CROTONIC ACID (C4H6O2). Three acids of this empirical formula are known, viz. crotonic acid, isocrotonic acid and methacrylic acid; the constitutional formulae are--
HC·CO(2)H, HC·CO2H /CH3 ·· ·· CH2:C HC·CH3 CH3·CH \CO2H. Crotonic Acid. Isocrotonic Acid. Methacrylic Acid.
The isomerism of crotonic and isocrotonic acids is to be explained on the assumption of a different spatial arrangement of the atoms in the molecule (see STEREOCHEMISTRY).
Crotonic acid, so named from the fact that it was erroneously supposed to be a saponification product of croton oil, may be prepared by the oxidation of croton-aldehyde, CH3·CH:CH·CHO, obtained by dehydrating aldol, or by treating acetylene successively with sulphuric acid and water; by boiling allyl cyanide with caustic potash; by the distillation of ß-oxybutyric acid; by heating paraldehyde with malonic acid and acetic acid to 100° C. (T. Komnenos, _Ann._, 1883, 218, p. 149).
CH2(COOH)2 + CH3CHO -> CH3CH:C(COOH)2 -> CH3·CH:CH·COOH;
or by heating pyruvic acid with an excess of acetic anhydride and sodium acetate to 160-180° C. (B. Homolka, _Ber._, 1885, 18, p. 987). It crystallizes in needles (from hot water) which melt at 72° C. and boil at 180-181° C. It is moderately soluble in cold water. It combines directly with bromine, and, with fuming hydrobromic acid at 100° C., it gives chiefly [alpha]-brombutyric acid. With hydriodic acid it gives only ß-iodobutyric acid. Potash fusion converts it into acetic acid; nitric acid oxidizes it to acetic and oxalic acids; chromic acid mixture to acetaldehyde and acetic acid, and potassium permanganate to [alpha]ß-dioxybutyric acid.
Isocrotonic acid (Quartenylic acid) is obtained from ß-chlorisocrotonic acid, formed when acetoacetic ester is treated with phosphorus pentachloride and the product poured into water, by the action of sodium amalgam (A. Geuther). It is an oil, possessing a smell like that of butyric acid. It boils at 171.9° C., with partial conversion into crotonic acid; the transformation is complete when the acid is heated to 170-180° C. in a sealed tube. Potassium permanganate oxidizes it to ß[gamma]-dioxybutyric acid.
Methacrylic acid was first obtained in the form of its ethyl ester by E. Frankland and B. F. Duppa (_Annalen_, 1865, 136, p. 12) by acting with phosphorus pentachloride on oxyisobutyric ester (CH3)2·C(OH)·COOC2H5. It is, however, more readily obtained by boiling citra- or meso-brompyrotartaric acids with alkalis. It crystallizes in prisms, which are soluble in water, melt at 16° C., and boil at 160.5° C. When fused with an alkali, it forms propionic acid; with biomine it yields [alpha]ß-dibromisobutyric acid. Sodium amalgam reduces it to isobutyric acid. A polymeric form of methacrylic acid has been described by F. Engelhorn (_Ann._, 1880, 200, p. 70).