REARRANGEMENT OF 2-ARYL-5-ALKOXY-1,3,4-OX ADAZOLES IN A MELT - DEMONSTRATION OF AN ISOKINETIC RELATIONSHIP

Kinetic data and activation parameters are reported for the thermal re arrangement of seven 5-methoxy-2-aryl-1,3,4-oxadiazoles and three 5-et hoxy-2-aryl-1,3,4-oxadiazoles 3 in the melt. This reaction is a model for the thermal, uncatalyzed, 1,3-rearrangement of alkyl-imidates into N-alkyl amides. The rate law In (1 + root 1 - (1 - alpha)(2)/1 - alph a = k(obs) t, deduced from a plausible double ionic chain mechanism, i s well obeyed up to 50-70% completion. In the case of 5-methoxy deriva tives, a statistical analysis of experimental results demonstrates the existence of two different isokinetic relationships, corresponding to the same isokinetic temperature. Correlation between the activation e nergy and the sigma(alpha) and sigma(F) Taft parameters divides compou nds 3 into the same two groups : compounds with X = H, OCH3, CN, NO2 a re more reactive than those with X = F, CI, Br. Kinetic constants for the analogous rearrangement of 5-ethoxy derivatives confirms that this rearrangement occurs via a S(N)2 mechanism.