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Allyl group: Wikis

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Structure of the allyl group.

An allyl group is a hydrocarbon group with the structural formula H2C=CH-CH2. It is made up of a vinyl group, CH2=CH, attached to methylene CH2. The name derived from the Latin word for garlic, Allium sativum. Theodor Wertheim isolated an allyl derivative from garlic oil and named it Schwefelallyl.[1]

Contents

Nomenclature and bonding

Allyl is a widely used term in organic chemistry.[2] Allyl itself is a relatively stable free radical with the formula C3H5. It contains three sp²-hybridized carbon centers. The unpaired electron is delocalized. Allylic anions and cations are often discussed as intermediates in reactions.

MO diagram for pi-orbitals on allyl. The middle MO labeled Ψ2 is singly occupied in the allyl radical. In the allyl cation Ψ2 is unoccupied and in the allyl anion it is doubly occupied. Hydrogen atoms are omitted from this picture.

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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=CH2CH2OH "has an allylic hydroxyl group." Allylic C-H bonds are about 20% 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 Trost asymmetric allylic alkylation.

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 lacrymatory.

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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:[3]

2 Ni(CO)4 + 2 ClCH2CH=CH2 → Ni2(μ-Cl)23-C3H5) 2 + 8 CO

References

  1. ^ Wertheim, Theodor (1844). "Untersuchung des Knoblauchöls". Annalen der Chemie und Pharmacie 51: 289. doi:10.1002/jlac.18440510302.  
  2. ^ March, J. “Advanced Organic Chemistry” $th Ed. J. Wiley and Sons, 1992: New York. ISBN 0-471-60180-2.
  3. ^ Martin F. Semmelhack and Paul M. Helquist (1988), "Reaction of Aryl Halides with π-Allylnickel Halides: Methallylbenzene", Org. Synth., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv6p0722  ; Coll. Vol. 6: 722  

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