Effect of acid-base properties of the medium on the reactions in the 2-furaldehyde-H2O2-H2O system with and without VOSO4

Article abstract of DOI:10.1134/S1070363214060140

Data on the effect of acid-base properties of the medium in the pH range 0-9 on the intensity of transformation of 2-furaldehyde and the direction of the multi-stage reactions in the 2-furaldehyde-H₂O₂-H ₂O system in the presence of VOSO₄ and without it are presented. Significant influence of the medium pH value on the reaction pathway in the system under investigation hase been found. Mechanism of these reactions considering the effect of acid-base properties of the medium has been suggested.

Full text of DOI:10.1134/S1070363214060140

ISSN 1070-3632, Russian Journal of General Chemistry, 2014, Vol. 84, No. 6, pp. 1133–1140. © Pleiades Publishing, Ltd., 2014.
Original Russian Text © L.A. Badovskaya, V.V. Poskonin, R.I. Ponomarenko, 2014, published in Zhurnal Obshchei Khimii, 2014, Vol. 84, No. 6, pp. 952–
959.
Effect of Acid–Base Properties of the Medium
on the Reactions in the 2-Furaldehyde–H2O2–H2O System
with and without VOSO4
L. A. Badovskaya, V. V. Poskonin, and R. I. Ponomarenko
Kuban State Technological University, ul. Moskovskaya 2, Krasnodar, 350072 Russia
e-mail: vposkonin@mail.ru
Received September 16, 2013
Abstract—Data on the effect of acid-base properties of the medium in the pH range 0–9 on the intensity of
transformation of 2-furaldehyde and the direction of the multi-stage reactions in the 2-furaldehyde–H2O2–H2O
system in the presence of VOSO4 and without it are presented. Significant influence of the medium pH value on
the reaction pathway in the system under investigation hase been found. Mechanism of these reactions
considering the effect of acid-base properties of the medium has been suggested.
Key words: 2-furaldehyde, hydrogen peroxide, oxidation, vanadyl sulfate, mechanism
DOI: 10.1134/S1070363214060140
We have found previously [1–3] that the reaction of
2-furaldehyde I with hydrogen peroxide in water
medium without special catalysts proceeds through the
formation of 2-furaldehyde hydroxyhydroperoxide II
and its subsequent transformations (Scheme 1).
Their yields are comparable and reach 35–40% for
each product [3]. The carbonyl group of 2-furaldehyde
quantitatively converts to formic acid IX. The
formation of compounds VI, XI is accompanied by
small amounts of substances VI, VIII, X, and XII–XIV.
2-Furyl formiate III and a small amount of furoic
acid IV were found in the reaction mixture. These
substances are the products of of peroxide II
rearrangement according to the Baeyer-Villiger
mechanism [1, 2]. Through the intermediate 2-
hydroxyfuran V a mixture of isomeric 2(5H)- and
2(3H)-furanones VI, VII, 5-hydroxy-2(5H)-furanone
VIII, and such carbon acids as formic IX, β-
formylpropionic X, succinic XI, cis- and trans-β-
formylacrylic XII, XIIa, as well as maleic XIII and
fumaric XIV acids is formed (Scheme 1). Lactol VIII
and acid XII are tautomeric forms.
The addition of vanadyl sulfate to the 2-
furaldehyde–H2O2–H2O reaction system also leads to
the formation of intermediate peroxides [4–5]. At the
same time the conversion of 2-furaldehyde and
hydrogen peroxide significantly accelerates, and the
direction of oxidation turns to the side of prevailing
formation of tautomeric forms of cis-β-formylacrylic
acid VIII, IX with the total yield 30–40%. Lactone VI
and acid XI are side products in this system.
Depending on the reaction conditions their yields are
6–8 and 2–10% respectively. Yields of acids XIII,
XIV do not exceed 5–9%.
In the course of the reaction pH of the medium
decreases to 1-2, and the acids formed play the role of
catalysts of the processes taking place in the 2-fur-
aldehyde–H2O2–H2O system.
