AULA DE ACIDO GLIOXILICO

1. The gas-phase photochemistry and thermal decomposition of glyoxylic acid'
R. A. BACKAND S. YAMAMOTO'
Division of Chetnisrry, National Research Council of Canada. 100 Su.sse.r Drive, Orrawa, Onr., Canada KIA OR6
Received April 24, 1984
R. A. BACKand S. YAMAMOTO.Can. J. Chem. 63, 542 (1985).
The photolysis of glyoxylic acid vapour has been studied at five wavelengths, 382, 366,346,275, and 239 nm, and pressures
from about I to 6 Torr, at a temperature of 355 K. Major products were C02 and CHZO, initially formed in almost equal
amounts, while minor products were CO and Hz. Except at 382 nm, the system was complicated by the rapid secondary
photolysis of CHzO. Three primary processes are suggested, each involving internal H-atom transfer followed by dissociation.
The absorption spectrum is reported and shows the three distinct absorption systems. A finely-structured spectrum from
about 320 to 400 nm is attributed to a transition to the first excited -rr* + n+ singlet state; a more diffuse absorption ranging
from about 290 nm to a maximum at 239 nm is assigned to the -rr* + n state, while a much stronger absorption beginning
below 230 nm is attributed to the -rr* + -rr transition. Product ratios vary with wavelength and depend on which excited state
is involved.
The thermal decomposition was studied briefly in a static system at temperatures from 470 to 710 K and pressures from 0.4
to 8 Torr. Major products were again COz and CHZO,but the latter was always less than stoichiometric. First-order rate
constants for the apparently homogeneous formation of COz are described by Arrhenius parameters log A (s-I) = 7.80 and
E = 30.8 kcal/mol. Carbon monoxide and H2 were minor products, and the CO/COz ratio increased with increasing
temperature and showed some surface enhancement at lower temperatures. The SF,-sensitized thermal decomposition of
glyoxylic acid, induced by a pulsed COz laser, was briefly studied, with temperatures estimated to be in the 1100- 1600 K
range, and the CO/C02 ratio increased with increasing temperature, continuing the trend observed in the static system.
R. A. BACKet S. YAMAMOTO.Can. J. Chem. 63, 542 (1985).
Opkrant a des pressions allant de I 6 Torr et une tempCrature de 355 K, on a CtudiC la photolyse de I'acide glyoxylique
en phase vapeur aux cinq longueurs d'onde suivantes: 382, 366, 346, 275 et 239 nm. Les produits principaux sont le COa et
le CH20 qui se forment initialement en quantitCs pratiquement Cgales; il se forme aussi du CO et du Hz comme produits
mineurs. Except6 B 382 nm, le systkme est compliquC par la photolyse secondaire rapide du CHzO. On suggkre ['existence
de trois processus primaires qui impliquent tous un transfert interne d'un atome d'hydrogkne qui serait suivi par une
dissociation.
On rapporte le spectre d'absorption qui prtsente trois systkmes distincts d'absorption. On attribue le spectre a structure fine,
qui s'Ctale de 320 B 400 nm, B une transition vers le premier Ctat singulet excitC -rr+ + n+; on attribue une absorption plus
diffuse, qui va de 290 nm jusqu'i un maximum a 239 nm, a 1'Ctat -rr* + n.. alors que I'absorption beaucoup plus forte qui
commence en dessous de 239 nm serait due a la transition -rr* +-rr. Les rapports des quantitCsde produits obtenus varient avec
la longueur d'onde utilisCe pour provoquer la photolyse et ils dCpendent de I'ttat excitC impliqui.
Opkrant a des temperatures allant de 470 a 710 K et a des pressions allant de 0,4 a 8 Torr, on a brikvement CtudiC la
dCcomposition thermique dans un systkme statique. De nouveau, les produits majeurs sont le COz et le CHzO; toutefois, la
quantitC formCe de ce dernier compost5 est toujours infkrieure a la stoYchiomt5trie. Les constantes de vitesse du premier ordre,
pour la formation apparement homogkne de COz, est dtcrite par les paramktres d'ArrhCnius suivants: log A (s-') = 7,80 et
E = 30.8 kcal/mol. Le CO et le Hzsont formts comme produits mineurs; le rapport CO/COr augmente avec une augmentation
de la tempkrature et, basse temptrature, il est rehaussC par la prCsence de surfaces. On a brikvement CtudiC la dCcomposition
thermique de I'acide glyoxylique, sensibilisCepar du SF, et induite par un laser pulsC au COr; on a CvaluC que les tempkratures
se situaient entre 1100 et 1600 K et, comme dans le systkme statique, le rapport CO/COz augmente avec une augmentation
de la tempkrature.
