Phenyl isocyanate, reaction rates with methanol

The reaction of phenyl isocyanate with methanol in dibutyl ether at 20°C was found to be second order, but the value of the rate coefficient increased slowly with time, indicating catalysis by the product of the reaction. The addition of methyl carbanilate to the initial reaction mixture increased the rate of reaction, confirming catalysis by the urethane, a weak base. The rate coefficient for the initial uncatalysed reaction was 0.28 x lO" 1 mole sec . 

Other important features of the reaction were also established. Primary alcohols reacted faster than secondary, which were much more reactive than tertiary alcohols. Electron-withdrawing substituents on the benzene ring of phenyl isocyanate enhanced the reactivity, while electron-releasing groups retarded it. The reaction was faster in non-polar than in polar solvents. Alcohol-solvent association was found when dibutyl ether was the solvent, based on infrared data. This association was considered to reduce the rate primarily by reducing the magnitude ofk. The activation energy for the uncatalysed reaction of phenyl isocyanate with methanol in dibutyl ether was 10 kcal mole , and that for the triethylamine-catalysed reaction was 5 kcal mole . ... 

On the other hand, there was little correlation between the initial rate of the phenyl isocyanate-methanol reaction and the hydrogen bonding capacity of the solvent. With the exception of nitrobenzene and dioxane, the logarithms of the apparent second-order rate constant were found to decrease linearly with an increase in the quantity D/(yiD -j- 1), where D is the dielectric constant of the solvent. 

The initial series of runs were carried out by these authors in dibutyl ether solution with phenyl isocyanate and methanol in the presence and absence of triethylamine and of other tertiary amines. The reaction was found to follow second-order kinetics, first order with respect to the isocyanate and first order with respect to the alcohol, the second-order rate constant 2 being given by the relation... 

Table 6.2.3 compares the relative reaction rate of phenyl isocyanate with methanol. 

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