The pH of the reaction medium in the presence of
vanadyl sulfate during the oxidation decreases from 4
at the beginning to 1.5 units at the moment of complete
conversion of 2-furaldehyde and hydrogen peroxide.
The pH value at the beginning of the reaction is due to
the hydrolysis of vanadyl sulfate in the reaction
mixture, and the final pH value of the oxidate is caused
by the formation of carbon acids IX, XI–XIV. Hence,
a double catalysis of the process with the vanadium
Main products of the reaction of 2-furaldehyde with
hydrogen peroxide under the conditions of auto-
catalysis with acids which are formed in the course of
the reaction (without controlling the pH of the
medium) are 2(5H)-furanone VI and succinic acid XI.
1133

1134
BADOVSKAYA et al.
Scheme 1.
O
OH
H2O2
C
O
H
X
O
H
CH
C
O
O
O
H
OOH
H O
H
II
I
−H2O
Iа
−H2O
HO
O
OH
OH
COOH
O
C
O
H2O2
O
C
O
O
O
V+4,+5
OH
OH
XV
III
IV
+H2O
HOOC
OHC
COOH
XIII
XII
+ HCOOH
OH
O
O
O
IX
VI
V
H2O2
H2O2
H2O
H2O
H2O2
COOH
COOH
HO
O
OHC
COOH
O
VIII
O
VII
O
X
H2O2
HOOC
COOH
XI
COOH
H2O2
OHC
HOOC
XIIa
XIV
compound and autocatalysis with the acids formed is
observed in this reaction system.
absence of 2-furaldehyde and VOSO4 is comparatively
stable in the pH range 1–7 within 4 h, but it intensely
decomposes at pH below 1 and above 7. In the
presence of vanadyl sulfate decomposition of hydrogen
peroxide takes place to a certain degree at any pH
value, but the most intense reaction proceeds at pH
below 1 and above 7.
As seen, the oxidation of 2-furaldehyde with hydro-
gen peroxide without controlling the oxidate pH value
in the presence as well as without vanadyl sulfate pro-
ceeds through various directions and leads to the forma-
tion of the multi-component mixture of substances.
We established a significant influence of the
reaction medium pH value on the intensity of 2-
furaldehyde and hydrogen peroxide transformations in
the presence of vanadyl sulfate as well as without it. In
weak acidic media (pH 5–7) the process is slow, but it
accelerates in more acidic and more basic media.
Consumption of 2-furaldehyde and hydrogen peroxide
are most intensive when the pH value is maintained
below 3 or above 7. The effect of pH range 7–8 on the
intensity of reagents consumption is more significant
To determine the pH value effect on the processes
taking place in the 2-furaldehyde–H2O2–H2O system
and to extend the synthetic possibilities the reaction
mentioned we for the first time have studied these
reactions under the conditions of controlled pH value
of the reaction medium in the range 0.07–9.
It was established preliminary that no trans-
ormations of 2-furaldehyde in water without H2O2 take
place in this pH range. Hydrogen peroxide in the
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EFFECT OF ACID–BASE PROPERTIES OF THE MEDIUM
1135
Table 1. Effect of the medium pH value in the range 0–7.5 on the formation of furanone VI and carbon acids in the 2-
furaldehyde–hydrogen peroxide–water system (exp. 1–9 in the Experimental)
Yields of oxidation products, % b,c
2-Furaldehyde
Exp. no.
pH
consumptiona
furanone VI
acid IV
acid XI
acid XIII
1
2
3
4
5
6
7
8
9
0
1
100
100
98
38
50
40
22
13
–
–
–
40
40
40
43
45
50
40
20
4
7
5
2
–
3
3
80
–
–
4
65
–
–
5
52
30
50
65
45
–
6
48
–
6
7
50
–
9
7.5
58
–
12
a
b
Consumption in 4 h. In final oxidates formic acid in the yield about 100%, a mixture of compounds VIII, XII, XIIa, XIV, and also
malonic, oxalic, and malic acid in the total yield up to 15% are present. c With respect to consumed 2-furaldehyde.
