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 stabilizing 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.

    Due to the stability of these allylic cations, they are readily formed as intermediates during chemical reactions, for example S_N1 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 stabilizing effect.

• Allylic bonds are often weaker and are 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 S_N1 or S_N2 pathways.

Summary:

• When treated with Br₂ or Cl₂, radical substitution of allylic C-H generates the allyl halide and HX.
• The process is very similar to that of alkanes (ch 4 review)
• Reaction proceeds via an radical chain mechanism (review) which involves radical intermediates. (review)
• The stability of the allylic radical (due to resonance) favours substitution at the allylic position.
• N-bromosuccinimide (NBS) can be used as an alternative source of Br₂.

• Halogenation of alkanes
• Halogenation of benzylic systems

RADICAL CHAIN MECHANISM FOR ALLYLIC BROMINATION
Step 1 (Initiation) Heat or uv light causes the weak halogen bond to undergo homolytic cleavage to generate two bromine radicals and starting the chain process.
Step 2 (Propagation) (a) A bromine radical abstracts a hydrogen to form HBr and an allyl radical, then (b) The allyl radical abstracts a bromine atom from another molecule of Br2 to form the allyl bromide product and another bromine radical,  which can then itself undergo reaction 2(a) creating a cycle that can repeat.
Step 3 (Termination) Various reactions between the possible pairs of radicals allow for the formation of  Br2 or the product, allyl bromide. These reactions remove radicals and do not perpetuate the cycle.

Substitution Reactions of Allyl Halides

• Allyl chlorides, bromides and iodides are good substrates for substitution reactions.
• A variety of nucleophiles can be used to generate a range of new functional groups.
• The process can be complicated by the allylic rearrangement where the nucleophile can attack either of the deficient sites.

Can you account for why the 3^o alcohol is the major product ?  
What does the fact that both the reactions shown give the same product mixture suggest ? 

• Substitution reactions of alkyl halides
• Substitution reactions of benzylic halides

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