Allyl



An allyl group is a substituent with the structural formula H2C=CH-CH2R, where R is the connection to the rest of the molecule. It is made up of a methylene (-CH2-), attached to a vinyl group (-CH=CH2). The name is derived from the Latin word for garlic, Allium sativum. Theodor Wertheim isolated an allyl derivative from garlic oil and named it Schwefelallyl. The term allyl applies to many compounds related to H2C=CH-CH2, which includes thousands of different chemical compounds, some of which are of practical or everyday importance.

Nomenclature and bonding
Allyl is a widely used term in organic chemistry. The unpaired electron is delocalized. Allylic radicals, anions, and cations are often discussed as intermediates in reactions. All feature three contiguous sp²-hybridized carbon centers.

Allylic sites
A site on the saturated carbon atom is called the "allylic position" or "allylic site." A group attached at this site is sometimes described as "allylic." Thus, CH2=CHundefinedCH2OH "has an allylic hydroxyl group." Allylic C-H bonds are about 15% weaker than the C-H bonds in ordinary sp3 carbon centers and are thus more reactive. As one practical consequence of this heightened reactivity, the production of acrylonitrile exploits the easy oxidation of the allylic C-H centers in propene. Benzylic and allylic are related in terms of structure, bond strength, and reactivity. Other reactions that tend to occur with allylic compounds are allylic oxidations, ene reactions, and the Tsuji–Trost reaction. Benzylic groups are related to allyl groups; both show enhanced reactivity.

Pentadienyl
A 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 allylic. The weakened C-H bonds reflect the high stability of the resulting "pentadienyl" radicals. Compounds containing the C=C-CH2-C=C linkages, e.g. linoleic acid derivatives, are prone to autoxidation, which can lead to polymerization or form semisolids. This reactivity pattern is fundamentally related to the film-forming behavior of the "drying oils" which are components of oil paints and varnishes.

Homoallylic
The term homoallylic refers to the position on a carbon skeleton next to an allylic position. In CH2=CHCH2CH2Cl, the chloride occupies a homoallylic position.

Allyl compounds
Many substituents can be attached to the allyl group to give stable compounds. Allyl alcohol has the structure H2C=CH-CH2OH. It contains two sp²-hybridized carbon centers and one sp3-hybridized carbon center. Another example of a simple allyl compound is allyl chloride. Substituted versions of the parent allyl group, such as the trans-but-2-en-1-yl or crotyl group (CH3CH=CH-CH2-), may also be referred to as allylic groups. Many allylic compounds are lachrymatory.

Allyl ligands in organometallic chemistry
The allyl ligand is commonly encountered in organometallic chemistry. Most commonly, allyl ligands bind to metals via all three carbon centers, the so-called η3-binding mode. η1-allyl complexes are also known. The most popular allyl reagent is probably allyl palladium chloride. They are usually generated by oxidative addition of allylic halides. This route affords pi-allyl nickel compounds, such as (allyl)2Ni2Cl2:
 * 2 Ni(CO)4 +  2 ClCH2CH=CH2   →   Ni2(μ-Cl)2(η3-C3H5)2  +  8 CO