Table 2. Composition of substances formed at the oxidation of 2-furaldehyde and the acid VI with hydrogen peroxide in the
presence of vanadyl sulfate at various pH values of reaction medium
Yields of oxidation products (see Scheme 1), %a
Exp. no.
рНа
VIII + XII
XIII + XIV
XI
IV
VI
XV
Oxidation of 2-furaldehyde
10
11
12
13
14
0–3
4–6
55
40
5
26
8
6
10
35
3
–
–
7
8
9
–
–
–
–
6.5–7
8–9
8–9b
17
18
29
6
4
2
6
60
39
–
3
13
Oxidation of furoic acid
15
16
6.5–7
8–9
14
4
10
22
20
8
14c
2c
1
–
1
40
a
b
pH ranges where the composition of products in the reaction mixtures is close are presented. Experiment performed under analogous
conditions without vanadyl sulfate. c Remaining amount of acid IV.
in the presence of vanadyl sulfate. At pH below 1 even
in the absence of vanadyl sulfate 100% conversion of
2-furaldehyde is achieved within 1 h.
From Table 1 it follows that furanone VI is formed
in the oxidation of 2-furaldehyde only at pH below 4,
and in the largest amount only at pH about 1. At the
same time acid IV is formed only in the processes
taking place at pH above 4, and its highest yield is
achieved in the reaction systems with pH about 7.
These facts on the one hand open the way to
optimization of methods of obtaining compounds IV
and VI, and on the other hand show the effect of pH on
The significant effect of the medium pH value on
the direction of reactions in the 2-furaldehyde–
hydrogen peroxide–H2O system in the presence or
without vanadyl sulfate has been established for the
first time (Tables 1, 2).
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1136
BADOVSKAYA et al.
the direction of rearrangement of peroxide II (Scheme 1).
The obtained results show that at pH 0–3 the
rearranegement of peroxide II into ester III and further
to lactone VI prevails. At pH 6–7 the main product of
peroxide II rearrangement is acid IV. Such effect of
the acid-base properties of the medium on the direction
of Baeyer–Villiger rearrangement is known also for
the other hydroxyhydroperoxides [1].
A common specific feature of processes in the 2-
furaldehyde–hydrogen peroxide–H2O–vanadyl sulfate
system in all the pH range under consideration as
compared to the process without vanadyl sulfate is the
formation of noticeable amounts of oxalic and malonic
acids and carbon dioxide as well as increased yield of
formic acid. It may be the result of oxidative cleavage
of molecules of the main reaction products in the
presence of VOSO4.
Alongside with compounds IV and VI succinic XI
and maleic XIII acids are also formed. They are the
products of transformation of isomeric lactone VII and
the tautomeric form of lactone VI, hydroxyfuran V,
respectively (Scheme 1).
It is noticeable that in the reaction of 2-furaldehyde
with hydrogen peroxide in the presence of vanadyl
sulfate in the conditions under consideration content of
furanone VI among the final reaction products is
insignificant. It may be explained by its quantitative
transformation to the acid XI (Scheme 1) which is
significantly activated in the presence of vanadyl
sulfate [5].
It may be due to the fact that during the pH increase
from 4 to 7.5 the transformation of lactone VI to
lactone VII and hydrolytic transformation of the latter
to acid X is facilitated. The compound formed is
oxidized to acid XI (Scheme 1). We proved it while
studying the reactions of furanone VI with hydrogen
peroxide in water media in the pH range 0–8 [6]. Maleic
acid XIII is the side product of the reaction at these pH
values. It shows that at the oxidation of 2-furaldehyde
in the absence of vanadyl sulfate the reaction pathway
leading to formation of hydroxyfuran V and then
compounds VIII and XII is insignificant.
The comparison of data presented in Tables 1 and 2
shows that the influence of the medium pH on the
composition of products, and hence on the direction of
oxidation in the 2-furaldehyde–hydrogen peroxide–
H2O system is more significant than in the presence of
vanadyl sulfate. This means that the main role in this
system at pH 0–7.5 belongs to vanadyl sulfate catalyst
while the contribution of acid–base catalysis is less
significant. The preferred formation of tautomeric
forms VIII, XII and acid XIII in the presence of
vanadyl sulfate may be due to the activation of
oxidation with hydrogen peroxide first of hydroxy-
furan V and then of tautomers VIII, XII into acid XIII
(Scheme 1). The preferred oxidation of acids VIII, XII
to acid XIII we established previously [2, 5]. In its
turn it favors the shift of equilibrium transformations
of compounds V, VI, VII to the isomer V.
