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Conclusions
The experimental data reported in this paper allow exclusion of a
4CP nitration process that takes place via ∑OH + ∑NO2. The same
process could be excluded for the photonitration of phenol.16 In
contrast, nitration of both phenolic compounds should involve
∑NO2 alone. There are two possible pathways that could account
for the nitration of phenol and of 4CP by ∑NO2. Path A proceeds via
H abstraction by ∑NO2 on the phenolic oxygen to give HNO2 and
the corresponding phenoxy radical. The latter reacts with another
∑NO2 to finally yield the nitrophenol. Path B involves addition of
∑NO2 to the aromatic ring, followed by H abstraction operated by
∑
∑
O2, NO2 or OH to give the nitrophenol. Quantum mechanical
calculations, carried out following the DFT theory, indicate that
path A would be strongly favoured over B because of the much
lower activation energy barrier. Moreover, the energetics of path
B suggests that the reaction intermediate would be decomposed
back to the reactants far more easily than it can evolve into
the nitrophenol product. Finally, path A correctly predicts that
the nitration of 4CP is faster than that of phenol,16 because the
chlorine substituent on the ring makes the phenolic hydrogen
more acidic and favours the formation of the phenoxy radical.
An additional issue is the lack of anisole nitration in the presence
of nitrate and/or nitrite under irradiation. It is consistent with the
occurrence of path A, because in anisole the phenolic hydrogen
is replaced by a methyl group. Nitration of 4CP via Path A also
allows the elaboration of a kinetic model that is in good agreement
with the experimental data.
15 S. Chiron, L. Comoretto, E. Rinaldi, V. Maurino, C. Minero and D.
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16 P. R. Maddigapu, D. Vione, B. Ravizzoli, C. Minero, V. Maurino,
L. Comoretto and S. Chiron, Laboratory and field evidence of the
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17 P. J. Squillace, J. C. Scott, M. J. Moran, B. T. Nolan and D. W. Kolpin,
VOCs, pesticides, nitrate, and their mixtures in groundwater used for
drinking water in the United States, Environ. Sci. Technol., 2002, 36,
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Nitration of phenolic compounds would thus involve two ∑NO2
radicals, which has implications for the use of phenol as a probe
molecule for nitrogen dioxide. Indeed, phenol photonitration can
be adopted to derive the formation rate of nitrogen dioxide in an
irradiated sample. The ∑NO2 formation rate calculated under the
∑
hypothesis that two NO2 are involved in nitration is exactly the
∑
double than for the case where nitration involves just one NO2.
Therefore, the involvement of two ∑NO2 in phenol photonitration
via Path A should be taken into account when using such a reaction
as a ∑NO2 probe in solution.
18 J. Mack and J. R. Bolton, Photochemistry of nitrite and nitrate in
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19 C. Minero, V. Maurino, E. Pelizzetti and D. Vione, Assessing the
steady-state [∑NO2] in environmental samples. Implication for aromatic
photonitration processes induced by nitrate and nitrite, Environ. Sci.
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Acknowledgements
Financial support from PNRA-Progetto Antartide and MIUR-
PRIN 2009 (project n◦510, area 02) is gratefully acknowledged.
We are thankful to Prof. Giovanni Ghigo for useful discussion.
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