Reduction of aromatic compounds
Study Notes
Catalytic hydrogenation of aromatic rings requires forcing conditions (high heat and hydrogen pressure).
Hydrogenation of benzene at high pressure
Although it does so less readily than simple alkenes or dienes, benzene adds hydrogen at high pressure in the presence of Pt, Pd or Ni catalysts. The product is cyclohexane and the heat of reaction provides evidence of benzene’s thermodynamic stability. Substituted benzene rings may also be reduced in this fashion, and hydroxy-substituted compounds such as phenol give carbonyl products resulting from the fast tautomerization of intermediate enols. Nickel or palladium on carbon catalysts are often used for this purpose, as noted in the following equations.
Under low pressure conditions at room temperature, H2/Pd will reduce a ketone carbonyl group when it is directly attached to an aromatic ring. This reduction of the $\ce{\sf{C=O}}$ group next to an aromatic ring is an important synthetic tool, as we saw in Section 16.2.
The Birch Reduction
Another way of adding hydrogen to the benzene ring is by treatment with the electron rich solution of alkali metals, usually lithium or sodium, in liquid ammonia. This general type of reaction is known as the Birch reduction after the Australian chemist, A. J. Birch. With benzene, reduction with metals leads to 1,4-cyclohexadiene:
The initial step of the Birch reduction is an electron transfer to the lowest unoccupied molecular orbital of the benzene [latex] \pi [/latex] system (see Figure 21-5) to form a radical anion:
Subsequent steps include a sequence of proton- and electron-transfer steps as follows:
Substituent effects observed for this reaction are entirely consistent with those described for electrophilic substitution and addition – only reversed. That is, the reactivity of an arene in metal reductions is increased by electron-withdrawing groups and decreased by electron-donating groups. Substituents that can stabilize the anion-radical intermediate facilitate the reduction.
Contributors
- John D. Robert and Marjorie C. Caserio (1977) Basic Principles of Organic Chemistry, second edition. W. A. Benjamin, Inc. , Menlo Park, CA. ISBN 0-8053-8329-8. This content is copyrighted under the following conditions, “You are granted permission for individual, educational, research and non-commercial reproduction, distribution, display and performance of this work in any format.”
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