Aldehydes, ketones, esters, and anhydrides

If the unknown neutral, oxygen-containing compound does not give the class reactions for aldehydes, ketones, esters and anhydrides, it is probably... 

If the unknown, neutral, oxygen-containing compound does not give the class reactions for aldehydes, ketones, esters and anhydrides, it is probably either an alcohol or an ether. Alcohols are readily identified by the intense characteristic hydroxyl adsorption which occurs as a broad band in the infrared spectrum at 3600-3300 cm-1 (O—H str.). In the nuclear magnetic resonance spectrum, the adsorption by the proton in the hydroxyl group gives rise to a broad peak the chemical shift of which is rather variable the peak disappears on deuteration. 

A major structural difference between aromatic aldehydes and most aliphatic analogues is that the former lack an a-hydrogen atom. As a consequence, they are unable to enolize and so enolates/carbanions cannot be generated from them. Nevertheless, aromatic aldehydes can react with carbanions derived from, for example, aldehydes, ketones, esters and anhydrides, and so undergo a range of condensation reactions. 

Carboxylic acids, anhydrides, aromatic and aliphatic hydrocarbons, alcohols, aldehydes, ketones, esters, and chlorinated solvents have only a slight effect. 

There are many classes of organic compounds that contain carbon-oxygen bonds, including alcohols, carboxylic acids, ethers, peroxides, aldehydes, ketones, esters, and acid anhydrides. Not all classes of compounds are included in the following discussion, so the interested smdent is advised to look at the more detailed references already mentioned for additional information. 

When the compound for identification fails to respond to test 4 (aldehyde or ketone), the next class reactions to apply are the hydroxatnic acid teat and saponification, i.e., hydrolysis in alkaline solution. These are the class reactions for esters and anhydrides the rarely-encountered lactones react similarly. 

The production of both an alcohol and the sodium salt of an acid might easily be confused with the hydrolysis products of an ester (in the above instance benzyl benzoate). Such an error would soon be discovered (e.g., by reference to the b.p. and other physical properties), but it would lead to an unnecessary expenditure of time and energy. The above example, however, emphasises the importance of conducting the class reactions of neutral oxygen-containing compounds in the proper order, viz., (1) aldehydes and ketones, (2) esters and anhydrides, (3) alcohols, and (4) ethers. 

Just as most other aldehydes do, furfural condenses with compounds possessing active methylene groups such as aUphatic carboxyUc esters and anhydrides, ketones, aldehydes, nitriles, and nitroparaffins. 

Esters are most commonly prepared by the reaction of a carboxyHc acid and an alcohol with the elimination of water. Esters are also formed by a number of other reactions utilizing acid anhydrides, acid chlorides, amides, nitriles, unsaturated hydrocarbons, ethers, aldehydes, ketones, alcohols, and esters (via ester interchange). Detailed reviews of esterification are given in References 1—9. 

AkoJwIs may be obtained from aldehydes, ketones, esters, acid chlorides, and anhydrides,... 

It is convenient to consider the indifferent or neutral oxygen derivatives of the hydrocarbons—(a) aldehydes and ketones, (b) esters and anhydrides, (c) alcohols and ethers— together. All of these, with the exception of the water-soluble members of low molecular weight, are soluble only in concentrated sulphuric acid, t.e., fall into Solubility Group V. The above classes of compounds must be tested for in the order in which they are listed, otherwise erroneous conclusions may be drawn from the reactions for functional groups about to be described. 

Only scare data is available in the literature on the application of rhenium containing mono- or bimetallic catalysts in the hydrogenolysis of esters to alcohols. Decades ago Broadbent and co-workers studied the hydrogenation of organic carbonyl compounds (aldehydes, ketones, esters, anhydrides, acids,... 

The reaction is often followed by protonation of the oxygen. The C=0 group may be present in an aldehyde, ketone, ester, anhydride, acyl halide, amide, and so forth. Acid catalysts may be necessary with weak nucleophiles. The acid protonates the oxygen, making the carbon more positive and thus, more electrophilic. 

For C=0 stretching vibrations of acid chlorides, acid anhydrides, carboxylic acids, esters, aldehydes, ketones, amides and salts, see tables 2.4 and 2.5. 

All kinds of carbonyl compounds, including aldehydes, ketones, esters amides, acid anhydrides, and nitriles, enter into condensation reactions, Nature uses these same carbonyl condensation reactions in the biosynthesis of many naturally occurring compounds. 

There is a strong single carbonyl band at 5.75 microns similar in shape and wave length to the carbonyl bands of aldehydes, ketones, peroxyacids, and esters, and different from the double carbonyl bands of acids, acid anhydrides, and diacyl peroxides. There is a neighboring, weaker band at 5.4 microns which might be considered a part of a double carbonyl band, but this seems doubtful because the two branches of a double carbonyl band are usually of approximately equal intensity and are usually closer together. The indications are that compound X from diacetyl is an acetyl compound—a logical product. 

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