1911 Encyclopædia Britannica/Crotonic Acid
CROTONIC ACID (C4H6O2). Three acids of this empirical formula are known, viz. crotonic acid, isocrotonic acid and methacrylic acid; the constitutional formulae are—
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 β-oxybutyric acid; by heating paraldehyde with malonic acid and acetic acid to 100° C. (T. Komnenos, Ann., 1883, 218, p. 149).
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 α-brombutyric acid. With hydriodic acid it gives only β-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 αβ-dioxybutyric acid.
Isocrotonic acid (Quartenylic acid) is obtained from β-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 βγ-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 αβ-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).