[Traduit par le journal]
Introduction Experimental
The photolysis and spectroscopy of glyoxal and biacetyl
have been the subjects of"much investigation and interest, but
few other a-dicarbonyl compounds have been studied in the gas
phase. We have recently completed studies of the spectroscopy
(1) and photochemistry (2) of oxalic acid vapour, and the
present paper describes the extension of this work to glyoxylic
0
acid, H-C-C-0-H. The thermal decomposition and the
II
0
decomposition induced by a pulsed infrared laser were also
studied for comparison with the photolysis.
'NRCC 23762.
'NRCC Research Associate, 1981-1983. Present address: Depart-
ment of Chemistry, Okayama University, Tsushima, Okayama 700,
Japan.
The apparatus and techniques were similar to those used with oxalic
acid (2). Photolyses were done in a cylindrical quartz vessel, 10 cm
long and 5 cm in diameter, with the vessel and gas-handling manifold
kept at a temperature of 355 K, high enough to permit an adequate
vapour pressure but low enough so that thermal dark reaction was
minimal, and no correction was needed. 'The thermal decomposition
was studied in a static system, a cylindrical quartz vessel 1I cm long
and 3 cm in diameter heated in a tube furnace. The laser-induced
pyrolysis was done in a cylindrical Pyrex vessel, I0 cm long and 4 cm
in diameter, fitted with NaCl windows, also held at 355 K.
The light source for the ultraviolet photolysis was a 1000W Hg-Xe
high pressure arc lamp with matching monochromator that gave a band
width of about 20 nm. For the laser-induced decomposition, a Lum-
onics Model 203 pulsed COr TEA laser was used, as previously
described (2, 3).
Glyoxylic acid (98% pure) was obtained from Aldrich as the hy-
drate, and was dehydrated by judicious heating and pumping until a
constant vapour pressure was attained; it was thoroughly pumped
to remove any decomposition products before each experiment.
Propylene was Phillips Research grade.
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2. BACK AND YAMAMOTO
A(nm)
FIG.I. Low-resolution absorption spectrum of glyoxylic acid vapour. E = molar decadic extinction coefficient.
TIME (rnin)
FIG.2. Time dependence of products in the photolysis of glyoxylic
acid at 366 nm and a pressure of 4 Torr.
Results
Absorption spectrum
The absorption spectrum of glyoxylic acid vapour, pre-
viously unreported, was measured in a modified Heath spec-
trophotometer fitted with a 4 m heated cell, and is shown in
Fig. 1. Three absorption systems are evident. The highly
structured spectrum extending from about 320 to 400 nm
undoubtedly corresponds to excitation of the T* + n+ first
excited singlet state. The absorption beginning at about 290 nm
and extending to a maximum at 239 nm, with broad diffuse
vibrational features, is probably due to the T* +n- excitation,
while the much stronger absorption setting in below about 230
nm is most probably from a T* +T excited state. The photo-
lysis was studied at the five wavelengths indicated in Fig. 1,
three in the first absorption system and two in the second. A
more detailed account and analysis of this spectrum will be
published elsewhere (4).
Ultravioletphotolysis
Major primary products of the photolysis were CO, and
formaldehyde, with much smaller amounts of Hzand CO. The
time dependence of products at 366 nm is shown in Fig. 2.
Production of CO, was linear with time at this and all other
wavelengths. Yields of H,CO, however, fell with increasing
time, and those of CO increased, and similar behaviour was
observed at 346,275, and 239 nm. In the photolysis at 382 nm,
however, both COz and CO were linear functions of time;
yields of HICO could not be measured accurately because the
amounts were quite small at this wavelength.
Experiments at 366 nm with added air or propylene at pres-
sures up to 50 Torr showed no difference in the production of
CO,, indicating that neither free radicals nor triplet states were
involved in its formation.
Some light emission was observed on excitation between
340 and 380 nm, but the excitation spectrum was almost ex-
actly that of formaldehyde and thus the emission was very
probably from excitation of formaldehyde product; there may
have been some weak underlying emission from glyoxylic acid
itself.
