Reaction kinetics in liquid ammonia up to 120°C: techniques and some solvolysis and substitution reactions

Abstract

Liquid ammonia is known to be a useful solvent for aromatic nucleophilic substitution reactions of reactive aromatics. Most of the prior work has been carried out at atmospheric pressure and low temperatures. The purpose of this work is to extend the scope of these studies to less reactive haloaromatic substrates that require elevated temperatures to give useful reaction rates. Equipment has been developed to permit the safe study of reactions in liquid ammonia in small (5 mL) stainless steel tube reactors at temperatures up to 120°C (90 bar). Experimental techniques have been developed to permit meaningful sampling of solutions at high temperature and pressure. Sample capture into closed systems, either into a syringe containing quench solution, or directly into an HPLC sample loop, has been used to overcome problems due to flash precipitation and aerosol formation when sampling from high pressure. Activation parameters for the solvolysis of 4-fluoronitrobenzene have been determined over the temperature range 20°-120°C, and the rates of solvolysis of 2-chloropyrimidine, 4,6-dichloropyrimidine and 2-chloro-5-trifluoromethylpyridine have been determined at appropriate temperatures. 2-Fluoropyridine is inert to solvolysis even at 120°C. Rates of some nucleophilic substitutions with sodium triazolate and sodium phenoxide have been measured. Useful selectivities towards nucleophilic substitution can be obtained for the reactions examined, and further work is planned to examine the wider scope of these reactions.

Lignin solubilisation and gentle fractionation in liquid ammonia

“…We expect that at high temperature also more volatiles are produced, which would explain the lowering of the mass balance to 70-85%, depending on the vapour pressure. 37 Note that the nitrogen content also increased. Only one nitrogen-containing compound was identified in the DCM soluble fraction of experiments d1-d3, in particular at high temperature (d2-d3).…”

“…The choice of solvent is important. 47,48 At low pressures on a lab scale, DCM is a suitable solvent since it dissolves well aromatic monomers 37 while dissolving lignin oligomers only to a limited extent, at least in the case of the P1000 soda lignin. However, it is very difficult to find a solvent which separates exclusively phenolic monomers in one single extraction step with high yield.…”

“…We feel that this can be the result of the different chemical behaviour of organic compounds in liquid ammonia, and possibly due to flash precipitation when the liquid ammonia is released. [36][37][38]…”

“…Liquid ammonia has received attention as an alternative to aprotic polar industrial solvents, which have toxicity concerns and are difficult to recycle. 37…”

“…36 It is known that liquid ammonia dissolves both ionic compounds and organic molecules. 37 Ammonia is basic, and has a stronger ionisation capacity than water. 36,38 Thus, carboxylic acids, phenols and imides form ammonium ion pairs in liquid ammonia.…”