Preparing Aldehydes and Ketones

If other functional groups are present and the doubly bonded oxygen is considered a substituent on a parent chain, the prefix oxo- is used. For example  

One of the best methods of aldehyde synthesis is by oxidation of primary alcohols, as we saw in Section 17.7. The reaction is often carried out using the Dess-Martin periodinane reagent in dichloromethane solvent at room temperature  

A second method of aldehyde synthesis is one that we ll mention here just briefly and then return to in Section 21.6. Certain carboxylic acid derivatives can be partially reduced to yield aldehydes. The partial reduction of an ester by diisobutylaluminum hydride (DIBAH, or DIBAL-H), for instance, is an important laboratory-scale method of aldehyde synthesis, and mechanistically related 

How would you prepare pentanal from the following starting materials (a) CH3CH2CH2CH2CH2OH (b) CH3CH2CH2CH2CH=CH2 

For the most part, methods of ketone synthesis are similar to those for aldehydes. Secondary alcohols are oxidized by a variety of reagents to give ketones (Section 17.7). The choice of oxidant depends on such factors as reaction scale, cost, and acid or base sensitivity of the alcohol. Either the Dess-Martin periodinane or a Cr(VI) regent such as Cr03 is a common choice. 

CHAPTER 14 ALDEHYDES AND KETONES N UCLEOPH I LIC ADDITION REACTIONS 

Aryl ketones can be prepared by Friedel-Crafts acylation of an aromatic ring with an acid chloride in the presence of AICI3 catalyst (Section 9.7)  

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To the synthetic chemist, the most important of the reactions in Table 17.1 are the last two the oxidation of primar-y alcohols to aldehydes and secondary alcohols to ketones. Indeed, when combined with reactions that yield alcohols, the oxidation methods are so versatile that it will not be necessary to introduce any new methods for preparing aldehydes and ketones in this chapter. A few examples will illustrate this point. 

The general procedure described here was originally published by the submitters,3 who have used this insoluble reagent to prepare aldehydes and ketones under Moffat oxidation conditions.4 A polymeric reagent offers two advantages (a) when an oxidation is complete, the urea... 

The Stiles-Sisti reaction (Scheme 12-17) is an azo coupling reaction in which an a-hydroxyalkyl residue is the electrofugic leaving group (Stiles and Sisti, 1960 Sisti et al., 1962). The reaction is used to prepare aldehydes and ketones (e. g., 2-methoxy-... 

Ozonoiysis is a reaction used with unsaturated hydrocarbons when preparing aldehydes and ketones, by reducing intermediate ozonide or acids by oxidation. The reducing agents used include hydrogen in the presence of palladium, and zinc in acid medium. 

In a similar fashion, chromyl chloride, normally a very vigorous oxidant, can be used to selectively prepare aldehydes and ketones once it has been adsorbed onto silica-alumina. Unfortunately, double bonds are still cleaved under the reaction conditions. 

A few of the many laboratory methods of preparing aldehydes and ketones are outlined below most of these are already familiar to us. Some of the methods involve oxidation or reduction in which an alcohol, hydrocarbon, or acid chloride is converted into an aldehyde or ketone of the same carbon number. Other methods involve the formation of new carbon-carbon bonds, and yield aldehydes or ketones of higher carbon number than the starting materials. 

The physical properties of aldehydes and ketones are discussed in Section 17.3 (see also Appendix I), and the methods used to prepare aldehydes and ketones are summarized in Appendix IV. 

This reaction has wide application in the analysis of carbohydrate structures and the cleavage of adjacent diols, a-keto alcohols, a-keto acids, and hydroxy amines. In addition, this oxidation also renders a method for preparing aldehydes and ketones. 

We see that the addition of hydrogen to an aldehyde produces a primary alcohol, whereas the addition to a ketone gives a secondary alcohol. Notice that these reactions are reductions that are essentially the reverse of the alcohol oxidations used to prepare aldehydes and ketones (Reactions 4.1 and 4.2). 

We have already seen, in previous chapters, several ways to prepare aldehydes and ketones. One of the most useful is the oxidation of alcohols. 

Preparing Aldehydes and Ketones A Review Introduction to Nucleophilic Addition Reactions Oxygen Nucleophiles Nitrogen Nucleophiles Mechanism Strategies Sulfur Nucleophiles Hydrogen Nucleophiles Carbon Nucleophiles Baeyer-Villiger Oxidation of Aldehydes and Ketones Synthesis Strategies... 

In previous chapters, we have studied a variety of methods for preparing aldehydes and ketones, which are summarized in Tables 20.1 and 20.2, respectively. 

Several ways to prepare aldehydes and ketones have already beai desoibed in comieclion with the chemistry of other functional groups (see the Reaction Sunamary Road Map on pp. 780-781). This section reviews the methods that we have studied, pointing out special features and additional examples. Other routes to aldehydes and ketones will be desoibed in lato clupters. 

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