Study of the photoinduced formose reaction by flash and stationary photolysis

Article abstract of DOI:10.1070/MC2006v016n01ABEH002188

The chemical condensation of formaldehyde into more complex aldehydes (glycolaldehyde and glyceraldehyde) and monosaccharides (glucose, lyxose, erythrose and erythrulose) under UV irradiation was found to proceed in acidic aqueous solutions in the absence of catalysts and initial primers.

Full text of DOI:10.1070/MC2006v016n01ABEH002188

Mendeleev Commun., 2006, 16(1), 9–11
COMMUNICATIONS
Study of the photoinduced formose reaction by flash and stationary photolysis
a
b
b
b
Olga A. Snytnikova, Alexandr N. Simonov, Oxana P. Pestunova, Valentin N. Parmon and
a
Yuri P. Tsentalovich*
a
International Tomography Center, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk, Russian Federation.
Fax: +7 3832 33 1399; e-mail: yura@tomo.nsc.ru
b
G. K. Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk,
Russian Federation. Fax: +7 3832 34 3056; e-mail: oxanap@catalysis.ru
DOI: 10.1070/MC2006v016n01ABEH002188
The chemical condensation of formaldehyde into more complex aldehydes (glycolaldehyde and glyceraldehyde) and monosaccharides
(
glucose, lyxose, erythrose and erythrulose) under UV irradiation was found to proceed in acidic aqueous solutions in the absence
of catalysts and initial primers.
The chemical condensation of formaldehyde (the formose reac-
tion) occurs in the presence of inorganic bases acting as catalysts
to form a complex mixture of carbohydrates.1–3 The formose
reaction is an autocatalytic process since the necessary condi-
tion for the reaction to proceed is the presence of an initiating
primer in the form of at least several molecules of monosac-
Table 1 Changes in the concentrations of the initial compound (formal-
dehyde) and the reaction products and in pH in an irradiated aqueous
formaldehyde solution.
Irradiation time/h
Compound
0
4
8
18
24
28
2
charides. Namely this feature of the formose reaction has recently
Formaldehyde, mol dm–3
pH
11.8 10.7 8.0
3.18 2.12 1.92 1.83 1.79 1.74
5.5
2.9
1.7
attracted special attention. The reaction, due to its autocatalytic
character and a possibility to produce the monosaccharide ribose,
could serve as one of the major ways of chemical evolution on
–3
Formic acid, mol dm
Glycolaldehyde, mol dm
–
–
–
–
–
–
–
0.32 0.80 1.22 1.46 1.84
0.074 0.105 0.115 0.100 0.110
–
3
4
the prebiotic Earth.
–
3
Glyceraldehyde, mmol dm
2
4
8
6
6
The most active primer for the formose reaction is glycol-
aldehyde. In the presence of even small amounts of glycolalde-
hyde in the initial solution, the formaldehyde consumption starts
without an induction period. In the course of the reaction, the
glycolaldehyde concentration increases, and its following reac-
tions result in the formation of a broad spectrum of carbo-
hydrates. A possible mechanism of the primer formation on the
prebiotic Earth might be based on the light-induced transforma-
tions of formaldehyde.5 The aim of this work was to study the
influence of UV irradiation on the formose reaction, first of all,
at its initial stage.
–3
Glucose, mmol dm
Lyxose, mmol dm
2.0
–
4.3
–
5.6
1.1
0.8
1.8
7.7
1.7
1.1
2.4
5.7
1.8
1.2
2.2
–3
Erythrose, mmol dm–3
Erythrulose, mmol dm
–
–
–
3
–
–
investigation of the reaction mechanism. The goal of this study
was to identify intermediates and products formed in the photolysis
of aqueous formaldehyde solutions. To avoid the influence of the
regular formose reaction on the proceeding of photochemical
reaction, all of the experiments were carried out in acidic solu-
tions and in the absence of catalysts, i.e., under conditions when
the regular formose reaction does not occur.
,6
The photochemistry of formaldehyde plays an important role
in the chemistry of atmosphere, and the photochemical properties
of formaldehyde in the gas phase were studied in detail.7 In
aqueous solution, formaldehyde exists mainly in the hydrated
form of methylene glycol, CH (OH) . Since the hydrated form
The transient absorption spectrum observed 1 µs after the laser
pulse irradiation (l = 308 nm) of a deaerated aqueous 10.4 M
formaldehyde solution (pH 3.2) is shown in Figure 2. At all
wavelengths the signal decays with the same rate, and, thus, can
be attributed to the same intermediate. Under argon, the signal
decay obeys a second-order rate law, at the absorption maximum at
,8
2
2
of formaldehyde is a very weak chromophore, the mechanism of
formaldehyde photolysis in aqueous solutions is poorly under-
stood. Nevertheless, at high formaldehyde concentrations, one can
achieve the optical densities of solutions of about 0.1–0.5 units
per centimeter in the wavelength range 250–350 nm (Figure 1),
6
–1
380 nm the decay is characterised by k /e = (1.3±0.2)ꢀ10 cm s ,
1
380
where k is the second-order rate constant, and e is the absorp-
1
380
tion coefficient of the intermediate. Under oxygen, the initial
intensity of the transient absorption signal does not change,
9
which allows the use of laser flash photolysis (LFP) for the
2
1
1
.0
.5
.0
2
0.020
1
0
0
.015
.010
1
2
0
0
.5
.0
0
0
.005
.000
200
250
300
350
400
300
350
400
l/nm
450
500
l/nm
Figure 1 Absorption spectra of an aqueous 12.6 M formaldehyde solu-
tion: (1) before irradiation; (2) after irradiation for 4 h followed by dilution
by a factor of 10.
Figure 2 Transient absorption spectra obtained 1 µs after pulse photolysis:
(1) 10.4 M formaldehyde solution in water; (2) same solution preirradiated
for 4 h and then diluted by a factor of 15.
Mendeleev Commun. 2006
9

