INFLUENCE OF NUCLEOPHILES ON THE HIGH-TEMPERATURE AQUEOUS ISOMERIZATION OF CIS-CINNAMIC TO TRANS-CINNAMIC ACID

The rate of isomerization of cis- to trans-cinnamic acid in water at 1 95-degrees-C was monitored as a function of added nucleophile. The exp ected nucleophile addition products were not observed but could be syn thesized and, when subjected to the reaction conditions, rapidly conve rted to trans-cinnamic acid. Deuterium labeling studies indicated that proton abstraction was not the rate-determining step, that the alpha- deuterium of alpha,beta-dideuterio-cis-cinnamic acid was rapidly excha nged for an alpha-hydrogen atom without loss of the cis geometry, and that some direct isomerization of cis- to trans-cinnamic acid occurred without the loss of vinyl deuterium atoms. The isomerization appears to involve an addition-elimination mechanism in the case of HO- and HS - ions, with the former more effective. Anthrahydroquinone ion was qui te effective, meaning that it was a superior nucleophile or was promot ing reactions by another mechanism (possibly electron transfer chemist ry). The results have bearing on the rate-determining step in one of t he delignification mechanisms occurring during the pulping of wood.