Thermal decomposition
The thermal decomposition was studied in a static system at
pressures from 0.4 to 8 Torr and temperatures from 470 to 710
K. Products observed and measured were those observed in the
photolysis, COz, H,CO, CO, and Hz, all volatile at dry-ice
temperature. No systematic search was made for higher prod-
ucts. Figure 3 shows an Arrhenius plot of the first-order rate-
constant for the formation of CO,. No trend with pressure was
observed between 2.5 and 8 Torr, and all the data shown were
obtained in this range. At lower pressures the rate constant
decreased substantially (Fig. 4). Formaldehyde was always less
than COz,with the ratio reaching a constant value of about 0.8
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3. 544 CAN. I. CHEM. 'OL. 63. 1985
acid and variable SF6pressure. Results are shown in Figs. 7 and
o E=29.6 KCAL /MOL
logloA (s-I)=7 . 6 8
UNSEASONED
SEASONED
PACKED,UNSEASONED
FIG. 3. Arrhenius plot for the first-order rate constant for the
formation of COr in the thermal decomposition of glyoxylic acid.
PRESSURE OF GLYOXYLIC ACID (Torr)
FIG. 4. Pressure dependence of the first-order rate constant for the
formation of COz.
above 625 K, and falling markedly at lower temperatures
(Fig. 5). Yields of CO and H, were each less than 10% of the
C02 and decreased with decreasing temperature (Fig. 6). All
products were linear functions of time at the low conversions
attained in the experiments.
Decomposition induced by a pulsed infrared laser
Glyoxylic acid did not itself absorb infrared radiation at the
wavelengths of the CO?laser, but its decomposition was easily
sensitized by a small pressure of SF6,which is a strong absorb-
er. Two sets of experiments were done, one with a constant
pressure of 1.2 Torr of SF6and varying pressures of glyoxylic
acid, the other with a constant pressure of 2.8 Torr of glyoxylic
8. Although the extent of decomposition of the reageit in the
laser beam (1.2 cm diameter) was quite high in some of these
experiments, product yields were proportional to the number of
pulses up to about four pulses, with the reagent in the beam
presumably replenished in the 2 s between pulses by diffusion
and convection. All the data shown were obtained with two
pulses per experiment.
The probability of decomposition, P, and the CO/CO, ratlo,
behaved in a parallel manner (Figs. 7 and 8), reflecting changes
in the effective temperature of the system with changing gas
composition. The major effect is that caused by dilution of the
absorbing SF6 by non-absorbing glyoxylic acid with con-
sequent decrease in temperature. The decrease in P in Fig. 7 is
less than expected, however, and the increase in P in Fig. 8
more than expected on this basis, as shown more clearly in a
plot of P vs. mole fraction of SF6 (Fig. 9). The explanation
probably lies in a pressure dependence of the multiphoton ab-
sorption by SF6, evident from the dependence of P on total
pressure (indicated on Fig. 9 beside each experimental point).
There is also probably an additional increasein P with pressure
due to an increase in reaction time, as the hot gas cools more
slowly after the pulse at higher pressure. A reaction time of the
order of 1 ms can be estimated. and if the decom~osition
follows the Arrhenius parameters' found in the static ihermal
decomposition (Fig. 3), reaction temperatures between about
1100 and 1500 K are indicated for the experiments shown in
Figs. 7 and 8.
Figure 10 shows the effect of added air on the laser-induced
decomposition. The decrease in P and in the CO/C02 ratio are
again less than expected from simple dilution, and counter-
vailing effects of total pressure on energy absorption and reac-
tion time are again probably responsible. Figure 11 shows the
effect of laser fluence (varied by means of a long-focus lens
(4))on the CO/C02 ratio.
Discussion
The photolysis products, their time dependence, and the lack
of effect of O2and propylene, suggests strongly that the major
primary process at all wavelengths is a simple photo-
dissociation into C02 and formaldehyde. The increase in CO
and the decrease in H,CO production with increasing time
points to a secondary photolysis of formaldehyde, a conclusion
supported by the absence of this behaviour at 382 nm where
formaldehyde is almost transparent. The secondary CO was not
accompanied by Hzin comparable amounts, and it appears that
disproportionation reactions involving excited HzCO and
HCO, similar to those invoked in the photolysis of for-
maldehyde alone (5), are probably involved in CO formation.
There were also smaller yields of CO and Hzformed as primary
products, and the following set of reactions accounts for our
observations.
131 + CO + HCOOH (or 2C0 + HzO)
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4. BACK AND YAMAMOTO
I o UNSEASONED
FIG.5. Arrhenius plot of the ratio of formaldehyde to COzfrom the thermal decomposition of glyoxylic acid.
