Azaarenes with Radicals in Water. 1
J. Phys. Chem. A, Vol. 102, No. 34, 1998 6761
TABLE 1: Concentrations of the Reactants
reactions
azaarene [mM]
radical forming system [mM]
Fenton reaction, H2O2
Fenton reaction, S2O8
0.01-1.0
0.01-1.0
0.01-1.0
0.01-1.0
0.0028
FeSO4, Na2C2O4, H2O2 in 1 and 5 molar excess
FeSO4, Na2C2O4, K2S2O8 in 1 and 5 molar excess
H2O2: 100 molar excess
K2S2O8: 100 molar excess
H2O2: 280 µM-2.8 mM + K2CO3: 84.0 mM
2-
photoreaction, H2O2
2-
photoreaction, S2O8
kinetics, carbonate radical
Experimental Section
Materials. All reactants were purchased from Aldrich in
maximum purity grade. The identification of most of the
reaction products was proved using native samples, which were
commercial products of Aldrich and Merck (quinoline-N-oxide;
2-, 4-, 5-, 6-, 8-hydroxyquinoline; isoquinoline-N-oxide; 1-, 3-,
5-hydroxyisoquinoline; 1-, 2-hydroxynaphthalene; 4-hydroxy-
quinazoline; 2-hydroxyquinoxaline; 1(2H)-phthalazinone; 1H-,
3H-quinazoline-2,4-dione; 2,3-dihydro-1,4-phthalazinedione;
2,3-dihydroxyquinoxaline; 2,4-, 2,6-dihydroxyquinoline; 8-hy-
droxyquinoline-N-oxide; phthalimide; 2-cumaranone; 1,2-phthal-
dialdehyde; cumarine; dihydrocumarine; 1,2-dicyanobenzene;
2-cyanobenzaldehyde; pyridine-2-aldehyde; pyridine-3-alde-
hyde; pyridine-4-aldehyde; and anisole). Ethyl acetate and
dichlormethane used for solvent extraction (resi analyzed),
hexane (GC grade), acetonitrile, and methanol (HPLC gradient
grade) were obtained from Baker. Ultrapure water was prepared
in a Milli-Q-plus-apparatus (Millipore).
Figure 1. Reaction of formation of carbonate radicals with the
according rate constants: reaction 1, k ) 4.2 × 108 (M s)-1; reaction
3 k ) 2 × 107 (M s)-1
.
conjugated acid, the HCO3 radical, is of minor importance. The
OH radicals quantitatively react with the carbonate ions, which
are present in excess (Figure 1).
The carbonate radicals quickly react in the self-quenching
reaction 3 or in ETRs with electron rich inorganic and organic
compounds comparable with reaction 2. The continuous
irradiation leads to a steady state concentration of carbonate
-
radicals [‚CO3 ]SS. The influence of organic compounds on
the lifetime of the radicals is low if the concentration of the
organics is low. Then the kinetics follows that of pseudo-first-
order reactions (correlation coefficients > 0.99):
Procedure. A high-pressure mercury lamp TQ 150 (150 W)
and a low-pressure mercury lamp (80 W) from Heraeus are used
as immersion lamps in a photo reactor thermostated at 25 °C.
The monochromatic radiation at λ ) 313 nm is isolated by a
filter solution (aqueous sodium dichromate solution) from the
complex line spectrum of the high pressure mercury lamp TQ
150. The low pressure mercury lamp mainly provides light of
λ ) 254 nm. The dark reactions are performed in reactors of
100 mL solution, the photo reactions in 400 mL. The reaction
mixtures are air saturated (for concentrations, see Table 1).
For extraction experiments, several solvents with different
polarity are tested: cyclohexane, toluene, methylene chloride,
and ethyl acetate. Additionally the SPME (solid phase mi-
croextraction) as a new extraction technique is tested using two
different phases (polyacrylate and polydimethylsiloxane). The
highest analytical sensitivity has been achieved with the classical
liquid-liquid extraction with methylene chloride and ethyl
acetate as solvents. Therefore, the aqueous samples (pH ) 7)
are extracted with ethyl acetate (+NaCl). An aliquot of the
solvent extract is evaporated to dryness and dissolved in 500
µL hexane for GC injection. The products are identified by
GC/MS (GC HP 5890 series II and MSD HP 5971, Hewlett-
Packard). The identification of most of the reaction products
was proved using native commercial samples (Aldrich and
Merck). The products were determined in SCAN and SIM
mode.
The experimental determination of rate constants is carried
out directly without any special sample preparation by HPLC
(HP 1050 and UV-diode array detector HP 1040M series II,
Hewlett-Packard) measuring the disappearance of the azaarenes.
A short column and isocratic mode are sufficient for separa-
tion of the reactants. Trimethylamine is added to suppress a
possible tailing of azaarenes.
Kinetic Method. The carbonate radicals are prepared in
redox reactions of hydroxyl radicals and carbonate anions.16-20
Hydroxyl radicals are obtained by photolysis of H2O2 (λ ) 313
nm). The reaction needs a highly basic milieu (pH ) 11). The
pKa value of the radical is between 7.0 and 8.2. Therefore, its
-
k ) [‚CO3 ]SSk(‚CO3- + RH)
-
k(‚CO3 + RH) represents the second-order rate constant for
the reaction between the carbonate radical and the organic
-
compound. k(‚CO3 + RH) is proportional to the rise of the
-
k/C(H2O2) plot. The direct determination of k(‚CO3 + RH)
-
is not possible as the [‚CO3
]
is unknown. By means of
SS
-
anisole as a reference substance with k(‚CO3 + anisole) )
2.8 × 105 L(mol s)-1 16
, relative rate constants for the azaarenes
are calculated.
Theoretical Calculations. Electrostatic charges and coef-
ficients of the frontier orbitals of the azaarenes are calculated
using a workstation RS 6000 (IBM) and the chemical modeling
software Sybyl (Tripos).
Results
Dark Reactions of Quinoline and Isoquinoline. Main
products in the Fenton reaction of quinoline with peroxodisulfate
are quinoline-5,8-dione and three products with a fragment peak
at 132, which could not be clearly identified. Pichat et al.1 also
found these three compounds (M ) 161, CI) during the
photocatalysis (TiO2, λ ) 365 nm) of quinoline, but only one
was identified as N-formyl-2-aminobenzaldehyde. After several
tests, the authors proceed on the assumption that the remaining
isomers are the two pyridinealdehydes resulting from the
cleavage of the benzene ring. Our investigations on the Fenton
reaction with naphthalene support this hypothesis. In the
reaction, two products with mass spectra analogously are
obtained showing the mass peaks at m/z(naphthalene) )
m/z(quinoline) - 1. In the Fenton reaction with isoquinoline,
the formation of three products with fragment peaks at 132 and
in the reactions with the benzodiazines analogous products with
mass peaks at m/z(benzodiazines) ) m/z(quinoline) + 1 are also
formed. In all cases, the formation of twice oxidized products
formed by cleavage of the benzene rings are likely. Beside the
traces indicating main products, traces indicating 5-, 8-hydroxy-
quinoline and indol-2,3-dione are found. The latter compound