Erythrogenic acid, C18H26O2, is an acetylenic fatty acid that turns avivid red on exposure to light. On catalytic hydrogenation over a palladium catalyst, 5 equivalents of H2 are absorbed, and stearic acid, CH3(CH2)16CO2H, is produced. Ozonolysis of erythrogenic acid givesfour products: formaldehyde, CH2O; oxalic acid, HO2CCO2H; azelaicacid, HO2C(CH2)7CO2H; and the aldehyde acid OHC(CH2)4CO2H. Drawtwo possible structures for erythrogenic acid, and suggest a way to tellthem apart by carrying out some simple reactions.