High-temperature photolysis and the pyrolysis of formamide vapour, and the thermal decomposition of the carbamyl radical

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Abstract
The photolysis of formamide vapour has been studied at temperatures from 262 to 500°C, at formamide pressures ranging from 6 to 33 Torr. A radical-chain reaction was observed, yielding CO and NH3 in equal amounts. It was concluded that the chain was initiated by photodissociation of formamide and propagated by the steps, NH2+ NH2CHO → NH3+ NH2CO and NH2CO + M → NH2+CO + M. The kinetics of this chain reaction were examined, including its dependence on temperature, light intensity, formamide pressure, and pressure of added inert gas. Under suitable conditions, the decomposition of the carbamyl (NH2CO) radical was rate controlling. It was also pressure dependent, and limiting high and low pressure Arrhenius expressions were obtained for the rate constant: k∞= 3.5 × 1012 exp (–25 000/RT)s–1, k0= 6.2 × 1013 exp (–21 700/RT)(M s)–1. From these data, D(NH2—CO) was estimated to be 21 and ΔH° f(NH2CO) to be –7 kcal/mol. A comparison is made with other RCO radicals. A relatively slow thermal decomposition of formamide vapour accompanied the photolysis, yielding CO and NH3 as major products and H2 as a minor one. Rates of production of CO and H2 were approximately first-order in formamide pressure and were given by the rate constants kCO= 3.9 exp (–15 000/RT) s–1, kH2 = 0.30 exp (–15 000/RT)s–1. This decomposition is probably a surface reaction.

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