The carbonyl group is a structural feature of many different types of compounds. It is present in carbon dioxide and in methanal, which represent respectively the high and low extremes in the level of oxidation of a carbonyl carbon. In between, there are carbonyl compounds ranging from aldehydes and ketones to carboxylic acids and their derivatives (esters, amides, anhydrides, and acyl halides). In this and succeeding chapters we describe the chemistry of these compounds with the intent of emphasizing the similarities that exist between them. The differences turn out to be more in degree than in kind. Even so, it is convenient to discuss aldehydes and ketones separately from carboxylic acids and, following some general observations about the carbonyl group, this chapter mainly is concerned with aldehydes and ketones.
- 16.1: Prelude to Aldehydes and Ketones
- The carbonyl group, −C=O, is a structural feature of many different types of compounds. It is present in carbon dioxide and in methanal, which represent respectively the high and low extremes in the level of oxidation of a carbonyl carbon In between, there are carbonyl compounds ranging from aldehydes and ketones to carboxylic acids and their derivatives (esters, amides, anhydrides, and acyl halides).
- 16.2: The Carbonyl Bond
- The carbonyl bond is both a strong bond and a reactive bond. The bond energy varies widely with structure. Methanal has the weakest bond (166 kcal) and carbon monoxide the strongest (237.3kcal). Irrespective of these variations, the carbonyl bond not only is significantly stronger but also is more reactive than a carbon-carbon double bond.
- 16.3: Physical Properties
- The polarity of the carbonyl group is manifest in the physical properties of carbonyl compounds. Boiling points for the lower members of a series of aldehydes and ketones are 50-80o higher than for hydrocarbons of the same molecular weight. The water solubility of the lower-molecular-weight aldehydes and ketones is pronounced, which is the consequence of hydrogen-bonding between the water and the electronegative oxygen of the carbonyl group.
- 16.4: Spectroscopic Properties
- A carbonyl group in a compound can be positively identified by the strong infrared absorption band in the region 1650-1850cm−1, which corresponds to the stretching vibration of the carbon-oxygen double bond. The position of the band within this frequency range depends on the molecular environment of the carbonyl group. As a result, we frequently can tell from the band position whether the structure is an aldehyde, ketone, carboxylic acid, ester, amide, or anhydride.
- 16.5: Typical Carbonyl-Addition Reactions
- We turn now to discuss a few specific addition reactions of the carbonyl groups of aldehydes and ketones. We shall not attempt to provide an extensive catalog of reactions, but will try to emphasize the principles involved with especially important reactions that are useful in synthesis.
- 16.6: Catalytic Hydrogenation
- The simplest large-scale procedure for reduction of aldehydes and ketones to alcohols is by catalytic hydrogenation since the product can be obtained simply by filtration from the catalyst and then distillation. The common catalysts are nickel, palladium, copper chromite, or platinum activated with ferrous ion. Hydrogenation of aldehyde and ketone carbonyl groups is much slower than of carbon-carbon double bonds so more strenuous conditions are needed.
- 16.7: Reduction of Carbonyl Compounds to Hydrocarbons
- There are several methods of reducing carbonyl groups to hydrocarbons. In some cases, a three-step sequence of conventional reactions may be useful with alcohol and alkene intermediates. Alternative approach include the Clemmensen and Wolff-Kishner reductions.
- 16.8: Oxidation of Carbonyl Compounds
- Aldehdyes are oxidized easily by moist silver oxide or by potassium permanganate solution to the corresponding acids. The mechanism of the permanganate oxidation has some resemblance to the chromic acid oxidation of alcohols. The oxidation of benzenecarbaldehyde with peroxybenzenecarboxylic acid is an example of a reaction of wide applicability in which aldehydes are oxidized to carboxylic acids, and ketones are oxidized to esters. The reaction is known as a Baeyer-Villiger oxidation.
- 16.9: Protection of Carbonyl Groups
- There are few reactions of aldehydes and ketones that do not in some way affect the carbonyl function. For this reason, it may be necessary to protect the carbonyl function when it is desirable to avoid reaction at this function.
- 16.10: Preparative Methods for Aldehydes and Ketones
- A number of useful reactions for the preparation of aldehydes and ketones, such as ozonization of alkenes and hydration of alkynes, have been considered in previous chapters. Only a few rather general methods that we have not discussed will be taken up here.
- 16.E: Carbonyl Compounds I (Exercises)
- These are the homework exercises to accompany Chapter 16 of the Textmap for Basic Principles of Organic Chemistry (Roberts and Caserio).
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."