Return to Contents Chapter 10: Conjugation in Alkadienes and Allylic Systems Ch 10 contents

Allylic systems

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






   





Allylic carbocations
 

Allyl cation resonance forms The π system of a double bond can stabilise an adjacent carbocation by donating electron density through resonance. Remember that delocalising charge is a stabilising effect. This stabilisation 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. electrostatic potential of allyl cation

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
 

allyl radical resonance forms The π system of a double bond can also stabilise an adjacent radical  through resonance. Remember that delocalising the radical is a stabilising effect. 
Due to the stability of these allylic radicals, they are readily formed as intermediates during chemical reactions, for example allylic halogenation.

Reactivity
both are primary C-H but the allylic C-H bond is weaker

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Dr. Ian Hunt, Department of Chemistry, University of Calgary