Correspondence
Comment on “Formation of Nitroaromatic
Compounds in Advanced Oxidation Processes:
Photolysis versus Photocatalysis”
SIR: The recent paper by Dzengel et al. (1) is a valuable
contribution to the knowledge on scope and limitations of
advanced oxidation technologies for the cleanup of polluted
groundwaters, surface waters, and process waters. The
treatment of waters containing phenol(s) and nitrate can
lead to the formation of nitrophenols. For the base case,
with phenol and nitrate at pH )5, the authors discuss possible
mechanismsscentered on eqs 13 and 14 in their paper. The
authors rejected a pathway via addition of OH to phenol
FIGURE 1. Chemical structures of 1-3.
followed by combination with NO
2
radical (their reaction
4
b) on good grounds. We also note that this mechanism
would result in the formation of 3-nitrophenol as a rel-
atively important product, by OH addition at an ortho or
2
para position, entrance of NO next to this OH, and loss of
water; however, this 3-isomer has not been observed. So, the
authors state that “reaction 4b indeed can be neglected in
comparison to reaction 13”, meaning that the addition of a
NO
first step.
We wish to point out that this cannot possibly be true on
thermochemical grounds. σ-bonding of a NO radical to a
benzene ring would imply a ‘negative’ C-N bond strength
2
radical to a phenol molecule is believed to be the real
2
FIGURE 2. Thermochemistry of radical addition of NO
(numbers in parentheses are estimated values, in kcal mol ).
2
to benzene
-
1
-1
of at least ∼10 kcal mol
.
This can be underscored by using known gas-phase
thermochemical data (2, 3), as follows (Figure 1). For 1a, the
standard enthalpy of formation at 298 K, ∆
f
H°298, is 50 kcal
-
1
mol (4). Common thermochemical approaches lead to a
-
1
difference in ∆
one arrives at ∆
above the sum of the values for benzene (3a, 20 kcal mol-1)
f
H°298 between 2a and 2b of -8 kcal mol , so
f
-1 -1
H°298 (1b) ) 42 kcal mol , 14 kcal mol
-
1
and NO
Note that the mentioned value for ∆
heat of addition for an H atom, ∆ H°298 ) 52 kcal mol , to
2
(8 kcal mol ).
f
H°298 (1b) implies a
-
1
f
the ipso position in nitrobenzene (3b, ∆ H°298 ) 16.5 kcal
f
-
1
-1
mol ) of 26.5 kcal mol (as shown in Figure 2). Corre-
sponding values for other Z range from around 18 (Z ) F,
-
1
NH , OH), via ca. 22 (Z ) H, alkyl) to around 25 kcal mol
2
for Z ) Cl, Br, and I. Such differences can be understood on
structural-chemical grounds (interaction of Z, especially in
FIGURE 3. Proposed mechanism of formation of 2- and 4-nitrophenol.
3
, with the benzene ring).
Introducing a para-OH substituent in 3b and in the
corresponding 1b certainly will bring some increase in the
products are long-lived; not so, of course with respect to
enolization (by rapid proton transfers with water) giving 2-
and 4-nitrophenol (Figure 3).
radical stabilization enthalpy of the latter (which amounts
-
1
to ca. 21 kcal mol in the base case 1a), but this will amount
-
1
at most to 5 kcal mol (5). Hence, the addition of NO
2
to
Literature Cited
-
1
phenol shown in eq 13 of ref 1 will be at least ca. 9 kcal mol
(1) Dzengel, J.; Theurich, J.; Bahnemann, D. W. Environ. Sci.
Technol. 1999, 33, 294-300.
endothermal. Transfer from the gas phase to aqueous solu-
tion will not change much on thermochemical balances
either. So, the thermochemistry of 1b or its hydroxyl
derivatives in aqueous solution will differ somewhat from
the gas phase, but their formationslet alone with sufficient
(2) NIST Chemistry WebBook; Mallard, W. G., Ed.; NIST Standard
Reference Database 69sNovember 1998 Release; National
Institute of Standards and Technology: Gaithersburg, MD,
1
998.
(
3) Stull, D. R.; Westrum, E. F., Jr.; Sinke, G. C. The Chemical
Thermodynamics of Organic Compounds; Wiley: New York,
lifetime to allow bimolecular reaction with dissolved O
to be excluded.
2
sis
1
969.
The question remains, then, how are nitrophenols really
formed? Accepting that both phenoxyl radicals and NO are
(4) Tsang, W. J. Phys. Chem. 1986, 90, 1152-1155.
(5) McMillen, D. F.; Golden, D. M. Annu. Rev. Phys. Chem. 1982,
33, 493-532.
2
present, we propose simply combination of theseswith a
rate constant near the diffusion limitsto make in first instance
the keto forms of 2- or 4-nitrophenol. The corresponding
C-N bond strengths will amount to roughly 45 and 40 kcal
Robert Louw* and Danilo Santoro
Center for Chemistry and the Environment
Leiden Institute of Chemistry
Leiden University, P.O. Box 9502
2300 RA Leiden, The Netherlands
ES990163R
-
1
mol , respectively (2). In this respect, the intermediate
*
Corresponding author fax: +31 71 527 4492; e-mail: R.Louw@
chem.leidenuniv.nl.
1
0.1021/es990163r CCC: $18.00
1999 Am erican Chem ical Society
VOL. 33, NO. 18, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3 2 8 1
Published on Web 08/13/1999
Louw
