2.9: Reactions at the Benzylic Position

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Page ID: 500377

Sol Parajon Puenzo
Cañada College

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Objectives

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.)

Key Terms

Make certain that you can define, and use in context, the key terms below.

benzylic oxidation
benzylic position
side-chain oxidation

Oxidation of Alkyl Side-Chains

Despite its unsaturation, the benzene ring is inert to strong oxidizing agents such as KMnO₄, 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, –CO₂H.The net effect is conversion of an alkylbenzene into a benzoic acid, $Ar–R \to Ar -$ CO2HAr–R→Ar–CO2H. Butylbenzene is oxidized by aqueous KMnO₄ to give benzoic acid, for instance.

Butylbenzene reacts with potassium permanganate in the presence of water to form benzoic acid in 85 percent yield.

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.

Tertiary-butylbenzene does not react with potassium permanganate in water

Exercise $\PageIndex{1}$

Predict the products of the following two reactions.

In the first reaction: 1-nitro-4-(pentan-3-yl)benzene reacts with water and KmnO4.

Answer

It one leads to no reaction because it requires a hydrogen just off the phenyl ring.

Exercise $\PageIndex{2}$

What aromatic products would you obtain from the KMnO₄ oxidation of the following substances?

a. in= b. In a benzene ring, C 1 is bonded to C (C H 3) 3 group. C 4 is bonded to a methyl group.

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, (PhCO₂)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.

In C C l 4, propylbenzene reacts with N-bromosuccinimide in the presence of (P h C O 2) 2 to form (1-Bromopropyl)benzene in 97 percent yield and succinimide.

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 Br₂ in step 2 to yield product and a $Br \cdot$ radical, which cycles back into the reaction to carry on the chain shown below as a summary. The Br₂ needed for reaction with the benzylic radical is produced in step 3 by a concurrent reaction of HBr with NBS.

Three-step reaction of a B r plus ion wih a benzene ring bearing a C H 2 R group, in the presence of N bromosuccinimide.

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.

Four benzylic radicals with double-headed arrows in-between. To the right, the ball-and-stick model with the electrostatic potential map of benzylic radical is depicted. — Figure $\PageIndex{1}$: A resonance-stabilized benzylic radical. The spin-density surface shows that the **unpaired electron** is shared by the carbons in ortho and para positions.

Study Notes

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

organobenzene with halogen on first carbon external to the ring

(where X = halogen) will be referred to as a “benzylic halide.”

Compounds of the type

methyl hydrogen of toluene replaced by a halogen atom

are actually called benzyl chloride, benzyl bromide, etc.

The compound

methyl hydrogen of toluene replaced by a hydroxide group

is called benzyl alcohol.

Exercise $\PageIndex{3}$

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.

Exercise $\PageIndex{4}$

How would you make the following molecule?

Bond line drawing of (prop-1-en-1-yl)benzene

Answer

The following is just one possibility.

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