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Originally appearing in Volume V07, Page 511 of the 1911 Encyclopedia Britannica.
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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).
End of Article: CROTONIC ACID (C4H6O2)

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