The thermal decomposition of 4-bromobutyric acid in the gas phase: A quantum chemical theory calculation

The gas-phase elimination kinetic of 4-bromobutyric acid to give butyrolactone, and hydrogen bromide was studied using Density Functional Theory (DFT) and Moller-Plesset Perturbation Theory of Second Order (MP2) to investigate the more reasonable reaction mechanism. Good agreement of calculated activation parameters with the experimental values was obtained when using PBEPBE/6-31++G(d,p) level of theory. Analysis of the calculated thermodynamic and kinetic parameters suggested the reaction mechanism is unimolecular, with involvement of the hydroxyl oxygen of the carboxylic moiety of the substrate assisting the exit of bromide in nucleophilic substitution. The alternate mechanism with the participation of the carbonyl oxygen in a slow step to give an intimate ion-pair intermediate was disregarded due to the high energy of activation. Bond order analysis shows the process is dominated by the breaking of the C-Br bond. The reaction can be described as unimolecular and moderately non-synchronous process.