 Angelic and tiglic acid are "cis" and "trans" isomers of 2-methyl-2-butenoic acid, respectively.The former can be entirely converted to the latter by boiling for about 40 hours, by reaction with sulfuric and other acids, by heating with a base to a temperature above 100 °C, or simply by storing the acid for about 25 years.The reverse transformation occurs much less readily; it can be induced by ultraviolet light, but not with visible light.