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17.7: Ketenes

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    Preparation of Ketenes

    Substances with cumulated carbonyl and carbon-carbon double bonds, Roberts and Caserio Screenshot 17-6-1.png, are called ketenes and, as may be expected, have interesting and unusual properties. Ketene itself, \(\ce{CH_2=C=O}\), and its monosubstitution products, \(\ce{RCH=C=O}\) (\(\ce{R} =\) alkyl or aryl), are called aldoketenes, whereas disubstituted ketenes, \(\ce{R_2C=C=O}\), are called ketoketenes.

    There are relatively few general methods for preparing ketenes. The simplest procedure is to treat an \(\alpha\)-bromoacyl bromide with zinc, but the yields usually are not very good:

    Roberts and Caserio Screenshot 17-6-2.png

    Several special methods are available for the preparation of ketene itself. The most convenient laboratory preparation is to pass 2-propanone vapor over a coil of resistance wire heated electrically to a dull red heat; air is excluded to avoid simple combustion:

    Roberts and Caserio Screenshot 17-6-3.png

    The weakest bonds are the \(\ce{C-C}\) bonds and, at \(750^\text{o}\), fragmentation yields a methyl radical and an ethanoyl radical:

    Roberts and Caserio Screenshot 17-6-4.png

    Transfer of a hydrogen atom (i.e., disproportionation) gives methane and ketene. Industrially, ketene is best prepared by dehydration of ethanoic acid:

    Roberts and Caserio Screenshot 17-6-5.png

    Reactions of Ketenes

    Ketene has a boiling point of \(-56^\text{o}\) and normally would be stored under pressure in steel cylinders. However, this is not possible because ketene is unstable with respect to formation of a dimer known as "diketene":

    Roberts and Caserio Screenshot 17-6-6.png

    The dimer also is a highly reactive substance with such unusual characteristics that its structure was not firmly established until 1956, almost 48 years after it first was prepared.

    Ketenes in general are useful reagents for acylating alcohols, \(\ce{ROH}\), and amines, \(\ce{RNH_2}\), because the reactions involve additions; there are no by-products to be separated:

    Roberts and Caserio Screenshot 17-6-7.png

    Ketenes also can be used for the synthesis of cyclobutane derivatives through [2 + 2] cycloadditions with suitably active alkenes (Section 13-3D):

    Roberts and Caserio Screenshot 17-7-7.png

    Diketene is very useful in synthesis, particularly through its reactions with alcohols and amines to give derivatives of 3-oxobutanoic acid:

    Roberts and Caserio Screenshot 17-6-9.png

    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 17.7: Ketenes 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.