Ketones

KETONES, R'.CO.R", are products of the oxidation of secondary alcohols, R'CH(OH)R" (R', R" are radicals, residues of hydrocarbons) (see Alcohols). Ketones represent compounds of two radicals with a carbonyl group -CO. With identical radicals, simple ketones are obtained; with different ones, mixed ketones. The isomerism of ketones is determined by the structure of the radicals and the distribution of carbon atoms between the two radicals (metamerism - see Isomerism). Thus, the ketone CH₃.CO.C₃H₇ (I) is metameric with the ketone C₂H₅.CO.C₂H₅ (II). Ketones are usually named according to the radicals forming them. Thus, ketone (I) is called methyl-propyl ketone; ketone (II) is called diethyl ketone. According to the international (Geneva) nomenclature, ketones are named after the hydrocarbons that lie at their base, adding the ending "-one" with a number indicating the position of the carbon atom in the form of the CO group. Thus, the ketone CH₃.CO.CH₂.CH₂.CH₃ is called "pentan-one-2". For some ketones, empirical names are retained; thus, the first ketone CH₃.CO.CH₃ is called acetone. Due to the low availability of secondary alcohols, ketones are rarely obtained by their oxidation. More often, ketones are obtained by dry distillation (sometimes under reduced pressure) of calcium or barium salts of fatty acids. For example, CH₃.CO.O.Ca.O.OC.CH₃ = CH₃.CO.CH₃ + CaCO₃. When using a mixture of two salts, mixed ketones are obtained. There are other methods for obtaining ketones, for example, from keto-acids (hydrolysis): CH₃.CO.CH₂.CO.OH + HOH = CH₃.CO.CH₃ + HOCO.OH (H₂O + CO₂). Such is the formation of acetone from acetoacetic acid in the organism. Some ketones are found in the essential oils of plants. The chemical properties of ketones are determined by the radicals and the carbonyl group. The radicals are capable of all transformations that hydrocarbons are capable of. In many cases, these transformations are facilitated due to the transition of ketones into the alcoholic enol form: CH₃.CO.CH₃ -> CH₂:C(OH).CH₃, which reduces substitution reactions to addition reactions [CH₂:C(OH).CH₃ + Br₂ = CH₂Br.CBr(OH).CH₃ -> CH₂Br.CO.CH₃ + HBr]. The carbonyl group (CO) causes addition reactions: 1) of hydrogen - with the formation of a secondary alcohol: R'CO.R" + 2H = R'.CH(OH).R"; 2) of hydrocyanic acid - with the formation of an oxynitrile (cyanohydrin): R'.CO.R" + HCN = R'.C(OH).CN.R"; 3) of sodium bisulfite (NaHSO₃): R'.CO.R" + NaHSO₃ = R'C(OH)(SO₃Na)R". The latter reaction is often used for the isolation and purification of ketones. From bisulfite compounds, ketones are obtained again upon heating with acids or alkali carbonates. Furthermore, ketones give characteristic compounds with substituted hydrazines, for example, phenylhydrazine C₆H₅.NH.NH₂, p-nitrophenylhydrazine NO₂.C₆H₄.NH.NH₂, etc.: R'.CO.R" + C₆H₅.NH.NH₂ = H₂O + R'C(N.NH.C₆H₅).R". Hydroxylamine reacts similarly: R'.CO.R" + NH₂OH = R'.C(NOH).R" + H₂O, forming a ketoxime. For the characterization of ketones, the reaction with semicarbazide NH₂CO.NH.NH₂ is also often used; in this case, the formation of a semicarbazone occurs: R'.CO.R" + NH₂CO.NH.NH₂ = H₂O + R'C(N.NH.CO.NH₂).R". Ketones are oxidized more difficultly than aldehydes (see). Thus, for example, an ammoniacal solution of silver oxide does not act on ketones; strong oxidizing agents lead to the decomposition of the molecule. Ketones differ from aldehydes also by the absence of the ability to polymerize; but like aldehydes, they are capable of condensation reactions: R'.CO.R" + CH₃.CO.CH₃ = R'.R"C(OH).CH₂.CO.CH₃ + H₂O.