Carbamates and monothiocarbamates of basic aliphatic amines undergo rate-determining C-N cleavage after a rapid equilibrium protonation step, as shown most directly by inverse solvent deuterium isotope effects of kD/kH = 3.6-4.8 for 0.0-and 0,s-N-n-butylcarbamates and by rapid acid-catalyzed exchange of the N H proton of n-BuNHCOS-with k e x c h = 5 X lo7 M-I s-I. The lifetimes of substituted N-protonated carbamates have been estimated to range down to <1O-Io s. It is concluded that general acid catalysis of the cleavage of carbamates of weakly basic anilines ( a = 0.84) occurs through an enforced preassociation mechanism with hydrogen bonding to the leaving protonated nitrogen atom and C-N cleavage in the rate-determining step. There is more proton transfer in the transition state (larger a ) and a smaller / ?I, with more basic amines and upon substitution of sulfur for oxygen. The low pK, values of N-protonated carbamates and monothiocarbamates illustrate the strong electron-accepting ability of -COO-and -COS-.
average of two independent determinations. Acid concentrations were measured by titrating aliquots of indicator solutions; the primary determination was equivalents of acid per kilogram of solution, and this was then converted to equivalents of acid per liter when needed by calculations using the recently measured12 densities of these solutions.Principal features of the ultraviolet spectra of the carbazole bases and their conjugate acids are given in Table II.
The heat of formation of the Meisenheimer complex formed from sodium thiophenoxide and 1,3,5-trinitrobenzene has been measured in methanol-DMSO mixtures. These data were combined with free energies from the literature to calculate AS for this reaction. Except at very high DMSO concentrations, this reaction is isoentropic across the methanol-DMSO solvent continuum. The heats of transfer of the starting materials and product from methanol to methanol-DMSO mixtures have been measured. The heat of reaction between thiophenoxide and 1,3,5-trinitrobenzene becomes more exothermic as the concentration of DMSO increases.
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