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Chapter 41 Suggested solutions for Chapter 41 367 Mechanism 3 requires the exchange of at least one hydrogen atom with the solvent so, if D₂O were used as the solvent or, better, deuterated starting material were used, the exchange of one whole deuterium atom would indicate mechanism 3 while no exchange or minor amounts from the inevitable enolization would show mechanisms 1 or 2. In mechanisms 1 and 3, the added OH ends up in the acid group but in mechanism 2 it ends up as the phenol. Using labelled or, better, labelling the ester oxygen as would separate mechanisms 1 and 3 from 2. D D H D D D mechanisms 1 or 2 mechanism 3 0 OH 0 OH CO₂ mechanisms 1 or 3 mechanism 2 0 OH = 0 = 180 OH Other experiments we might do could include trying to trap the ketene in a [2 + cycloaddition, perhaps with dihydropyran as in Problem 35.1, studying the reaction by UV, hoping to see the release of p-nitrophenolate in the slow step, changing the structure of the starting material so that one or other of the mechanisms would become more difficult, even measuring the kinetics and studying the effects on the rate of aryl substituents (pp. 1090-100), or looking for a deuterium isotope effect in the labelled lactone. Problem 2 Explain the stereochemistry and labelling pattern in this reaction. t-Bu = 180 t-Bu 0 HOAc 0 enantiomerically pure racemic Purpose of the problem A combination of labelling and stereochemistry reveals the details of an interesting rearrangement. Suggested solution The randomization of the label and the racemization suggest that the carboxylate falls off the allyl cation and comes back again at either end. While they are detached, the distinction between the two ends of both cation and anion disappears as they are delocalized. t-Bu t-Bu t-Bu t-Bu 0 + 0 0 0 0 0 The product is racemic because the two intermediates have planes of symmetry and are achiral. The retention of relative stereochemistry (formation of trans product from trans starting material) could result from stereoselective recombination (the two faces of the allylic cation are not the same) or from the two ions sticking together as an ion pair so that the acetate anion slides across the cation