Allylic Position

The so-called allylic position is located at the carbon next to a C=C double bond. Propene is the smallest molecule bearing an allylic position. In cyclohexene there are two equivalent allylic positions, flanking the C=C double bond.

C–H bond dissociation energies: allylic 87, tertiary 93, secondary 95 and primary 98 kcal/mol

Up to now, a tertiary position is the most advantageous one to locate either a positive charge or a radical. Not any more! The allylic position is even better. For instance, the strength of a C-H at the allylic position is lower because an allylic radical is less unstable.

Methanolysis of 1-chloropropane is extremely slow because methanol is a relatively poor nucleophile and the heterolytic cleavage of the C-Cl bond is not favored because a primary carbocation is obtained, the most unstable of them all. However, the very same reaction performed on 2-propenyl (allyl) chloride is much faster. Why is that?

Methanolysis of 1-chloropropane is very slow and that of allyl chloride, fast

Acidity of propane (K = 10^−50) and of the allylic position of propene (K = 10^−40)

Alkanes display an extraordinarily reduced acidity. The heterolytic cleavage of any alkane's C-H bond is very costly in energy terms. A very unstable carbanion has to be formed that way. Even though propene also bears a very small acidity, the allylic position is ten orders of magnitude more acidic. This clearly suggests that an allylic carbanion is less unstable than expected.

These very findings are clearly pointing to the fact that the production of either a radical, a cation or an anion in allylic position is less difficult than it could have been anticipated:
The neighbouring C=C double bond is able to stabilize by resonance the generated species. Let's write the corresponding resonance forms.

Resonance forms of the allyl radical, cation and anion: the charge or the electron is shared between the two end carbons

Geometry of propene and of the allyl cation, radical and anion: the two C–C bonds become equal (1.37-1.38 Å)

These resonant structures are indicating that the double C=C bond delocalizes the created species. A simple molecular orbital diagram enlightens us:

π molecular orbitals of the allyl system (bonding, non-bonding and antibonding) and their occupation in the cation, radical and anion

The three carbons of a allylic radical, carbocation or carbanion possess sp² hybridization. The "p" orbitals on each of the three carbon overlap giving rise to three molecular orbitals where the charge or radical are delocalized. This would not be possible without the C=C double bond.