The Reaction of Nitriles under High Pressure, I. The Formation of Triphenyl-1,3,5-triazine via the Equilibrium among Benzonitrile, Alcohols, and Benziminoethers

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Benziminoethers were formed from benzonitrile and alcohols to the equilibrium extent, and then 3 mol of benziminoethers were reacted with each other to afford triphenyl-1,3,5-triazine at 2–10kbar and 100–120°C. The equilibrium shifted toward benziminoether with an increase in the pressure. The equilibrium constant decreased progressively in the order of methanol>ethanol≈n-propanol≈n-butanol>isopropanol≈sec-butanol, which accords with the increasing number of substituents on the α-position of the alcohols. The formation of triphenyl-1,3,5-triazine obeyed the second-order reaction mechanism and was also retarded in the same order. The reaction was found to proceed in the following manner: Benziminoethers produced by the addition of alcohols to benzonitrile dimerize through a rate-determining step and then trimerized into triphenyl-1,3,5-triazine. The apparent activation volume of the formation of triphenyl-1,3,5-triazine amounted to −31 cm3 mol−1 at 110°C when methanol was used as the alcohol. The accelerating effect of the pressure on the formation of triphenyl-1,3,5-triazine is concluded to be mainly caused by the shift of equilibrium toward benziminoether. Three isomers of cyanopyridine were converted in good yields into the corresponding triazines under high pressure.
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