Mendeleev Commun., 2006, 16(1), 9–11
while the decay significantly accelerates and becomes exponen-
0
.025
tial (Figure 3). The rate constant of the oxygen quenching, k =
2
9
3
–1 –1
=
(6.1±0.8)ꢀ10 dm mol s , was determined from the depend-
ence of the decay rate on oxygen concentration. Thus, the
observed intermediate with the absorption maximum at 380 nm
is the formaldehyde triplet state. Unfortunately, the quenching
mechanism remains unknown: whether the reaction between the
triplet formaldehyde and oxygen is electron transfer yielding a
formaldehyde cation radical and superoxide, or the triplet energy
transfer from formaldehyde to oxygen takes place, resulting in
the singlet oxygen formation. Nevertheless, taking into account
that by the completion of quenching (about 1 µs with a
dissolved oxygen concentration of 1 mM) practically no
residual absorp-tion is observed at all wavelengths, one can
presume that the triplet energy transfer is a more probable
quenching mechanism. Thus, the primary reactions taking place
in the direct 308 nm photolysis of aqueous formaldehyde
solution can be described by the following scheme:
0.020
0.015
0.010
Ar
0
0
.005
.000
O2
–
1
0
1
2
3
4
t/µs
HCHO + H O
CH (OH)2
(1)
(2)
(3)
(4)
(5)
Figure 3 Transient absorption kinetics observed at 380 nm in the photolysis
2
2
of an aqueous 10.4 M formaldehyde solution under argon and oxygen.
hv
HCHO
THCHO*
k1
the spectrum recorded with the fresh formaldehyde solution
2THCHO*
2HCHO
(
Figure 2). The spectrum becomes narrower, and its maximum
k2
THCHO* + O2
HCHO + O2
S
shifts to 340 nm. Oxygen quenches this intermediate with the
rate constant k = (8±2)ꢀ10 dm mol s . Most likely, this short-
k3
8
3
–1 –1
T
· ·
CH OH + CH(OH)
2 2
4
HCHO* + CH (OH)2
2
lived intermediate should be attributed to the triplet state of a
photoactive product formed in the photolysis.
Note that under our experimental conditions the decay of the
triplet formaldehyde signal remains bimolecular at the highest
concentrations of the initial solution (up to 12 M) and the lowest
intensities of laser flashes (down to 7 mJ per pulse). From that
The influence of oxygen on the photochemical reaction was
studied by performing the photolysis of a 12.6 M formaldehyde
solution with bubbling argon or oxygen followed by the analysis
of the irradiated samples. Under oxygen, both formaldehyde
consumption and product formation decreased by a factor of 2–3.
This result additionally testifies that the reaction under study
occurs via the formation of triplet states of formaldehyde and
photoactive photolysis products, and that the oxygen quenching
evidently proceeds through triplet energy transfer.
one can conclude that the rate constant k of reaction (5) does
3
4
3
–1 –1
not exceed 10 dm mol s .
The formation of gas bubbles in the solution was observed
during steady irradiation. One can assume that the gases are
CO, CH , CO , H , and minor amounts of ethane and ethylene,
4
2
2
6
as was shown earlier.
Thus, the chemical condensation of formaldehyde into more
complex monosaccharides can proceed under UV irradiation even
in acidic solutions without catalysts and initial primers. At the
initial stage of irradiation, the photolysis proceeds through the
formation of a fomaldehyde triplet state, the subsequent reac-
tions of the latter result in the formation of photoactive products.
Under further irradiation, the primary products absorb the majority
of light, and the formation of complex carbohydrates (glucose,
lyxose, erythrose and erythrulose) seems to be the result of these
secondary photochemical processes.
A sample placed in a quartz cell was irradiated for several
hours with full light from a DRSh-500 high-pressure mercury
lamp. Then, the irradiated solution was treated with dinitrophenyl-
10
hydrazine, and monosaccharides were analysed by HPLC after
derivatization. The formaldehyde concentrations were determined
optically by the reaction with chromotropic acid.11 Table 1 shows
the product yields for different irradiation times. The concen-
trations of glycolaldehyde and glyceraldehyde grow up only at
the initial stage of the irradiation, and then remain almost
constant. These intermediate products participate in secondary
photochemical reactions yielding more complex monosaccha-
rides. A significant decrease in pH of the irradiated solution
was also observed. The acid analysis performed with the use of
a Nucleosil SA ion-exchange column, has shown that formic
and acetic acids in the ratio of ~15:1 are accumulating in the
solution.
This work was supported by the Russian Foundation for Basic
Research (project no. 06-03-32149-a) and the Presidium of the
Russian Academy of Sciences (program ‘Origin and Evolution
of Biosphere’).
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The formation of methanol or ethanol was not detected in the
solution. Thus, the Cannizzaro reaction
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,
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1
Received: 20th May 2005; Com. 05/2513
Mendeleev Commun. 2006 11