A computational study on the mechanism and the kinetics of urethane formation

The reaction of n-butanol with several aromatic diisocyanates - 2,4 toluene diisocyanate (2,4 TDI), 2,6 toluene diisocyanate (2,6 TDI), 2,4′ dibenzyl diisocyanate (2,4′ DBDI), 4,4′ stilbene diisocyanate, (4,4′ SBDI), m-phenylene diisocyanate (m-PDI), and p-phenylene diisocyanate (p-PDI) - resulting in the production of urethane is studied by using density functional theory (DFT) calculations (B3LYP/6-31 + G(d,p)). The addition of alcohol to isocyanate follows either a concerted or stepwise path. The obtained product differs in two mechanisms. Single point solvent calculations were carried out in benzene, by means of the polarizable continuum model (PCM) at the B3LYP/6-31 + G(d,p) level of theory. The calculated free energy profiles showed that the concerted path is more likely to occur than the stepwise route. Natural bond orbital analysis was performed to provide an insight into charge delocalization and the stabilization energies of the molecules. The calculated rate constant ratios (k1/k2) of the two consecutive alcoholysis reactions of aromatic diisocyanates agree strongly with the experimental results.