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16.7: Reduction of Carbonyl Compounds to Hydrocarbons

  • Page ID
    22275
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    There are several methods of transforming Roberts and Caserio Screenshot 16-6-1.png to Roberts and Caserio Screenshot 16-6-2.png. In some cases, the following three-step sequence of conventional reactions may be useful:

    Roberts and Caserio Screenshot 16-6-3.png

    This route requires a hydrogen \(\alpha\) to the carbonyl function and may give rearrangement in the dehydration step (Sections 8-9B and 15-5E). Alternatively, the hydroxyl can be converted to a better leaving group (halogen or sulfonate ester), which then may be displaced by \(\ce{H}^\ominus\) (as \(\ce{LiAlH_4}\); see Table 16-6):

    Roberts and Caserio Screenshot 16-6-4.png

    More direct methods may be used, depending on the character of the \(\ce{R}\) groups of the carbonyl compound. If the \(\ce{R}\) groups are stable to a variety of reagents there is no problem, but with sensitive \(\ce{R}\) groups not all methods are equally applicable. When the \(\ce{R}\) groups are stable to acid but unstable to base, the Clemmensen reduction with amalgamated zinc and hydrochloric acid is often very useful.

    Roberts and Caserio Screenshot 16-6-5.png

    The mechanism of the Clemmensen reduction is not well understood. It is clear that in most cases the alcohol is not an intermediate, because the Clemmensen conditions do not suffice to reduce most alcohols to hydrocarbons.

    When the \(\ce{R}\) groups of the carbonyl compound are stable to base but not to acid, the Huang-Minlon modification of the Wolff-Kishner reduction usually gives good results. This procedure involves heating the carbonyl compound in a high-boiling polar solvent, such as 1,2-ethanediol, with hydrazine and potassium hydroxide and driving the reaction to completion by distilling out the water formed:

    Roberts and Caserio Screenshot 16-6-6.png

    When the carbonyl compound is sensitive to both acids and bases, or for other reasons gives poor yields in both the Clemmensen and Wolff-Kishner reductions, a recourse may be reduction of the corresponding thioacetal or thioketal with hydrogen-saturated Raney nickel (Section 11-2B):

    Roberts and Caserio Screenshot 16-6-7.png

    Thioketals, unlike ordinary ketals, are formed readily from ketones and thiols \(\left( \ce{RSH} \right)\) in the presence of acid catalysts. The desulfurization procedure usually goes well, but the product is rather difficult to separate by extraction from the large excess of Raney nickel required for optimum yields.

    Contributors and Attributions

    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."


    This page titled 16.7: Reduction of Carbonyl Compounds to Hydrocarbons is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by John D. Roberts and Marjorie C. Caserio.