It is convenient first to consider the effect of introducing one, two, or three hydroxyl (OH) groups into the -CH3, >CH2, and ->CH groups, which we have seen to characterize the different types of hydrocarbons. It may be noticed here that cyclic nuclei can only contain the groups >CH2 and ->CH, the first characterizing the polymethylene and reduced heterocyclic compounds, the second true aromatic compounds.

Substituting one hydroxyl group into each of these residues, we obtain radicals of the type -CH2·OH, >CH·OH, and ->C·OH; these compounds are known as alcohols (q.v.), and are termed primary, secondary, and tertiary respectively. Polymethylenes can give only secondary and tertiary alcohols, benzene only tertiary; these latter compounds are known as phenols. A second hydroxyl group may be introduced into the residues -CH2·OH and >CH·OH, with the production of radicals of the form -CH(OH)2 and >C(OH)2. Compounds containing these groupings are, however, rarely observed (see [Chloral]), and it is generally found that when compounds of these types are expected, the elements of water are split off, and the typical groupings are reduced to -CH:O and >C:O. Compounds containing the group -CH:O are known as aldehydes (q.v.), while the group >C:O (sometimes termed the carbonyl or keto group) characterizes the ketones (q.v.). A third hydroxyl group may be introduced into the -CH:O residue with the formation of the radical -C(OH):O; this is known as the carboxyl group, and characterizes the organic acids.

Sulphur analogues of these oxygen compounds are known. Thus the thio-alcohols or mercaptans (q.v.) contain the group -CH2·SH; and the elimination of the elements of sulphuretted hydrogen between two molecules of a thio-alcohol results in the formation of a thio-ether or sulphide, R2S. Oxidation of thio-ethers results in the formation of sulphoxides, R2:S:O, and sulphones, R2:SO2; oxidation of mercaptans yields sulphonic acids, R·SO3H, and of sodium mercaptides sulphinic acids, R·SO(OH). We may also notice that thio-ethers combine with alkyl iodides to form sulphine or sulphonium compounds, R3÷SI. Thio-aldehydes, thio-ketones and thio-acids also exist.

We proceed to consider various simple derivatives of the alcohols, which we may here regard as hydroxy hydrocarbons, R·OH, where R is an alkyl radical, either aliphatic or cyclic in nature.

Of these, undoubtedly the simplest are the ethers (q.v.), formed by the elimination of the elements of water between two molecules of the same alcohol, “simple ethers,” or of different alcohols, “mixed ethers.” These compounds may be regarded as oxides in just the same way as the alcohols are regarded as hydroxides. In fact, the analogy between the alkyl groups and metallic elements forms a convenient basis from which to consider many derivatives. Thus from ethyl alcohol there can be prepared compounds, termed esters (q.v.), or ethereal salts, exactly comparable in structure with corresponding salts of, say, potassium; by the action of the phosphorus haloids, the hydroxyl group is replaced by a halogen atom with the formation of derivatives of the type R·Cl(Br,I); nitric acid forms nitrates, R·O·NO2; nitrous acid, nitrites, R·O·NO; sulphuric acid gives normal sulphates R2SO4, or acid sulphates, R·SO4H. Organic acids also condense with alcohols to form similar compounds: the fats, waxes, and essential oils are naturally occurring substances of this class.

An important class of compounds, termed amines (q.v.), results from the condensation of alcohols with ammonia, water being eliminated between the alcoholic hydroxyl group and a hydrogen atom of the ammonia. Three types of amines are possible and have been prepared: primary, R·NH2; secondary, R2:NH; and tertiary, R3÷N; the oxamines, R3N:O, are closely related to the tertiary ammonias, which also unite with a molecule of alkyl iodide to form salts of quaternary ammonium bases, e.g. R4N·I. It is worthy of note that phosphorus and arsenic bases analogous to the amines are known (see [Phosphorus] and [Arsenic]). From the primary amines are derived the diazo compounds (q.v.) and azo compounds (q.v.); closely related are the hydrazines (q.v.). Secondary amines yield nitrosamines, R2N·NO, with nitrous acid. By the action of hydroxylamine or phenylhydrazine on aldehydes or ketones, condensation occurs between the carbonyl oxygen of the aldehyde or ketone and the amino group of the hydroxylamine or hydrazine. Thus with hydroxylamine aldehydes yield aldoximes, R·CH:N·OH, and ketones, ketoximes, R2C:N·OH (see [Oximes]), while phenyl hydrazine gives phenylhydrazones, R2C:N·NH·C6H5 (see [Hydrazones]). Oxyaldehydes and oxyketones (viz. compounds containing an oxy in addition to an aldehydic or ketonic group) undergo both condensation and oxidation when treated with phenylhydrazine, forming compounds known as osozones; these are of great importance in characterizing the sugars (q.v.).

The carboxyl group constitutes another convenient starting-point for the orientation of many types of organic compounds. This group may be considered as resulting from the fusion of a carbonyl (:CO) and a hydroxyl (HO·) group; and we may expect to meet with compounds bearing structural resemblances to the derivatives of alcohols and aldehydes (or ketones).

Considering derivatives primarily concerned with transformations of the hydroxyl group, we may regard our typical acid as a fusion of a radical R·CO- (named acetyl, propionyl, butyl, &c., generally according to the name of the hydrocarbon containing the same number of carbon atoms) and a hydroxyl group. By replacing the hydroxyl group by a halogen, acid-haloids result; by the elimination of the elements of water between two molecules, acid-anhydrides, which may be oxidized to acid-peroxides; by replacing the hydroxyl group by the group ·SH, thio-acids; by replacing it by the amino group, acid-amides (q.v.); by replacing it by the group -NH·NH2, acid-hydrazides. The structural relations of these compounds are here shown:

R·CO·OH;R·CO·Cl;(R·CO)2O;R·CO·SH;
acid;acid-chloride;acid-anhydride;thio-acid;
R·CO·NH2;R·CO·NH·NH2.
acid-amide;acid-hydrazide.