Specific acid catalysis inverse solvent isotope effect

An inverse solvent isotope effect (kjDjO] > k[H20]) is indicative of specific acid catalysis. 

The kinetic solvent isotope effects, h.o/ d,o> range between 2 and 4 in the case of compounds 2 (Noyce et al., 1965, 1967 Noyce and Schiavelli 1968b Noyce and De Bruin, 1968) and between 1-7 and 2 0 in the case of compounds 3 (Stamhuis and Drenth, 1963a Drenth and Hogeveen, 1960). The observed values accord with the idea of a rate-limiting proton transfer to the triple bond a small inverse isotope effect would have been observed in the case of specific acid catalysis (Noyce et al., 1967). 

The inverse solvent deuterium isotope effect indicates specific acid catalysis (pp. 1102-4) and the modest negative entropy of activation suggests some bimolecular involvement. There are various mechanisms you might propose but the right one must surely involve cleavage of the three-membered ring in the protonated amine. The second or possibly the third step could be rate-determining. 

The racemizalion doesn t include that last step, rate-determining in the dehydration. Now w. an inverse solvent deuterium isotope effect indicating specific acid catalysis. The loss of wa -. form the same cation is now Ure slow step. The large Hammett p value also suggests the fonr.i the racem/zat/on mechanism... 

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