Aldehydes and ketones have the carbonyl group >C=O as a functional group and they are classified as carbonyl compounds. The carbonyl group is also a constituent of other functional groups generally expressed as -C(O)Z, where Z is a substituent that has an atom with the unshared electron pair, i. e. halogen or oxygen-, nitrogenor sulfur-containing groups. Such compounds are known rather as carboxylic acids and their derivatives, though they are sometimes referred to as carbonyl compounds too. We will discuss them later, and this chapter deals with aldehydes and ketones.
ДХД Aldehydes are compounds in which the carbonyl group is bonded to at least one hydrogen. Ketones contain the carbonyl group bonded to two hydrocarbon residues. The general formulas of aldehydes and ketones are RCH=O and RC(O)R' (or simply RCOR'), respectively.
Carbonyl compounds are among the most important of all organic compounds, especially, in biological processes. Suffice it to say that carbohydrates contain a carbonyl group.
8.1. GENERAL CHARACTERISTICS OF ALDEHYDES AND KETONES
Chemical properties of aldehydes and ketones are very similar regardless of their specific structure. It is no wonder that both of the classes are considered together.
8.1.1. Classification and Nomenclature
Aldehydes may be classified as either aliphatic or aromatic depending on the R substituent. The only exception is the simplest representative - formaldehyde, HCH=O, where R is hydrogen in the general formula of aldehydes. Ketones are classified in a similar way, except for existence of cyclic ketones and a mixed type of the compounds.
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In the IUPAC nomenclature system, the aldehyde group in aliphatic compounds is indicated by the suffix -al, with elision of terminal -e from the name of the parent hydrocarbon. Note that carbon of the aldehyde group is a part of the hydrocarbon structure. The parent name benzaldehyde is used for aromatic aldehydes. Many aldehydes have semi-trivial names derived from the corresponding carboxylic acids and accepted by the IUPAC nomenclature, for example acetaldehyde - from acetic acid (as shown above), butyraldehyde, CH3CH2CH2CH=O - from butyric acid (C4 acid), and so on.