Note that in the processes without vanadyl sulfate
at pH 0–7.5 succinic acid XI is one of the main
products. At pH below 5 it is formed together with
lactone VI, and at higher pH values, along with acid
IV (Table 1).
Note also that in acidic and close to neutral media
in the presence and in the absence of vanadyl sulfate a
significant difference in the yields of main products of
oxidation, and hence in the direction of the processes
taking place is observed (Tables 1, 2).
It turned out unexpectedly that at pH 8–9 the
compositions of oxidation products in the systems
containing vanadyl sulfate and without it differ
insifnificantly. They are in striking contrast with the
com-position of oxidation products at pH 0–7.5
(Tables 1, 2). It permits a conclusion that in the
processes at pH 8–9 the main factor determining the
direction of oxidation is the high basicity of the
medium. Most probably in this case the same active
particles leading to similar oxidation products in the
presence or without vanadyl sulfate are formed from
H2O2.
In the systems with pH below 3 containing no
vanadyl sulfate furanone VI and the acid XI are the
main reaction products. In the presence of vanadyl
sulfate cis-formylacrylic acid existing as a mixture of
tautomeric forms VIII and XII prevails among the
oxidation products (Table 2). In the absence of vanadyl
sulfate their total yield is no more than 4%.
At pH 6–7 the main oxidation product of 2-fur-
aldehyde in the presence of vanadyl sulfate is acid XI,
while in its absence the acid IV is formed. It is
characteristic that in any other studied reaction
conditions in the presence of vanadyl sulfate acid IV is
not the main product.
One more peculiarity of oxidation processes of 2-
furaldehyde with hydrogen peroxide in highly acidic
media in the presence as well as in the absence of
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EFFECT OF ACID–BASE PROPERTIES OF THE MEDIUM
1137
Scheme 2.
COO−
pH > 7
COO−
OH
H2O
H
+ 1O2
COO−
HO
H2O2 or
O
O
O
O
O
[H2O2 + VOSO4]
XVIII
XVI
XVII
−H2O
O
COO−
OH
COO−
OH
H2O2
O
OH
−CO2
O
O
O
O
O
VIII
XIX
XX
2HO
H2O
H2O
HO
O
OH
COO−
OH
HO
OH
H+
COOH
OH
HOOC
COOH
XIII
O
O
O
O
O
O
XXII
XXI
XV
H2O
HOOC
COOH
XI
vanadyl sulfate is the preferred formation of 3,4,5-
trihydroxy-5-carboxy-2(3H,5H)-furanone XV. Its yield
significantly increases in the presence of vanadium
catalyst. We first obtained and described lactone XV in
[7].
process in the presence of vanadyl sulfate proceeds
significantly faster. At pH 6.5–7 insignificant amounts
of lactone XV, tautomeric forms of acids VII,XII,
acids XIII and XI with the prevalence of the latter are
formed. At pH 8–9 the main products of acid IV
oxidation are carboxylactone XV and acid XIII.
The lack of acid IV among the products of 2-
furaldehyde oxidation with hydrogen peroxide in the
presence of vanadyl sulfate at pH 0–6 requires to be
explanated. It is characteristic that at pH 6.5–9 it forms
only in insignificant amounts (Table 2). At the same
time in the absence of vanadyl sulfate acid IV is
present in the reaction mixtures obtained in the media
with pH 4–9. Its maximum yield is achieved at pH
about 7, and then its content decreases (Tables 1, 2).
The results presented permit to explain formation of
carboxylactone XV in the reactions of 2-furaldehyde
oxidation with hydrogen peroxide at pH 8–9 by the
transformations of a salt of acid IV according to
Scheme 2.
