Chapter 10: Conjugation in Alkadienes and Allylic Systems

Allylic systems

    The positions adjacent to C=C often show enhanced reactivity compared to simple alkanes due to the proximity of the adjacent p system.  Such positions are referred as "allylic". Recall that the term "vinylic" is used to described the atoms directly associated with the C=C unit.

Allylic carbocations
 

The p system of a double bond can stabilize an adjacent carbocation by donating electron density through resonance. Remember that delocalising charge is a stabilising effect. This stabilization is equivalent to that of two alkyl groups, so the allyl cation has similar stability to the 2-propyl cation.

Note that in the two resonance forms of the allylic cation, the positive charge is located on the terminal carbon atoms and never on the middle carbon.  This is reflected in the resonance hybrid and the positive areas of the electrostatic potential shown to the right (blue). Note that either of the carbons with +ve charge could be attacked by a nucleophile.

    Due to the stability of these allylic cations, they are readily formed as intermediates during chemical reactions, for example SN1 reactions of allylic halides.

Allylic radicals
 

The p system of a double bond can also stabilize an adjacent radical  through resonance. Remember that delocalising the radical is a stabilising effect.

• Allylic bonds are often weaker and more easily broken, for example compare the bond dissociation energies:

• The stability of the allylic radical can be utilised in the preparation of allylic halides (esp. -Cl and -Br)
• Allylic halides readily undergo substitution reactions via either SN1 or SN2 pathways.