The thermal decompositions of carbamates. IV. The reactions of isopropyl N,N-dimethylcarbamate and t-butyl N,N-dimethylcarbamate

Abstract

The thermal decompositions of isopropyl N,N-dimethylcarbamate and t- butyl N,N-dimethylcarbamate are shown to occur over the temperature range 485-602 K through the reactions

Me₂NCO₂Prⁱ → Me₂NH+CO₂+MeCH=CH₂

Me₂NCO₂Bu^t → Me₂NH+CO₂+Me₂C=CH₂

which are described as first-order unimolecular processes for which the rate equations are

isopropyl k = 10^13.04exp(-181209/8.314T) s^-1

 t-butyl k = 10^12.87exp(-157904/8.314T) s^-1

For both carbamates these rate equations describe the rates of formation of the amine and the appropriate alkene but apparently overestimate the rate of carbon dioxide formation. The discrepancy in the carbon dioxide data is explained in terms of the formation of an amine-carbon dioxide adduct during the condensation stage of the analyses. The adduct is described as an ammonium carbamate which undergoes hydrolysis in solution to free the original amine. The existence of transesterification in the gas phase is ruled out.