CROTONIC

ACID (C4H6O2)  . Three acids of this empirical formula are known, viz. crotonic acid, isocrotonic acid and methacrylic acid; the constitutional formulae are HC•CH3H' CHsCH ' CH2 . C0 H' 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 STEREOCIEMISTRY) . 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 boilhig allyl cyanide with caustic potash; by the distillation of 0-oxybutyric acid; by heating paraldehyde with malonic acid and acetic acid to 100° C . (T . Komnenos, Ann., 1883, 218, p . 149) . CH2(000H)2+CH 3CHO->CH 3 • CH: C(COOH)2-> CH s• CH:CH• COOH; or by heating pyruvic acid with an excess of acetic anhydride and sodium acetate to 16o-18o° 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 Too° C., it gives chiefly a-brombutyric acid . With hydriodic acid it gives only 0-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 a8-dioxybutyric acid . Isocrotonic acid (Quartenylic acid) is obtained from (3-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 17o-18o° C. in a sealed tube . Potassium permanganate oxidizes it to 0y-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 ob-tained 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 bromine it yields a(3-dibromisobutyric acid . Sodium amalgam reduces it to isobutyric acid . A polymeric form of methacrylic acid has been described by F . Engelhorn (Ann., 188o, 200, p . 70) .