Synthesis of Isocyanates

Abstract

The interaction of carbon monoxide with organic nitro compounds is a thermodynamically highly favourable process. For reaction (1):

$$PhN{O_{2\left( 1 \right)}} + 3C{O_{\left( g \right)}} \to PhNC{O_{\left( 1 \right)}} + 2C{O_{2\left( g \right)}}$$

(1)

a ΔH⁰ = − 128.8 Kcal/mol can be calculated assuming ΔH⁰ _f = − 16.1 KcaUmol for PhNCO₍₁₎ [1]. Of course, the carbonylation of a dinitro derivative is an even more exothermic reaction (ΔH° = − 228 Kcal/mol for 2,4-dinitrotoluene to give 2,4-toluenediisocyanate [2]). However, in the absence of a catalyst, this reaction requires drastic conditions in order to occur [3]. Moreover, azobenzene, instead of phenylisocyanate, was obtained from nitrobenzene in 97 % yield only at 250 °C and under 3000 atm of carbon monoxide [3]. However, it should be noted that some Fe(CO)₅ was surely present under so forcing conditions, since this last complex has been reported to be formed by reaction of CO with the stainless steel walls of an autoclave even under far milder conditions [4]. Note also that Fe(CO)₅ is now known to be a selective catalyst for the reduction of nitrobenzene to azobenzene ([5], see also Chapter 4), although this fact was not known yet when the original report on the “uncatalysed” reaction was published.