Some of the most useful reactions of carbonyl compounds involve carbon-hydrogen bonds adjacent to the carbonyl group. Such reactions, which can be regarded as the backbone of much synthetic organic chemistry, usually result in the replacement of the hydrogen by some other atom or group. The important examples we will consider in this chapter are halogenation, alkylation, and aldol reactions of aldehydes and ketones.
- 17.1: Prelude to Enols and Enolate Anions, Unsaturated, and Polycarbonyl Compounds
- Some of the most useful reactions of carbonyl compounds involve carbon-hydrogen bonds adjacent to the carbonyl group. Such reactions, which can be regarded as the backbone of much synthetic organic chemistry, usually result in the replacement of the hydrogen by some other atom or group. The important examples we will consider in this chapter are halogenation, alkylation, and aldol reactions of aldehydes and ketones.
- 17.2: Enolization of Aldehydes and Ketones
- Transformation of a carbonyl compound to an enol at a useful rate normally requires either a basic catalyst or an acidic catalyst and, of course, at least one hydrogen on the αα carbon. The features of each type of catalysis follow.
- 17.3: Halogenation of Aldehydes and Ketones
- Halogenation of saturated aldehydes and ketones usually occurs exclusively by replacement of hydrogens alpha to the carbonyl group. The reagents that commonly are used to halogenate carbonyl compounds are those that are used to halogenate alkanes. However, the mechanism of the two types of halogenation normally are very different. When one attempts E2 reactions with α-halo ketones using strong bases such as alkoxides, an interesting rearrangement may occur called the Favorskii rearrangement
- 17.4: Nucleophilic Addition Reactions of Enolate Anions
- A most important property of enolate anions, at least as far as synthesis is concerned, is their excellent nucleophilicity, which enables them to add to double bonds and to participate in nucleophilic substitution. When the addition is to a carbonyl double bond, it is called an aldol addition. Additions of enolate anions to carbon-carbon double bonds usually are classified as Michael additions.
- 17.5: Nucleophilic Substitution with Enolate Anions
- The synthetic chemistry of enolate anions is centered on their nucleophilic and basic properties. Accordingly these ions participate in SN2 reactions with suitable alkyl compounds. However, there are a number of complicating factors to consider. First, the basic conditions needed to form the enolate ions often lead to side reactions such as aldol addition and E2 elimination of RX compounds.
- 17.6: α,β-Unsaturated Aldehydes and Ketones
- The most generally useful preparation of α,β-unsaturated carbonyl compounds is by dehydration of aldol addition products. There are many addition reactions of α, β -unsaturated aldehydes, ketones, and related compounds that are the same as the carbonyl addition reactions described previously. Others are quite different and result in addition to the alkene double bond.
- 17.7: Ketenes
- Substances with cumulated carbonyl and carbon-carbon double bonds are called ketenes and, as may be expected, have interesting and unusual properties. Ketene has a boiling point of −56° and normally would be stored under pressure in steel cylinders. Ketenes in general are useful reagents for acylating alcohols, and amines, because the reactions involve additions; there are no by-products to be separated:
- 17.8: 1,2-Dicarbonyl Compounds
- Most of the 1,2-dicarbonyl compounds are yellow. Ethanedial is unusual in being yellow in the liquid state, but green in the vapor state. It has very reactive aldehyde groups and is employed in the manufacture of plastics and as a component of embalming fluids to harden proteins by linking together their amino groups through imine formation.
- 17.9: 1,3-Dicarbonyl Compounds
- Much of the chemistry of 1,3-dialdehydes, aldehyde ketones, and diketones already has been mentioned in this chapter. The reactions discussed in this chapter that depend on the formation of enolate anions (i.e., halogenation, aldol addition, and alkylation) often proceed smoothly and under milder conditions with 1,3-diketones than with monoketones. This is because the 1,3-diketones are stronger acids and therefore can form the enolate anion with weaker bases.
- 17.10: 1,4-Dicarbonyl Compounds
- Most of the reactions of the 1,4-dicarbonyl compounds are the conventional reactions expected for isolated carbonyl groups. These reactions are reasonably general and also can be used to prepare compounds with oxygen and sulfur in five-membered rings.
- 17.11: Tricarbonyl Compounds
- The properties of tricarbonyl compounds are for the most part as expected, except when the three groups are contiguous to one another, as in diphenylpropanetrione. With such compounds, the central carbonyl group is highly reactive; it is lost, as carbon monoxide, in the presence of acidic catalysts such as aluminum chloride, and adds water readily to give a monohydrate.
- 17.12: Cyclopropanones and Cyclopropenones
- ones deserve special comment, not because of their practical importance, but because of their novel behavior and reactivity. No unambiguous synthesis of cyclopropanones was known prior to 1965, and the older textbooks usually contained statements such as "cyclopropanones apparently cannot exist". However, they had been postulated as intermediates in various reactions, but until recently had defied isolation and identification.
- 17.E: Carbonyl Compounds II (Exercises)
- These are the homework exercises to accompany Chapter 17 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."