An allyl group is three-carbon substituted propene, also called a propenyl group (prop-2-en-1-yl).
It has a structural formula CH2=CH-CH2-X, where the X can be an alkyl/aryl group or a functional group.
The -CH2- carbon linker connects alkene with the rest of the molecule, called the allylic carbon, and it is the most reactive position in a molecule, called the allylic position. It is a sp3 tetrahedral methylene carbon connected to a sp2 double bond alkene carbon.
The hydrogens attached to the allylic carbon are called allylic hydrogens.
Significance of the allylic position
The allyl group can accommodate and stabilize carbocation, carbanion, and free radical formation and, therefore, is a good reactive intermediate in many organic reactions. The allylic hydrogens are especially reactive in radical substitution reactions where the allyl hydrogen is replaced with a halogen, forming allyl halides.
The stabilization is due to the extended delocalization of the positive and negative charges and the radical on three more carbon atoms.
The delocalization is possible due to the conjugation of the pie bonds with the p-orbital of the carbocation, carbanion, and free radical. The planarity and the orbital overlap allow for the delocalization or the spread of the charges/radical onto several atoms.