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

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 k_CO= 3.9 exp (–15 000/RT) s^–1, k_H2= 0.30 exp (–15 000/RT)s^–1. This decomposition is probably a surface reaction.