Density functional theory has been employed to investigate the effects of hydrogen bonding on the aminolysis of 6-chloropyrimidine, i.e., the reaction of NH3 with 6-chloropyrimidine to give HCl and 6-aminopyrimidine. The isolated aminolysis of 6-chloropyrimidine requires 138.1 kJ mol(-1). If the electrondonating species H2CO is hydrogen bonded to the NH3 moiety during aminolysis, the barrier is I educed to 112.2 kJ mol(-1). When H2NCHO is added to NH3 + 6-chloropyrimidine, the electron-donating -CHO group is able to hydrogen bond to the NH3 moiety while the electron-accepting -NH2 group hydrogen bonds to the N of the pyrimidine ring that is adjacent to the carbon at which substitution occurs. This results in the barrier to aminolysis of 6-chloropyrimidine being reduced to 95.3 kJ mol(-1). Thus, the aminolysis of 6-chloropyrimidine, as mediated by hydrogen bonding, provides a clear and simple example of the catalytic possibilities of well-chosen hydrogen bonding.