Benzylic systems

Chapter 11 : Arenes and Aromaticity

    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:
 
 

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Bromobenzene
(phenyl bromide)
Benzyl bromide

Benzylic carbocations
 

benzyl cation resonance forms
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 SN1 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.

Reaction of Alkyl Benzenes with X2

radical halogenation of benzylic CH
Reaction type: Radical Substitution

Summary:

an alternate source of Br2 for radical bromination      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.
initiation, formation of Br radical by homolytic cleavage
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.
propagation : benzyl radical formed and converted to product
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.
termination steps : radicals combine

 

Oxidation of Alkyl Benzenes.

oxidation of benzylic CH

Reaction type: Oxidation

Summary:

Nucleophilic Substitution of Benzylic Halides.

nucleophilic substitution of benzylic systems

Reaction type: Nucleophilic substitution (SN1 or SN2)

Summary:

Example:

nucleophilic substitution of benzyl bromide

Related reactions

Eliminations of Benzylic Systems.

elimination of benzylic systems
Reaction type: Elimination (E1 or E2)

Summary:

Example:
elimination of a benzylic alcohol and bromide to styrene

Related reactions

Additions to Alkenylbenzenes.

additions to alkenylbenzenes
Reaction type: Electrophilic addition

Summary:

Example:
hydration of styrene
hydrobromination of styrene
Related reactions