In the molecule of salt XVI the electron density
distribution significantly differs from that in 2-
furaldehyde. The anionic substituent significantly in-
creases the electron density in the furan ring. It distorts
its aromaticity and favors the reactions of the furan
ring as the diene system. As known [8–10], at pH > 7
an active decomposition of hydrogen peroxide
molecules takes place. It leads to the formation of a set
of active particles, among them of HO·, HOO·, HO2–,
O22–, and singlet oxygen 1O2. In the presence of vanadyl
sulfate this process is activated to form peroxo-
complexes of VO5– type as well as peroxocomplexes
containing a larger number of peroxoligands [11–13].
Note that at pH 8–9 in the reactions in the presence
as well as in the absence of vanadyl sulfate
carboxylactone XV is mainly formed (Table 2).
Apparently acid IV is transformed to lactone XV at
these pH values. Taking this fact in consideration we
carried out the oxidation of the acid IV with hydrogen
peroxide in the media with definite pH values varying
from 6.5 to 9 (Table 2). It occurred that the
compositions of the products in the presence as well as
in the absence of vanadyl sulfate are similar, but the
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1138
BADOVSKAYA et al.
These vanadium compounds are significantly more
active oxidants than H2O2, and their decomposition
leads to intense formation of singlet oxygen 1O2. A lot
of reactions reactions of furan compounds with singlet
oxygen are known [14]. The lifetime of singlet oxygen
is sufficient for its entering the reaction with
substances present in the reaction mixture. These facts
permit the suggestion of the formation of ozonides
XVII and their trans-formation to compounds XVIII,
XIX during the reaction of the acid IV with hydrogen
peroxide in basic media. In the course of our studies of
the reaction of the acid IV with hydrogen peroxide in
basic media the formation of several peroxo-
compounds was established chromatographically, but
we failed to reveal their structure. Analogous trans-
formations and their products were found in the studies
of photochemical reactions of furan compounds with
molecular oxygen [15, 16].
reaction systems under study. The acids protonate the
carbonyl oxygen of 2-furaldehyde increasing the
electrophilicity of the carbonyl carbon atom in the
reactions with hydrogen peroxide. Analogous effect
produces the complex formation of 2-furaldehyde with
VOSO4. Bases favor the increase in the nucleophilicity
of hydrogen peroxide in its reactions with the carbonyl
group of 2-furaldehyde and also the formation of
HOO– nucleophilic species from hydrogen peroxide.
Due to that the rate of oxidation of 2-furaldehyde
increases in the presence of acids as well as of bases.
Acids protonate the peroxide bond oxygen in peroxide
II favoring its cleavage and the formation of
intermediate through which proceeds the Baeyer-
Villiger rearrangement of the peroxide including the
transfer of furyl radical and the formation of
hydroxyfuran formiate III. The latter in the presence
of acids as well as of bases readily hydrolyses to form
hydroxyfuran V (Scheme 1).
Our results [7] permit to suggest that unsaturated
carboxylactone XIX is epoxized to lactone XX. The
latter by hydrolysis converts to compound XXI which
at acidifying the reaction medium forms lactone XV
(Scheme 2). The formation of compound XXI may be
regarded also as the result of hydroxylation of double
bond of lactone XIX with hydroxyl radicals formed
from hydrogen peroxide. This pathway of acid IV
reaction with hydrogen peroxide leading to lactone XV
prevails at pH > 7. The directions leading to the acids
XI, XIII are developed in parallel. Most probably they
form through the stage of decarboxylation of lactone
XIX giving lactol VIII followed by its subsequent
oxidation to acid XIII, and also by isomerization to
acid XI. Recently we have found such isomerization,
and its dependence on the pH of the medium was
established [17]. It was found also that the preferred
product of oxidation of compounds VIII, XII, and
XIIa is acid XIII [2, 5].
As known [1], the rearranegement of hydroxy-
hydroperoxides to carboxylic acids facilitates in basic
media. It is also observed in the reaction under study
where at pH > 6 among the compounds forming at the
oxidation of furoic acid VI or the products of its
transformation prevail (Tables 1, 2; Schemes 1 and 2).