IOOO/T(K)
FIG.6. Arrhenius plot of product ratios CO/COI and H2/C02from
the thermal decomposition of glyoxylic acid.
-I
0
161 H2CO*+ other products
- 0. . . .O,o ..O0 0KO.0-0- C0/CO2 0
'0
Reaction [l] was the predominant primary process at all wave-
lengths, while reaction [2], measured by the yield of H,, was
consistently about 3% of reaction [l] under all conditions.
Reaction [3] is suggested to"account for the primary production
of CO unaccompanied by Hz.This process was clearly evident
at 382 nm and easily measured directly. At 366 and 346 nm it
was obscured by secondary production of CO via reaction [5]
(Fig. 2). Reactions [5] and [6] are not elementary reactions, but
simply represent formally the photochemical loss of for-
maldehyde with and without formation of CO. A simple math-
ematical modelling of reactions [I] to [6] to give the best fit to
the time dependence of the CO/CO, ratio (Fig. 12) led to
estimates of reaction [3] at these two wavelengths by extrapo-
lation to zero time. At 275 and 239 nm a simple empirical
extrapolation was used. Values for the yields of reactions [2]
and [3], relative to that of 111, are shown in Table 1. Values of
+3 are based on formation of CO + HCOOH; if 2C0 + H,O
are formed instead, these would be halved. Neither HCOOH
nor H,O was measured in these systems, as the absolute
amounts were small and easily lost by adsorption in the vacuum
-
-2
0 UNPACKED
PACKED
GLYOXYLIC ACID PRESSURE (Torr)
FIG.7. Probability, P, decomposition of glyoxylic acid in the laser
beam in a single pulse, and the CO/COI product ratio, for the
SF6-sensitized thermal decomposition induced by a pulsed COz laser.
Pressure of SF, = 1.2 Torr.
line.
Absolute quantum yields of CO, were estimated at the three
longest wavelengths using azomethane as an actinometer, and
values of 0.08, 0.66, and 0.76 were obtained at 382, 366, and
346 nm, respectively. The trend with wavelength is probably
real, but absolute values are rather uncertain, especially at 382
nm, because of the difficulty in estimating the light absorbed by
the glyoxylic acid. Absorption was too weak to be measured
directly in the 10-cm photolysis cell (the spectrum in Fig. 1 was
obtained in a 4-m cell), and the sharp structure of the spectrum,
especially at the longest wavelengths, made it hard to estimate
effective extinction coefficients for the photolytic light. There
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5. 546 BACK AND YAMAMOTO
SF6 PRESSURE (Torr 1
FIG.8. Dependence of P and CO/C02 product ratio on pressure of
added SF, in the laser-induced thermal decomposition of 2.8 Tom of
glyoxylic acid.
was no evidence for collisional quenching of the photo-
dissociation by added gas (air or propylene) up to 50 Torr at
366 nm, and no strong fluorescence was seen, so it seems
probable that the absolute quantum yield at 366 nm and shorter
wavelengths may have been close to unity; the lower value at
382 nm is probably real.
The photochemistry of glyoxylic acid differs from that of
oxalic acid (2) in that the ratio @,/a,, and thence the CO/C02
ratio, depends strongly on wavelength (Table 1). At the three
longest wavelengths, the CO/C02 ratio decreases with in-
creasing photon energy, although in the thermal decomposition
the same ratio increases with increasing temperature (see be-
low). This indicates that the photolysis does not proceed
through internal conversion and dissociation of the
vibrationally-excitedground state, but probably occurs directly
from the T* +n+ singlet state or perhaps a short-lived triplet.
As in oxalic acid, internal transfer of H to the carbonyl oxygen
may be involved, with rearrangement of the resulting carbenes
leading to HzCOand HCOOH.
MOLE FRACTION SF,
FIG.9. Probability, P, of decomposition of glyoxylic acid vs.
mole fraction of SF,. Numbers show total pressure in Tom for each
experiment.
PRESSURE OF AIR (Torr)
FIG.10. Effect of added air on P and on the product ratio CO/C02
in the laser-induced thermal decomposition of glyoxylic acid.
Glyoxylic acid = 2.8 Tom, SF, = 1.2 Torr, P,, = probability of
decomposition in the absence of air.
The observed minor formation of Hz(reaction [2]) might arise
from an alternative decomposition of HCOH into Hz and CO.
