Hydrogen-oxygen reactions mercury-sensitized reaction

In this article both unsensitized and mercury vapor sensitized reactions are discussed. The plan has been to proceed from the chemically simplest system, oxygen molecule, to the chemically most complex, ozone plus hydrogen peroxide. Unless otherwise stated, thermochemical values have been taken from compilations of the National Bureau of Standards (68) and are reported in kcal. for the reaction represented in mole amounts. Conversion factors for energy units, spectroscopic notation, and unless noted, spectroscopically based values have been taken from... 

It is not possible from the information available to choose between a number of mechanisms which may lead to hydrogen and oxygen formation. An interesting difference between the results obtained in the sensitized reaction and those obtained in the unsensitized reaction is the failure to find H202 in the sensitized reaction in the flow experiments. However, it may be observed that the low quantum yield of water decomposition indicates that there must be a relatively large quantity of mercury vapor carried over to the freeze out traps. A relatively small amount of H202 could react completely with mercury when the contents of the trap are thawed for analysis (88). 

This leaves unexplained the anomalously high reactivities for both mercuriation and protiodemercuriation. For the latter, it may be relevant that with thiophene and substituted thiophenes kinetics were second order for only 20% of reaction, and there were marked differences in entropies of activation for the heterocyclic and phenyl derivatives. Even for benzenoid compounds the reaction is not well behaved as (1) the Hammett correlation shows marked curvature, suggesting that reactive compounds undergo a different mechanism to unreactive ones (2) the reaction is sensitive to oxygen and (3) it is sensitive to added chloride ion, due possibly to salt effects and reaction via hydrogen chloride ion pairs (65AJC1521). A reexamination of both mercuriation and protiodemercuriation of thiophene would seem desirable. 

Mercury-sensitized photochemical oxidation has been studied by many authors for hydrogen, methane, ethane, propane, etc. The main product of hydrogen oxidation at 14 °C has been found to be [24] H2O2 in a yield > 85%. This supports the conclusion that oxygen atoms are not the primary products of the interaction between an excited mercury atom and an O2 molecule since the main reaction product would then be water. 

No peroxide has found practical use as an explosive, a consequence of the weak oxygen-oxygen bond leading to poor thermal and chemical stability and a high sensitivity to impact. Hexamethylenetriperoxidediamine (HMTD) (46) is synthesized from the reaction of hexamine with 30 % hydrogen peroxide in the presence of citric acid. HMTD is a more powerful initiating explosive than mercury fulminate but its poor thermal and chemical stability prevents its use in detonators. 

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