As we saw in chapter 10, the positions adjacent to C=C, the allylic position, often show enhanced reactivity compared to simple alkanes due to the proximity of the adjacent p system.  Similarly, the positions adjacent to a benzene ring, known as the benzylic position also show enhanced reactivity compared to simple alkanes.
Students often confuse the term benzyl with phenyl, for example compare the location of the bromine atoms in Bromobenzene and benzyl bromide:
 
 

Benzylic carbocations
 

The p system of a benzene ring can stabilize an adjacent carbocation by donating electron density through resonance. Remember that delocalising charge is a stabilizing effect.

    Note that in the resonance forms of the benzylic cation, the positive charge is located on the ortho and para positions of the benzene ring, but not the meta positions.  This is reflected in the resonance hybrid.
    Due to the stability of these benzylic cations, they are readily formed as intermediates during chemical reactions, for example S_N1 reactions of benzylic halides. Note that 2-chloro-2-phenylpropane is 600 times more reactive that the 2-methyl analogue.

Benzylic radicals

Benzyl radicals can also be stabilized by resonance in the same manner as shown above for carbocations.

Reactions of the Benzylic position

The functional groups in a benzylic position are generally more reactive than the related isolated functional group.

• Benzylic C-H can easily be radically halogenated since the benzylic radical is resonance stabilized.
• Benzylic C-H bonds of alkyl benzenes can be oxidized to give benzoic acids
• Benzylic halides readily undergo nucleophilic substitution reactions even with weak nucleophiles.
• Benzylic halides or alcohols readily eliminate to give conjugated alkenes.
• Alkenylbenznes undergo addition reactions to give typical addition products.

Summary:

• When treated with Br₂ or Cl₂ radical substitution of benzylic-H generates the benzyl halide and HX.
• The benzyl radical is quite stable so bromination will often by selective for the benzyl position.
• Note that there is no substitution of the benzene ring under these conditions.
• Only chlorination and bromination are useful in the laboratory.
• NBS is an alternate source for bromine.
• Reaction proceeds via a radical chain mechanism similar to that of alkanes.

N-Bromosuccinimide

RADICAL CHAIN MECHANISM FOR REACTION OF TOLUENE WITH Br2
Step 1 (Initiation) Heat or uv light cause 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 a benzyl radical, then  (b) The benzyl radical abstracts a bromine atom from another molecule of Br2 to form the benzyl 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, benzyl bromide. These reactions remove radicals and do not perpetuate the cycle.

Oxidation of Alkyl Benzenes.

Reaction type: Oxidation

Summary:

• When treated under strong oxidising conditions, benzylic-H are oxidized all the way to the carboxylic acid.
• The alkyl substituent can be methyl-, 1^o or 2^o alkyl.
• 3^o alkyl are not oxidized because they lack a benzylic-H.
• Simple alkane C-H can not be oxidized in this manner.

Reaction type: Nucleophilic substitution (S_N1 or S_N2)

Summary:

• Benzylic halides undergo nucleophilic substitution reactions very readily (review)
• 1^o benzylic halides typically react via an S_N2 pathway (review), and there is no competition from elimination.
• 2^o or 3^o benzylic halides typically react via an S_N1 pathway (review), via the resonance stabilized carbocation.
• Unlike allylic systems, there is no "benzylic rearrangement" since that would result in loss of aromaticity.

Related reactions

• Nucleophilic substitution of alkyl halides
• Nucleophilic substitution of allyl halides 

Eliminations of Benzylic Systems.

Summary:

• Benzylic alcohols undergo acid catalyzed dehydration readily via an E1 pathway (review) to form conjugated alkenes (review)
• Benzylic halides undergo base catalyzed dehydrohalogenation via an E2 pathway (review) to form conjugated alkenes (review)
• The stability of the conjugated system usually favours the conjugated product.

Related reactions

• Dehydration of alcohols
• Dehydrohalogenation of allyl halides

Additions to Alkenylbenzenes.

Summary:

• Alkenylbenzenes undergo many of the electrophilic addition reactions of simple alkenes (review)
• Addition to the alkene portion is usually faster than reaction of the aromatic C=C.
• Regioselectivity is usually dictated by the stability of the benzylic carbocation.

• Electrophilic additions reactions of alkenes