Solid-supported aromatic nitrations.
The efficacy of the claycop system in the nitration and the dinitration of toluene has been
investigated.Comparison of regioselectivity and of rate constant with homogeneous
nitric acid nitration was made.
The use of ion-exchanged montmorillonite clay as a catalyst for acetyl nitrate nitration
was studied. The system was found to enhance Para selectivity in the nitration of toluene
and to catalyse the reaction. Additionally, the catalyst was shown to be recyclable. The
reaction of toluene was too fast to allow kinetic study. However, kinetic study was
possible,using chlorobenzene as substrate.Again, the system was found to decrease both
the reaction time and the o-/ p- ratio. It was possible to demonstrate a dependence of
rate constant upon mass of clay. The effect of the counter ion was investigated and the
use of trifluoroacetyl anhydride was studied.
A new system for aromatic nitration has been developed in this work using zeolite with
dinitrogenp entoxide.A series of zeolites were screened in the nitration of onitrotoluene,
with the H-faujasites showing the quickest reaction times and highest
degree of regioselectivity. The silica/ alumina ratio of the faujasite used was varied and
this was shown to have an effect on regioselectivity. However, reactions were too fast
for kinetic study.
The faujasite/ dinitrogen pentoxide system was used in the nitration of some deactivated
benzenes using these substrates allowed kinetic studies to be made and the order in each
component to be determined. It was found that nitration was zeroth order in dinitrogen
pentoxide, that the reaction obeyed a first order rate law, and that the first order rate
constant was proportional to the mass of zeolite used. Comparison of the relative rates
of nitration of 1-chloro-2-nitrobenzene1, -chloro-4-nitrobenzene and nitrobenzeneb y this
system to the relative rate constants of nitronium ion nitration was made. The similarity
suggested that the mechanism of nitration by faujasite/d initrogen pentoxide might
involve nitronium ion transfer.Amongst the H-faujasites of different silica/ alumina ratio, the rate constant was shown
to increase with the aluminium content. A mechanism is proposed in which zeolite
protons (present in proportion to the aluminium atoms) are first replaced by nitronium
ions. The latter are transferred to the aromatic in the rate-determining step.
The use of non-chlorinated solvents and the recycling of zeolite were both studied, and
the nitration of 2,6-dinitrotoluene was attempted.
The use of the faujasite/ dinitrogen pentoxide system was extended to the nitration of
nitrogen containing aromatic heterocycles direct C-nitration of quinolone was not
possible, only N-nitration. However, pyrazole was converted to 1,4-dinitropyrazole
quickly and cleanly under mild conditions. It was shown that the N-nitration occurred
instantly, followed by slower C-nitration. The kinetics of the second nitration were
studied and comparison with deactivated benzenes was made.