Despite its unsaturation, the benzene ring is inert to strong oxidizing agents such as KMnO
4
, which will cleave alkene carbon–carbon bonds (Section 8.8). It turns out, however, that the presence of the aromatic ring has a dramatic effect on the reactivity of alkyl side chains. These side chains react rapidly with oxidizing agents and are converted into carboxyl groups, –CO
2
H.The net effect is conversion of an alkylbenzene into a benzoic acid,
2.8: Reactions at the Benzylic Position
After completing this section, you should be able to
- write an equation to describe the oxidation of an alkylbenzene to a carboxylic acid.
- identify the reagents required to oxidize a given alkylbenzene to a carboxylic acid.
- identify the product formed from the side-chain oxidation of a given alkylbenzene.
- identify the aromatic compound needed to produce a given carboxylic acid through side-chain oxidation.
- write the equation for the bromination of an alkylbenzene side chain.
- identify the reagents and conditions necessary to bring about bromination in the side chain of an alkylbenzene.
- identify the product formed when a given alkylbenzene undergoes side-chain bromination.
- identify the alkylbenzene needed to prepare a given benzylic bromide by radical substitution.
- write the mechanism for the radical substitution at the benzylic position of an alkylbenzene.
- explain the stability of benzylic radicals in terms of resonance, and draw the resonance contributors of a given benzyl radical.
- explain, and illustrate with appropriate examples, the importance of benzylic bromides as intermediates in organic syntheses.
- arrange a given series of radicals (including benzylic type radicals) in order of increasing or decreasing stability. (Review Section 10.3 if necessary.)
Make certain that you can define, and use in context, the key terms below.
- benzylic oxidation
- benzylic position
- side-chain oxidation
As you can see from the examples, no matter what the length of the alkyl group in the arene substrate, the product is always a one-carbon carboxyl group. Thus, the benzylic carbon atom has been oxidized and the term benzylic oxidation is appropriate. The term side-chain oxidation is also commonly used.
In alkylbenzenes, the carbon atom which is attached to the aromatic ring is particularly reactive. Reactions taking place at this carbon atom are said to occur at the benzylic position .
You may wish to review allylic bromination using N-bromosuccinimide from CHEM 231.
Benzylic halides undergo the typical reactions of alkyl halides, so they are frequently used in multistep syntheses.
Note that we have adopted the terminology given below.
|
Any compound of the type
(where X = halogen) will be referred to as a “benzylic halide.” |
Compounds of the type
are actually called benzyl chloride, benzyl bromide, etc. |
The compound
is called benzyl alcohol. |
Oxidation of Alkyl Side-Chains
Two other examples of this reaction are given below, and illustrate its usefulness in preparing substituted benzoic acids.
The mechanism of side-chain oxidation is complex and involves reaction of C–H bonds at the position next to the aromatic ring to form intermediate benzylic radicals. tert -Butylbenzene has no benzylic hydrogens, however, and is therefore inert.
Predict the products of the following two reactions.
- Answer
-
The second one leads to no reaction because it requires a hydrogen just off the phenyl ring.
What aromatic products would you obtain from the KMnO 4 oxidation of the following substances?
a. b.
- Answer
-
- m -Nitrobenzoic acid
- p - tert -Butylbenzoic acid
Bromination of the Benzylic Carbon
Side-chain bromination at the benzylic position occurs when an alkylbenzene is treated with N -bromosuccinimide (NBS). For example, propylbenzene gives (1-bromopropyl)benzene in 97% yield on reaction with NBS in the presence of benzoyl peroxide, (PhCO 2 ) 2 , as a radical initiator. Bromination occurs exclusively in the benzylic position next to the aromatic ring and does not give a mixture of products.
The mechanism of benzylic bromination is similar to that discussed in Section 10.3 for allylic bromination of alkenes. Abstraction of a benzylic hydrogen atom first generates an intermediate benzylic radical, which then reacts with Br 2 in step 2 to yield product and a radical, which cycles back into the reaction to carry on the chain shown below as a summary. The Br 2 needed for reaction with the benzylic radical is produced in step 3 by a concurrent reaction of HBr with NBS.
Reaction occurs exclusively at the benzylic position because the benzylic radical intermediate is stabilized by resonance. Figure \(\PageIndex{1}\) shows how the benzyl radical is stabilized by overlap of its p orbital with the ringed \(\pi\) electron system.
Consider a benzyl radical. Would it be more stable than an alkyl radical? Explain.
- Answer
-
Yes, it would be more stable than an alkyl radical. The benzyl radical is stabilized through several resonance structures where the radical is moved through the ring via the pi system there.
How would you make the following molecule?
- Answer
-
The following is just one possibility.