Editing Allyl group

{{Short description|1=Chemical group (–CH₂–CH=CH₂)}}
[[File:Allyl.svg|thumb|180px|right|Structure of the allyl group]]

In [[organic chemistry]], an '''allyl group''' is a [[substituent]] with the [[structural formula]] {{chem2|\sCH2\sHC\dCH2}}. It consists of a [[methylene bridge]] ({{chem2|\sCH2\s}}) attached to a [[vinyl group]] ({{chem2|\sCH\dCH2}}).<ref name=March /><ref name=boyd /> The name is derived from the scientific name for [[garlic]], {{lang|la|Allium sativum}}. In 1844, [[Theodor Wertheim]] isolated an allyl derivative from [[garlic oil]] and named it "{{lang|de|Schwefelallyl}}".<ref>{{Cite journal | doi = 10.1002/jlac.18440510302 | title = Untersuchung des Knoblauchöls | year = 1844 | author = Theodor Wertheim | journal = Annalen der Chemie und Pharmacie | volume = 51 | pages = 289–315 | issue = 3| url = https://zenodo.org/record/1426988 }}</ref><ref Name="Block">{{cite book|author=Eric Block|title=Garlic and Other Alliums: The Lore and the Science|url=https://books.google.com/books?id=6AB89RHV9ucC|publisher=Royal Society of Chemistry|year=2010|isbn=978-0-85404-190-9}}</ref> The term allyl applies to many compounds related to {{chem2|H2C\dCH\sCH2}}, 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 (chemistry)|substrate]].<ref name=March />

==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]]
{{clear}}

==Bonding==
The allyl group is widely encountered in organic chemistry.<ref name=March>Jerry March, "Advanced Organic Chemistry" 4th Ed.  J. Wiley and Sons, 1992: New York.  {{ISBN|0-471-60180-2}}.</ref> Allylic [[free radical|radicals]], [[Carbanion|anions]], and [[Carbocation|cations]] are often discussed as [[Reactive intermediate|intermediates in reactions]].  All feature three contiguous sp²-hybridized carbon centers and all derive stability from resonance.<ref name=McMurry>Organic Chemistry John McMurry 2nd ed. 1988</ref> Each species can be presented by two [[resonance structures]] with the charge or unpaired electron distributed at both 1,3 positions.

:[[File:Allyl anion.svg|thumb|none|320px|[[Resonance (chemistry)|Resonance structure]] of the allyl anion.  The cation is identical, but carries an opposite-sign charge.<ref>{{cite book|pages=56–57|chapter=The properties of alkene carbonium ions and carbanions|first=Herman&nbsp;G.|last=Richey|title=The Chemistry of Alkenes|volume=2|series=The Chemistry of Functional Groups|editor-first=Jacob|editor-last=Zabicky|year=1970|publisher=Interscience / William Clowes&nbsp;& Sons|location=London|lccn=64-25218|isbn=0471980501}}</ref>]]

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.<ref name=boyd>{{cite book|title = Organic Chemistry |edition = 4th |last1= Morrison|first1 = Robert Thornton|last2= Boyd|first2= Robert Neilson|date =  1987|publisher = Allyn and Bacon}}</ref>

:[[File:AllylMO.png|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.]]

<ref>{{cite journal |doi=10.1021/acs.chemrev.0c00157|title=Carbon–Carbon Bond Cleavage at Allylic Positions: Retro-allylation and Deallylation |year=2021 |last1=Nogi |first1=Keisuke |last2=Yorimitsu |first2=Hideki |journal=Chemical Reviews |volume=121 |issue=1 |pages=345–364 |pmid=32396335 |s2cid=218617434 }}</ref>

