Selective Nitration of Toluene using Nitrogen(V) Oxide in Dichloromethane and Catalysts

A series of toluene nitration reactions was carried out using a nitrogen(V) oxide (N₂O₅)solution in dichloromethane (CH₂Cl₂), with and without the addition of solid catalysts. N₂O₅ in CH₂Cl₂ was found to be a more active and selective mixture compared to 100% nitric acid and a mixture of nitric acid(V) in CH₂Cl₂. Temperature has a significant effect on the composition of the products of nitration of toluene with N₂O₅ in CH₂Cl₂; as the reaction temperature was decreased, the amount of meta-nitrotoluene decreased to approx. 1-1.2%, at a nitration temperature below -40 °C. The use of molecular sieves with toluene adsorption resulted in a quite powerful para effect, demonstrated by a significant increase in the amount of the para isomer, while reducing the ortho- and meta-isomers. The most pertinent effect was obtained when using 13X molecular sieves. In addition to improving the selectivity, the use of molecular sieves also leads to a higher degree of nitration of toluene (toluene/N₂O₅ = 1:1) due to the resulting nitric acid being absorbed by the pores of the molecular sieve.