Properties
Angelic acid is a volatile solid with a biting taste and pungent sour odor. It crystallizes in colorless monoclinic prisms which dissolve rapidly in alcohol or hot water and slowly in cold water. Angelic and tiglic acid are trans and cis 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. The conversion rate is low and only 0.36 g of angelic acid could be obtained from 13 g of tiglic acid after 43-day irradiation with a 500-watt lamp. Despite being the trans isomer, angelic acid has a lower melting point and higher acid dissociation constant than tiglic acid, although the opposite trends are usually observed.
Angelic acid reacts with hydrobromic acid and bromine producing bromovaleric and dibromovaleric acids, respectively, with a yield of 60–70%. Chlorovaleric and iodovaleric acids are obtained using hydrochloric and hydroiodic acids, respectively. The salts of angelic acid are called angelates. Angelates of alkaline earth metals M have a general formula M(C5H7O2)2 and form white, water-soluble crystals.
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