The efficacy of ‘Claycop’ in the dinitration of toluene
N. Llewellyn Lancaster, Roy B. Moodie* and John P. B. Sandall
Department of Chemistry, University of Exeter, Exeter, UK EX4 4QD
Claycop and acetic anhydride in tetrachloromethane are
modestly catalytic and regioselective in the mononitration
of toluene, but are neither catalytic nor regioselective in the
nitration of 2-nitrotoluene.
The importance of regioselectivity in mononitration of reactive
aromatics and the relative success of recent methods using solid
supports have been recently summarised.1 One such procedure,
using ‘Claycop’ (montmorillonite clay impregnated with copper
nitrate) with acetic anhydride (and if necessary nitric acid) in
tetrachloromethane, has been reported to improve both yields
and selectivities not only in mono-, but also in poly-nitration of
aromatic compounds, relative to the same reactions carried
out under homogeneous conditions.2 As a prelude to a study
of the use of this reagent for polynitration of heteroaromatics,
we reinvestigated the dinitration of toluene. In order to show
whether or not the reaction rate was enhanced by the presence
Fig. 1 Yields as percentages of starting aromatic in the nitration of
of claycop the progress of the reaction with time was investi-
gated. Samples were taken at time intervals, quenched and
analysed by GC.
toluene in tetrachloromethane containing claycop, acetic anhydride and
nitric acid; () 2-nitrotoluene, (᭹) 4-nitrotoluene, (᭜) 2,4-DNT, (᭡)
2,6-DNT
Fig. 1 shows the results for a reaction mixture prepared
as described 2 and reported to give a 95% yield of 2,4-
dinitrotoluene (2,4-DNT) and 2,6-dinitrotoluene (2,6-DNT) in
a molar ratio of 6.6:1 in 4 h. It is clear that under the condi-
tions toluene is rapidly converted into a mixture of mononitro-
toluenes. (3-Nitrotoluene, not shown, was 4% of the early reac-
tion mixture; separate studies showed that its further nitration
was relatively slow and the products did not interfere with the
analyses reported.) Subsequent conversions (Fig. 1) of 2-
nitrotoluene into a mixture of 2,4-DNT and 2,6-DNT and of 4-
nitrotoluene into 2,4-DNT follow a first-order course (rate con-
stant ratio for the two processes was ca. 2.5:1) and dinitration
is almost complete in 4 h.
Since our main interest was in the facilitation or otherwise by
claycop of the second nitration step, we investigated separately
the reaction of 2-nitrotoluene under the conditions described,2
the same reaction but with dichloromethane in place of tetra-
chloromethane, and finally with dichloromethane without
claycop or acetic anhydride but with the same stoichiometric
concentration of nitric acid. Observed first-order rate con-
stants, yields and product ratios are given in Table 1.
It is clear from the results that the use of dichloromethane
rather than tetrachloromethane makes no difference and that
the presence of claycop and acetic anhydride does not affect
significantly the product ratio and does not enhance the rate
constant (indeed the reaction is nearly 4 times faster in their
absence than in their presence).
These reactions were compared with those in which toluene,
rather than 2-nitrotoluene, was the starting material under
otherwise identical conditions. Using the results of Table 1 to
deduce the fate of 2-nitrotoluene, the initial partitioning of the
toluene could be deduced from the observed yields of 2,4-DNT
Table 1 Yields expressed as percentages of initial aromatic in the reac-
tion of 2-nitrotoluene with nitric acid (stoichiometric concentration
4.2 mol dmϪ3) at 25 ЊC
2,4-DNT:
2,6-DNT
kobs/10Ϫ4
Conditions
Yield
sϪ1
CCl4, claycop, acetic anhydride
CH2Cl2, claycop, acetic anhydride
CH2Cl2 (no claycop or acetic
anhydride)
96%
98%
98%
2.1
2.1
2.2
7.7
8.4
30
and 2,6-DNT. Results are given in Table 2. These confirm 3 that
with claycop and acetic anhydride in tetrachloromethane the
para :ortho nitration ratio in the mononitration is somewhat
higher than normal. It is in this step that the higher than nor-
mal 2,4-DNT :2,6-DNT ratio in the overall dinitration is gener-
ated (though in our hands this ratio was 5.9, not 6.6).
The high para :ortho ratio in the first nitration step was not
observed using dichloromethane with or without claycop and
acetic anhydride, nor was it reported for the homogeneous reac-
tion with 0.3 mol dmϪ3 nitric acid in tetrachloromethane4 (see
Table 2). The reactions of toluene in tetrachloromethane with
claycop and acetic anhydride, but in the absence of added nitric
acid, were also investigated. They were not first-order, but
showed a marked induction period (Fig. 2) followed by more
rapid nitration which stopped at the mononitration stage. The
reaction gave an ortho:meta :para ratio of 43:5:52, very similar
to that deduced for the fast mononitration step under the con-
ditions described in Table 2.
The only source of NV is the nitrate in the claycop. If all this
were made available as nitric acid the concentration of nitric
acid in tetrachloromethane would have been 0.48 mol dmϪ3
under which conditions the reaction with toluene (which is
zeroth-order in toluene and fifth-order in nitric acid 4) would
© British Crown Copyright 1997, DERA Farnborough, Hants, UK.
Published with the permission of the controller of Her Britannic
Majesty’s Stationery Office.
J. Chem. Soc., Perkin Trans. 2, 1997
847
Lancaster, N. Llewellyn