==Reactions and applications==
{{See also|Allylic substitution}}
This heightened reactivity of allylic groups has many practical consequences. The [[sulfur vulcanization]] or various rubbers exploits the conversion of allylic {{chem2|CH2}} groups into {{chem2|CH\sS_{x}\sCH}} crosslinks.  Similarly [[drying oil]]s 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 [[Halogenation|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]],<ref>{{cite journal |doi=10.1021/cr400008h|title=Diastereoselective Allylation of Carbonyl Compounds and Imines: Application to the Synthesis of Natural Products |year=2013 |last1=Yus |first1=Miguel |last2=González-Gómez |first2=José C. |last3=Foubelo |first3=Francisco |journal=Chemical Reviews |volume=113 |issue=7 |pages=5595–5698 |pmid=23540914 |hdl=10045/38276 |hdl-access=free }}</ref><ref>{{cite journal |doi=10.1021/cr1002744|title=Transition Metal-Catalyzed Decarboxylative Allylation and Benzylation Reactions |year=2011 |last1=Weaver |first1=Jimmie D. |last2=Recio |first2=Antonio |last3=Grenning |first3=Alexander J. |last4=Tunge |first4=Jon A. |journal=Chemical Reviews |volume=111 |issue=3 |pages=1846–1913 |pmid=21235271 |pmc=3116714 }}</ref><ref>{{cite journal |doi=10.1021/cr1004474|title=Catalytic Enantioselective Allylation of Carbonyl Compounds and Imines |year=2011 |last1=Yus |first1=Miguel |last2=González-Gómez |first2=José C. |last3=Foubelo |first3=Francisco |journal=Chemical Reviews |volume=111 |issue=12 |pages=7774–7854 |pmid=21923136 }}</ref> 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 [[Alpha-beta Unsaturated carbonyl compounds|enone]]. The [[Sakurai reaction|Hosomi-Sakurai reaction]] is a common method for conjugate allylation.<ref>{{cite journal |doi=10.15227/orgsyn.062.0086|title=Conjugate Allylation of α,β-Unsaturated Ketones with Allylsilanes: 4-Phenyl-6-Hepten-2-one |journal=Organic Syntheses |year=1984 |volume=62 |page=86|author1=Sakurai Hideki|author2=Hosomi Akira|author3=Hayashi Josabro}}</ref>
[[File:Carbonyl Allylation Scheme 4.png|center|frameless|349x349px|insert a caption here]]

===Oxidation===
Allylic C-H bonds are susceptible to oxidation.<ref>{{cite journal |doi=10.1055/s-0033-1338491 |title=Allylic Oxidations of Olefins to Enones |date=2013 |last1=Maison |first1=Wolfgang |last2=Weidmann |first2=Verena |journal=Synthesis |volume=45 |issue=16 |pages=2201–2221 |s2cid=196767407 }}</ref>  One commercial application of '''allylic oxidation''' is the synthesis of [[nootkatone]], the fragrance of [[grapefruit]], from [[valencene]], a more abundantly available [[sesquiterpenoid]]:<ref>{{cite journal |doi=10.1038/nature17431 |title=Scalable and sustainable electrochemical allylic C–H oxidation |date=2016 |last1=Horn |first1=Evan J. |last2=Rosen |first2=Brandon R. |last3=Chen |first3=Yong |last4=Tang |first4=Jiaze |last5=Chen |first5=Ke |last6=Eastgate |first6=Martin D. |last7=Baran |first7=Phil S. |journal=Nature |volume=533 |issue=7601 |pages=77–81 |pmid=27096371 |pmc=4860034 |bibcode=2016Natur.533...77H }}</ref>  
[[File:ValenceneToNootkatone.svg|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:<ref>{{cite book |doi=10.1002/047084289X.rs008.pub3 |chapter=Selenium(IV) Oxide |title=Encyclopedia of Reagents for Organic Synthesis |date=2017 |last1=Hoekstra |first1=William J. |last2=Fairlamb |first2=Ian J. S. |last3=Giroux |first3=Simon |last4=Chen |first4=Yuzhong |pages=1–12 |isbn=978-0-470-84289-8 }}</ref>
: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]].<ref>{{cite journal |doi=10.1021/cr040170k |title=Free Radical Functionalization of Organic Compounds Catalyzed by ''N-'' Hydroxyphthalimide |date=2007 |last1=Recupero |first1=Francesco |last2=Punta |first2=Carlo |journal=Chemical Reviews |volume=107 |issue=9 |pages=3800–3842 |pmid=17848093 }}</ref>

==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 [[terpene]]s, a precursor to many natural products, including [[natural rubber]].
*[[Transition-metal allyl complex]]es, such as [[allylpalladium chloride dimer]]

==See also==
{{wikiquote}}

* [[Allylic strain]]
* [[Carroll rearrangement]]
* [[Organopalladium#Palladium allyl complexes|Allylic palladium complex]]
* [[Tsuji–Trost reaction]]
* [[Propargylic|Propargylic/Homopropargylic]]
* [[Benzylic#Benzylic position|Benzylic]]
* [[Vinylic]]
* [[Acetylenic]]
* [[Naloxone]]
* [[Allylic rearrangement]]

==References==
{{Reflist|3}}

{{Functional group}}
{{Authority control}}

[[Category:Alkenyl groups]]
[[Category:Allyl compounds]]