Jack Halpern

Abstract Jack Halpern was the leading expert in applying kinetics and thermodynamics to establish the intellectual framework of modern catalytic science and the constituent elementary steps in homogeneous catalysis. He had a most impressive ability to identify the core of a mechanistic problem and then to resolve it using both logic and experiment. His early research on alkene hydrogenation catalysts and, especially, his later work on asymmetric catalysis led to his formulation of the Halpern theorem: namely that the true intermediates along a catalytic cycle exist only fleetingly, and that those species with sufficient stability for direct observation or isolation are usually not active intermediates. Halpern's studies also strongly affected the then nascent field of bioinorganic chemistry by establishing that the reactivity of vitamin B12 cofactor was initiated by homolysis of its cobalt–carbon bond.