3.10: Reduction of Aromatic Compounds
After completing this section, you should be able to
- write an equation to represent the reduction of a substituted benzene to a substituted cyclohexane.
- identify the catalyst and reagents used to reduce aromatic rings.
- compare the ease of reduction of alkenes with the difficulty in reducing benzene rings, and show how this difference in reactivity can be used in organic synthesis.
- write an equation to illustrate the reduction of an aromatic ketone to an arene.
- explain why Friedel-Crafts acylation, followed by reduction, provides a better route to primary alkylbenzenes than does direct alkylation.
- show how a specified alkylbenzene may be prepared by a Friedel-Crafts acylation, followed by reduction. Specify all reagents, the structure of the intermediate ketone, and the necessary starting material.
Catalytic Hydrogenation of Aromatic Rings
Just as aromatic rings are generally inert to oxidation, they’re also inert to catalytic hydrogenation under conditions that reduce typical alkene double bonds. As a result, it’s possible to reduce an alkene double bond selectively in the presence of an aromatic ring. For example, 4-phenyl-3-buten-2-one is reduced to 4-phenyl-2-butanone using a palladium catalyst at room temperature and atmospheric pressure. Neither the benzene ring nor the ketone carbonyl group is affected.
To hydrogenate an aromatic ring, it’s necessary either to use a platinum catalyst with hydrogen gas at a pressure of several hundred atmospheres or to use a more effective catalyst such as rhodium on carbon. Under these conditions, aromatic rings are converted into cyclohexanes. For example, o -xylene yields 1,2-dimethylcyclohexane, and 4- tert -butylphenol gives 4- tert -butylcyclohexanol.
Reduction of Aryl Alkyl Ketones
In the same way that an aromatic ring activates a neighboring (benzylic) C–H toward oxidation, it also activates a benzylic carbonyl group toward reduction. Thus, an aryl alkyl ketone prepared by Friedel–Crafts acylation of an aromatic ring can be converted into an alkylbenzene by catalytic hydrogenation over a palladium catalyst. Propiophenone, for instance, is reduced to propylbenzene by catalytic hydrogenation. Because the net effect of Friedel–Crafts acylation followed by reduction is the preparation of a primary alkylbenzene, this two-step sequence of reactions makes it possible to circumvent the carbocation rearrangement problems associated with direct Friedel–Crafts alkylation using a primary alkyl halide (Section 16.3).
The conversion of a carbonyl group into a methylene group (
How would you make the following from benzene and an acid chloride?
- Answer
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Catalytic hydrogenation of aromatic rings requires forcing conditions (high heat and hydrogen pressure).
Under milder conditions it is possible to reduce the double-bond of an alkene without reducing the aromatic ring.
Notice in the above equation that H 2 /Pd does not reduce the keto-carbonyl group. Remember, however, that H 2 /Pd will reduce a keto-carbonyl group when it is directly attached to an aromatic ring (see equations 4 and 5 under Carbonyl Reductions).
This reduction of the (C=O) group next to an aromatic ring is an important synthetic tool. Recall the Friedel-Crafts alkylation from Section 16.3. When attaching larger alkyl groups to arenes there is a possibility of rearrangement of the alkyl group structure.
To generate the target compound (in this case n ‑propylbenzene) in a more controlled fashion, one can simply use the equivalent Friedel-Crafts acylation and then reduce the keto-carbonyl group next to the ring as a final step.