Chemistry Reference
In-Depth Information
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
10
20
30
40
50
60
70
Time (min)
Figure 3.8
Plot of (1
+
log A) versus t (min) for anthranilic acid.
of the products from the acid is often a difficult task and energy consuming process that habitually implies a
basic aqueous work up. Additionally, sulfuric acid is corrosive in nature and is difficult to transport and handle.
Naturally, it has been proposed to develop an alternative green chemical pathway to obtain nitro compounds
under ambient conditions. Thus photochemical aromatic nitration of anthranilic acid has been carried out in
the presence of ultraviolet radiations and the formation of the product could be studied spectrophotometrically.
Green laboratory proposal
[22]
0.1370 g of anthranilic acid was dissolved in 100 ml of doubly distilled
water. 0.690 g of sodium nitrite and 0.0680 g of sodium formate were dissolved separately in 100 ml of doubly
distilled water. All these three solutions were used as stock solutions; of which 6 ml of anthranilic acid solution,
8 ml of sodium nitrite solution and 0.7 ml of sodium formate solution were mixed together, so that the
concentration of each of them in the resulting solution was 2.4 × 10
−3
M, 3.2 × 10
−3
M and 2.8 × 10
−4
M
respectively. The reaction mixture was exposed to ultraviolet lamp (254 nm). The optical density of this
solution at different time intervals was determined at the wavelength of 380 nm for anthranilic acid with the
help of a spectrophotometer. Some control experiments were also carried out and it was confirmed that light
is necessary for photochemical nitration. The major product could be identified by gas chromatography
combined with mass spectrometry as 2-amino-5-hydroxy-3-nitrobenzoic acid. The results of a typical run for
photochemical nitration show absorbance (A) values increase with time of exposure which indicates the
formation of nitro compounds. A plot of (1
logA) versus exposure time was found to be linear (Figure 3.8)
and hence this reaction follows pseudo-first order kinetics (in two stages: the first stage being faster than
second stage). The rate constant was determined by the expression, k
+
=
2.303 × slope and the values were
k
1
=
1.228 × 10
−3
s
-1
, k
2
=
1.150 × 10
−4
s
−1
.
Comments
In the reaction shown in Scheme 3.3 formate ions were used as
·
OH radical scavengers in order
to increase the rate of nitration, but since
·
OH radicals are very reactive and present in higher concentrations
than
·
NO
2
radicals, so -OH group is also introduced in the ring even in the presence of formate ion.
Nevertheless, ordinary nitration of aromatic system is accompanied by use of corrosive nitric and sulfuric
acids and liberation of hazardous brown nitrous fumes which add to environmental pollution. The present
work reports a facile route for nitration. Although the yield of the product is 42
%
but the photochemical
nitration of aromatic compounds by alkali nitrite provides an eco-friendly route.
