PROPIOLIC ACID, CH⋮C·CO2H, acetylene mono-carboxylic acid, an unsaturated organic acid prepared by boiling acetylene dicarboxylic acid (obtained by the action of alcoholic potash on dibromsuccinic acid) or its acid potassium salt with water (E. v. Bandrowski, Ber., 1880, 13, p. 2340). It forms silky crystals which melt at 6° C., and boil at about 144º C. with decomposition. It is soluble in water and possesses an odour resembling that of acetic acid. Exposure to sunlight converts it into trimesic acid (benzene-1.3.5-tricarboxylic acid). Bromine converts it into dibromacrylic acid, and it gives with hydrochloric acid β-chloracrylic acid. It forms a characteristic explosive silver salt on the addition of ammoniacal silver nitrate to its aqueous solution, and an amorphous precipitate which explodes on warming with ammoniacal cuprous chloride. Its ethyl ester condenses with hydrazine to form pyrazolone (R. v. Rothenburg, Ber., 1893, 26, p. 1722). Phenylpropiolic acid, C6H5C⋮C·CO2H, formed by the action of alcoholic potash on cinnamic acid dibromide, C6H5·CHBr·CHBr·CO2H, crystallizes in long needles or prisms which melt at 136–137° C. When heated with water to 120° C. it yields phenyl acetylene C6H5·C⋮CH. Chromic acid oxidizes it to benzoic acid; zinc and acetic acid reduce it to cinnamic acid, C6H5·CH:CH·CO2H, whilst sodium amalgam reduces it to hydrocinnamic acid, C6H5·CH2·CO2H. Ortho-nitrophenylpropiolic acid, NO2·C6H4·C⋮C·CO2H, prepared by the action of alcoholic potash on ortho-nitrocinnamic acid dibromide (A. V. Baeyer, Ber., 1880, 13, p. 2258), crystallizes in needles which decompose when heated to 155–156° C. It is readily converted into indigo (q.v.).