Kinetics and reaction mechanisms: selected examples from the experience of forty years

Kinetics of parallel reactions contribute to the elucidation of poly-step mechanisms. Competing pathways in the reaction of benzenediazonium ion with azide anion helped to establish the occurrence of phenylpentazole. Steady state kinetics clarified the competition between the interception of an intermediate and its return to the starting material. The DielsAlder reactions of cyclooctatetraene and of diphenylbenzocyclobutenes, and the thermal equilibration of aziridines with azomethine ylides serve as examples. Kinetics and trapping experiments revealed a four-step sequence for the conversion of bromocyclooctatetraene to trans-8-bromostyrene. INTRODUCTION Physical organic chemistry is no longer the height of fashion as it has been 25 years ago. In the arsenal of methods and tools, chemical kinetics plays a kind of Cinderella role today. To the author's opinion, to this very day kinetics has been the most important instrument in the elucidation of reaction mechanisms. Why rate measurements ? In their pioneering work on cationic rearrangements and substitution mechanisms, Hans Meerwein and Christopher Ingold had demonstrated the power of the kinetic argument. At the time the author realized that many further reaction mechanisms required rate studies to be clarified beyond the stage of vagueness in which they were hovering. Unifying concepts, capable of bringing order into a chaos of facts, were expected to emerge from the exploration of reaction mechanisms. When foreign journals became accessible again in 1948 /50 , the author learned that many young scientists world-wide were directing their research activities toward mechanistic and physical organic chemistry. The application of chemical kinetics to mechanistic problems will be limited to selected examples of competing pathways. Competition belongs to the essence of life. Charles Darwin's "survival of the fittest" is evolution promoted by competition. In 1776 Adam Smith published his famous book on the wealth of nations. His dream is now reality: free enterprise, and compet i t ion have become the pillars of world economy and social structures. Competition pervades our individual life: at school, on the job, in sports, in scientific research. T h i s l e c t u r e i s d e d i c a t e d t o P r o f e s s o r P a u l D . B a r t l e t t , o n e of t h e p i o n e e r s of p h y s i c a l o r g a n i c c h e m i s t r y , on t h e o c c a s i o n of h i s 8 0 t h b i r t h d a y .