Allyl Cations, Anions and Radicals

Allyl Cations

Allyl cations are intermediates in SN1 reaction of allyl halides and in Electrophilic Addition to Dienes.  Its very important that you understand how to draw alternative resonance structures for ally cations, since this is how you will predict products.  On paper we would represent the allyl cation as follows.  Recall from our discussions of Resonance, that eac\h of these structures do not exist seprately, but rather the true structure is a combination (hybrid) of these two.

Combining these two structures gives us a hint at what this structure truly looks like.  Recall that this is called the resonance hybrid.  Notice the partial + charges (δ+) on carbons 1 and 3.  Each of these carbons is equally defieicent in electrons.  In reactions in which allylic cations are attacked by nucleophiles, each of these carbons would be attacked.

Allyl Radicals

Allylic radicals are observed in Allylic Bromination reactions and some others.  Again its critical that you are able to show the resonance structure for a given allyl radical. 

The allyl radical resonance hybrid is shown below.  In this case the radical electron is shared over carbons 1 and 3

Allyl Anions

Can you determine(sketch) the structure of the resonance hybrid for the allyl anion?

As discussed above allylic cations, radicals and anions are often intermediates in chemical reactions.  For example we will see allylic radicals as intermediates in Allylic Bromination reactions and allylic cations in the Electrophilic Addition to Dienes and in SN1 Reactions of Allylic Halides and Tosylates.