Ammonium amalgam, decomposition

Baranski and Lu [209] have carried out, applying microelectrodes, voltammetric studies on ammonium amalgam in propylene carbonate solutions at room temperatures. The sweep rates up to 80 V s were appropriate for the analysis of the formation kinetics of this compound. Experimental and numerical simulation results have shown that ammonium amalgam was formed via fast charge-transfer process and its first-order decomposition was characterized by the rate constant of about 0.6 s . Diffusion coefficient of NH4 radical in mercury was estimated to be about 1.8 X 10 cm s k The formal potential of NH4+ (aq)/NH4(Hg) couple was determined as—1.723 V (SHE). 

In extending our study of chemical kinetics, we sought systems as far removed as possible from ordinary aqueous and organic solutions. The decomposition of ammonium amalgam... 

The facts are readily explained on the theory that decomposition of the ammonium amalgam occurs very slowly as an homogeneous reaction at temperatures below zero and that a rapid, heterogeneous reaction takes place at the surface of little droplets of liquid ammonia which are dispersed throughout the mercury. The surface tension of the mercury is sufficient to cause liquefaction of the first ammonia which is liberated. The autocatalytic effect occurs when the reaction has had time to build up these droplets of liquid ammonia. 

It was found impossible to measure the rate of decomposition by the evolution of gases because the release of these gas bubbles is very slow and erratic. The course of the reaction was followed by analyzing samples for the ammonium ion. Small amounts of the decomposing amalgam were forced through a capillary tube into a chilled solution of an iodate. The ammonium reacted with iodate ion to give iodide ion. The solution was then acidified with acetic acid and the iodine distilled out, collected and titrated with sodium thiosulfate. The method was checked with samples... 

The reductive cleavage of quaternary ammonium salts to give a tertiary amine and a hydrocarbon by reaction with sodium amalgam in hydroxylic solvents is called the Emde degradation. However, saturated hydrocarbons are not cleaved under these conditions. Grovenstein suggested that the reason for this unreactivity is due to the fact that the sodium reacts with the alcohol much faster than with the ammonium salts. However, by operating in dioxane or dioxane-alcohol mixtures, the dealkylation reaction could be accomplished. The products of the decomposition of either tetramethylammonium chloride or bromide are methane, ethylene, trimethylamine, and dimethylethylamine. 

---