Photochemical Formation of Free Radicals
J. Phys. Chem. A, Vol. 102, No. 18, 1998 3119
TABLE 1: Retention Times tret and Concentrations c of
Products in a Solution of 2 M Malonic Acid after 6 h of UV
Irradiation
c/10-5
M
peak
tret/s
compound
in 1 M H2SO4
in water
1
2
3
4
5
6
7
570
600
800
MAMA
ETA
tartonic acid
ETRA
succinic acid
unidentified
acetic acid
0.62
1.9
0.23
4.7
0.26
0.8
840
3.4
1.5
1240
1345
1511
2.3
13
17
compound disappears completely within 1 h of UV irradiation.
The observed HPLC peaks of the irradiation products are
summarized in Table 1, and the concentrations of these products
were calculated from calibration curves of Sirimungkala.12
The identified products were monomalonyl malonate, 1,1,2,2-
ethanetetracarboxylic acid, tartronic acid, 1,1,2-ethanetricar-
boxylic acid (ETRA), succinic acid, and acetic acid. The
products of photochemical decomposition of malonic acid in
water and sulfuric acid are the same, but in water we got less
products (except acetic acid) according to smaller quantum
yield.8,13
Figure 1. HPLC spectra (absorption A versus time) of a 2 M malonic
acid solution in 1 M sulfuric acid: (a) Before illumination; (b) after 6
h of illumination with UV light. The peak at tret ) 1495 s corresponds
to a contamination in malonic acid which disappears under UV
irradiation within 1 h. Note that the new peak 7 appears at tret ) 1511
s.
Discussion
•
reactive OH radical would certainly abstract an H-atom from
the acid and in this way generate the observed alkyl malonyl
radical (R9). Direct evidence for this mechanism was not
possible, however, because the assumed acyl-type radical was
not detectable in their ESR experiments.
Our aim here is to generate malonyl radicals with UV
irradiation and to study in this way their reactions in the absence
of Ce4+. Naturally the HPLC technique applied here is not able
to detect the organic free radicals themselves but only their
recombination products. Knowledge of these secondary prod-
ucts, however, can help to identify the primary radicals
participating in these reactions.
Apparently acetic acid is the main reaction product. First
we tried to interpret our results accepting the reaction scheme
suggested by Kaiser et al.9-11 According to their theory, acetic
acid is formed in R6, a reaction between acetyl and carboxyl
radicals. Moreover, it is also assumed that these radicals are
formed in a 1:1 stoichiometric ratio from malonic acid in its
photochemical decomposition (R5). If their scheme is correct,
then recombination products of both radicals should appear.
Indeed, succinic acid, a recombination product of acetyl radicals
(see R7), was found both in Kaiser’s and in our experiments.
The other expected recombination product would be oxalic acid
(a recombination product of two carboxyl radicals). In contrast,
no oxalic acid was found either in our or in Kaiser’s experi-
ments. Within Kaiser’s scheme, this result can be explained
only if we assume that the recombination reaction between
carboxyl radicals is much slower than R6 or R7. This is not
very probable. Another assumption which is also hard to accept
is that CO2 is a product of R6. The main product of a
recombination reaction between acetyl and carboxyl radicals
should be malonic acid. On the other hand, without R6 we
have no explanation within this scheme for the experimentally
observed CO2 evolution.14
Experiments
The HPLC experiments were performed with Shimadzu
equipment (LC-10AS pump, CTO-10A column oven, SPD-10A
dual-wavelength UV detector working at 220 nm, 8-nm
bandwidth, 8-µL cell volume) using an ion-exchange column
(Merck, Polyspher OA KC column with a length of 30 cm and
diameter of 9.5 cm). The temperature was controlled at 45 °C
with the column oven. The eluent was 0.01 M H2SO4 (flow
rate of 0.40 mL/min.). The sample was injected using a
Rheodyne 7010 injector with a 20-µL sample loop.
The irradiation was performed in a closed quartz cell with
an optical path length of 1 cm. The solutions were purged with
nitrogen for 30 min before the irradiation. The light source
was a 200-W Hg lamp (the light intensity was 640 W/m2). The
samples were diluted 100 times with water before the injection
into the HPLC instrument.
H2SO4 (Merck 95%) was used without further purification.
Malonic acid (Fluka puriss.) was recrystallized twice from water.
All solutions were prepared with doubly distilled water.
A further problem of the scheme is the generation of malonyl
radicals. The appearance of MAMA and ETA among the
reaction products (see Table 1) proves that malonyl radicals
are important intermediates in the photochemical degradation
of malonic acid. In Kaiser’s scheme, malonyl radicals emerge
in a sequence where the first intermediates are hydroxyl and
acyl-type radicals. We did not observe, however, any recom-
bination products of either the hydroxyl or the acyl-type radicals.
Consequently, there is no direct (ESR) or indirect (HPLC)
evidence that alkyl malonyl radicals are formed according to
the sequence R8 and R9.
Results
The above considerations show that, at least in the case of
malonic acid, it is rather difficult to explain the experimental
findings with the mechanism proposed by Kaiser et al.9,10
Therefore, we suggest a new reaction path for the formation of
We studied the products of UV irradiation of malonic acid
in sulfuric acid (Figure 1) and in water. One problem was a
contaminant in the malonic acid resulting in a peak at a retention
time of 1495 s. We were not able to identify it. However, this
Szalai, Istvan