The acidity of the medium also influences mutual
tautomeric and isomeric transformations of compounds
V, VI, VII, and VIII, XII (Scheme 1) as well as the
oxidative and hydrolytic transformations of compounds
VIII, XII and lactones VI, VII as well as the oxidation
processes of compounds IV, V, X, XII and lactones VI,
VII (Scheme 1).
Hence, in the oxidation of 2-furaldehyde with
hydrogen peroxide the acid-base properties of the
reaction medium play significant role at all stages of
the process and determine its direction.
Results we have obtained show that pH of the
medium in the 2-furaldehyde–H2O2–water and the 2-
fural-dehyde–H2O2–VOSO4–water systems significantly
influences the rate and the direction of transformations
of substances in these systems.
EXPERIMENTAL
All compounds used were of “pure” or “chemically
pure” grade. 2-Furaldehyde was preliminary distilled
in a vacuum.
The established rules of the influence of the
medium pH value on the reactions in these systems is
the basis for optimization of methods for the syntheses
of lactones VI, VII, VIII, XV, and acids IV, X–XII
(Scheme 1). On the other hand, the obtained results
extend our knowledge on the mechanism and the effect
of acid–base properties of the reaction medium on the
majority of stages of multiple-directed reactions in the
Reactions were carried out under temperature
control and intense stirring. Every 20–40 min samples
were taken, and the content of 2-furaldehyde, hyd-
rogen peroxide, organic peroxides, general acidity, pH
of the medium, and also the content of acids IV, IX–
XIV, lactol VIII, and lactones VI, VII, XV was
evaluated. General acidity was evaluated by the acid-
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EFFECT OF ACID–BASE PROPERTIES OF THE MEDIUM
1139
base titration. pH value was determined by means of
pH-meter. Content of hydrogen peroxide and organic
peroxides in the case of their joint presence was
evaluated according to the procedure [18]. The content
of 2-furaldehyde and acid IV in oxidates was evaluated
spectrophotometrically using the calibrating curves at
278 and 245 nm in water solutions. The content and
concentration of unsaturated carboxylic acids was
evaluated polarographically using a mercury dropping
electrode [19–21] and chromatographically [22].
Evaluation of acids was also carried out by known and
previously described methods of acid-base [1], iodo-
metric, and cerimetric titration [3, 21], polarography
with a dropping mercury electrode [21], pH-metering
[3, 23], UV spectroscopy in the range 230–300 nm [3,
4], TLC, paper chromatography, GLC [1, 22, 25], and
chromatomass spectrometry. Yields of all compounds
are presented with respect to reacted 2-furaldehyde.
out at 60°C, 2-furaldehyde–hydrogen peroxide–VOSO4
molar ratio 1 : 3.2 : 0.005, and starting concentration
of 2-furaldehyde 1.2 mol/L.
Oxidation of 2-furaldehyde with hydrogen per-
oxide at pH 8-9 in the presence of VOSO4 (Table 2,
exp. 13). Reaction was carried out at 60°C, 2-fur-
aldehyde–H2O2–vanadyl sulfate molar ratio 1 : 5 : 0.005.
Oxidation of the acid IV with hydrogen peroxide
at pH 6.5–9 (exp. 15, 16). Reaction conditions are
analogous to that of the exp. 12, 13, but instead of 2-
furaldehyde acid IV is used.
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A three-necked flask equipped with a stirrer and a
reflux condenser placed in a thermostat was charged
with desired amounts of 2-furaldehyde, 30% hydrogen
peroxide solution, and buffer solutions for creating a
definite pH value of the medium. The desired pH value
in the range 0–7.5 was maintained by addition of
definite amount of phosphoric acid and various
amounts of potassium hydroxide. Constant value of
ionic power was maintained by addition of cor-
responding amount of potassium chloride. To obtain
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1.25 mol of sodium carbonate per 1 mol of 2-
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Reaction of 2-furaldehyde with hydrogen
peroxide at pH 8–9 without the addition of vanadyl
sulfate (Table 2, exp. 14). The process was carried out
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