At the two shortest wavelengths, the CO/C02 ratio increases
with increasing photon energy. It is clear from the absorption
spectrum (Fig. 1) that the excitation here involves the second
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6. BACK AND YAMAMOTO
TABLEI . Relative quantum yields of
primary processes in the photolysis of
glyoxylic acid
RELATIVE FLUENCE
FIG.I I. Effect of laser fluence on CO/COZ product ratio in the
laser-induced thermal decomposition of glyoxylic acid. Relative flu-
ence is in arbitrary units. = 2.8 Torr glyoxylic acid, 1.2 Torr SFh,
0= 2.8 Torr glyoxylic acid, 2.6 Torr SF,.
TIME (min)
FIG.12. Time dependence of the CO/CO, product ratio from the
photolysis of glyoxylic acid .at 366 nm. Curve is calculated from
model based on mechanism in text. Points are experimental.
excited singlet state, .rr* +-n-, and perhaps the .rr" +.rr state,
which probably has a strong absorption tail underlying the
former. A different decomposition mechanism, perhaps
through the vibrationally excited ground state, is probably
operative at these wavelengths.
The thermal decomposition is more complicated than the
photolysis, although there seems to be no secondary decom-
position of products as all yields were linear with time. At the
higher temperatures, the major process is again the formation
of C02 and H,CO, but the latter remains at only 80% of its
stoichiometric yield; at lower temperatures much more is miss-
ing (Fig. 5), perhaps lost through polymerization or reaction
with glyoxylic acid to give higher products. Production of COz
showed some enhancement in the packed vessel, in which the
surface/volume ratio was increased by a factor of 4 (Fig. 3). A
correction for surface reaction, assuming simple additivity with
the gas-phase decomposition, led to corrected Arrhenius pa-
rameters of log A (s-') = 7.80 and E = 30.8 kcal/mol, not
much changed from the values obtained before correction
(Fig. 3). If these parameters can be ascribed to a simple
homogeneous process,
they may be compared with those for the similar formation of
C02and HCOOH from oxalic acid (6). The activation energies
are almost the same (30.8 vs. 30 kcal/mol) but the frequency
factor for glyoxylic acid is four orders of magnitude smaller.
This is much too low for a normal simple unimolecular reac-
tion. As with the corresponding decomposition of oxalic acid
(2),one might expect reaction [9] to proceed by internal trans-
fer of the 0-H hydrogen through a four-membered transition
state:3
It is difficult to see why this should be a factor of 10' slower
in glyoxylic acid, or in simpler terms, why glyoxylic acid
should be so much more stable towards thermal decomposition
than oxalic acid.
Formation of CO, at least at the lower temperatures, showed
a strong surface enhancement while Hz, a very minor product,
was somewhat reduced (Fig. 6). Carbon monoxide was not a
product of the thermolysis of oxalic acid, which suggests that
in glyoxylic acid it is formed by transfer of the aldehydic
hydrogen:
'Transfer of H to the carbonyl oxygen as in reaction [7] is unlikely
in the thermal reaction because of the expected high energy required
to form hydroxymethylene.
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7. 548 CAN. J. CHEM. VOL. 63. 1985
Neither H,O nor HCOOH was detected, but their analysis in
this system in the amounts expected is difficult, and either
reaction [I 11 or [I21 may have occurred. The upward trend in
the CO/C02 ratio with increasing temperature evident in
Fig. 6 continues in the laser-induced pyrolysis, reaching values
as high as 0.71 at the highest fluence (Fig. 1I).
It has been shown (7) that the most stable structure of gly-
oxylic acid in the gas phase is planar, with the two carbonyl
groups in a trans configuration maintained by strong hydrogen
bonding. All the processes we have suggested, both in the
thermal and the photochemical decomposition, proceed from
this configuration except for reaction [I 11 which would require
a cis structure. The barrier to isomerization is probably less
than 10 kcal/mol, and would not hinder the occurrence of
reaction [l 11 in the thermal decomposition.
1. R. A. BACK.Can. J. Chem. 62, 1414 (1984).
2. S. YAMAMOTOand R. A. BACK.J. Phys. Chem. In press.
3. R. A. BACK.Can. J. Chem. 60, 2542 (1982).
4. R. A. BACKand J. M. PARSONS.To be published.
5. H. OKABE.Photochemistry of small molecules. Wiley, New
York. 1978.
6. G. LAPIDUS, D. BARTON,and P. E. YANKWICH.J. Phys. Chem.
68, 1863 (1964).
7. 1. CHRISTIANSEN,K.-M. MARSTOKK,and H. MOLLENDAL.
J. Mol. Struct. 30, 137 (1976).
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