The kinetics and mechanisms of organic reactions in liquid ammonia

Liquid ammonia is a potentially useful solvent for a variety of organic reactions and so understanding the kinetics and mechanisms of these processes is important. In contrast to the hydrolysis rates of the substituted benzyl halides in water which vary 107-fold, the rates of the solvolysis of substituted benzyl chlorides in liquid ammonia at 25 °C have little or no dependence upon ring substituents and vary only 2-fold between 4-methoxy- and 4-nitro- derivatives. The Hammett -value is practically zero, which suggests there is no significant charge developed on the central carbon in the transition state. Activation energies for solvolysis of 4-nitro-, 4-methoxy-, 4-chloro- and unsubstituted benzyl chloride in liquid ammonia vary from 40.3 to 43.8 kJ mol-1 in the order of NO2 < H < Cl < OMe and entropies of activation (S) are very negative varying from -188 J K-1 mol-1 to -202 J K-1 mol-1. The solvolysis rates of benzyl halides in liquid ammonia increase linearly with increasing concentration of potassium perchlorate and ammonium chloride. All of these observations are indicative of a bimolecular concerted SN2 mechanism proceeding through a transition state structure with complementary charge development on the entering and leaving groups and little or no change in charge on the central benzylic carbon. Benzyl chloride undergoes substitution with oxygen and nitrogen nucleophiles also by an SN2 process as shown by the dependence of the rate on the concentration of the nucleophile. A Brønsted type relationship for a series of amine nucleophiles show a nuc of 0.33 suggesting an early type transition state with a small amount of charge development on the amine nitrogen. Contrary to commonly accepted views, it appears that the liquid ammonia behaves like a dipolar aprotic solvent in nucleophilic substitution reactions. The nucleophilic substitution reaction in liquid ammonia shows high (>99%) selectivity towards O-benzylation of phenoxide ion, and with 1,2,4-triazolate anion gives predominantly (92%) substitution in the 1-position