| The nitration of benzene and mono-substituted benzenes was investigated by electronic structure theory. The nitronium ion NO2+ is the electrophile for the electrophilic attack of the aromatic ring. The detailed mechanism of benzene nitration was carried out by CCSD(T) calculations. This calculation approaches the computer power limit today. The potential energy profile was carefully plotted by examining all possible geometries and was confirmed by intrinsic reaction coordinate studies. A pi-complex as well as a sigma-complex was formed in the initial reaction stages, which were about 28 kcal/mol lower than the total energy of separated reactants. The sigma-complex, also called the Wheland intermediate, was converted into a protonated nitrated benzene product through a two step hydrogen migration. The single step migration for this process was ruled out as a probable reaction path by its high energy barrier of 20 kcal/mol.; The orientation of nitration in a series of mono-substituted benzenes was studied next. This work elucidated the basic relationship between the affinity of the substituted group and the product orientation. The sigma-complex was the predecessor to form the product. A Boltzmann distribution was applied to calculate the electrophiles orientation in the products distribution. The results agreed qualitatively with experimental observations. |
