH2Cl}}, the chloride is homoallylic because it is bonded to the homoallylic site.
[[File:Allyl,homo,doubly.png|thumb|left|540 px|The allylic, homoallylic and doubly allylic sites are highlighted in red]]
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