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It is significant that in every case studied only the mono addition product of carbon tetrabromide to the olefin was formed; there was prac-

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While it has been known for many years that the isomerization of maleic acid to fumaric acid may be catalyzed by inorganic acids and salts,1 the mechanism of the reaction is still a matter of controversy. It has been suggested2 that in cistrans isomerizations catalyzed by strong acids, one of the doubly bonded carbons shares a pair of electrons with a proton, leaving the other carbon with six electrons free to rotate about a single bond. Dissociation of the proton after rotation would then produce the more stable isomer. However, studies by Horrex,3 using deuterium chloride as the catalyst, eliminated such intermediates. He found that in the isomerization of maleic acid there was no deuterium exchange with the protons attached to the carbons of the double bond. Mayo and Walling4 have recently pointed out that in cases such as this, when a carbonyl group is conjugated with the ethylene linkage, the catalyst may associate with the carbonyl group. The double bond is then assumed to shift during isomerization, leaving an electron deficient carbon and a single bond around which rotation may occur.This mechanism satisfactorily explains Horrex's results.The mechanisms mentioned attribute the catalysis by inorganic acids to their ability to donate protons. It might thus be expected that in aqueous solution strong acids, due to nearly complete ionization, would exert similar catalytic effects. However, studies reported in this communication have shown that this expectation is not realized. It was found, for example, that hydrochloric acid is more than fifty times as effective a catalyst as perchloric acid. These results, together with the reported111 strong catalysis of isomerization by alkali thiocyanates, suggested that the anion also played an important role in the reaction.As a consequence of a study of bromine addition *