Investigation in the series of substituted butan- and butenolides: XV. Transformations of 4-hydroxy-2-butenolide in water media at various pH values

Article abstract of DOI:10.1023/A:1015578003374

The 4-hydroxy-2-butenolide was established by polarographic method to undergo in water solutions fast tautomeric and acid-base transformations. Depending on pH of the medium the compound is present either as a cyclic irreducible form (at pH 0-4) or in the open-chain (carbonyl-containing) reducible forms (at pH > 4 and < 0). It is presumed that in going from the basic to strongly acidic medium and backwards occur consecutive equilibrium transitions of four species: cis-β-formylacrylic acid anion (A ^-), its neutral molecule (HA) existing as linear and cyclic tautomers, and protonated forms (H₂A⁺ and H ₃A⁺). The formation mechanism thereof is considered.

Full text of DOI:10.1023/A:1015578003374

Russian Journal of Organic Chemistry, Vol. 38, No. 2, 2002, pp. 251 255. Translated from Zhurnal Organicheskoi Khimii, Vol. 38, No. 2, 2002,
pp. 273 277.
Original Russian Text Copyright
2002 by Strizhov, Poskonin, Badovskaya, Kupina.
Investigation in the Series of Substituted Butan-
*
and Butenolides: XV. Transformations
of 4-Hydroxy-2-butenolide in Water Media
at Various pH Values
N. K. Strizhov, V. V. Poskonin, L. A. Badovskaya, and E. P. Kupina
Kuban State Technological University, Krasnodar, 350072 Russia
Received August 16, 2000
Abstract The 4-hydroxy-2-butenolide was established by polarographic method to undergo in water
solutions fast tautomeric and acid-base transformations. Depending on pH of the medium the compound is
present either as a cyclic irreducible form (at pH 0 4) or in the open-chain (carbonyl-containing) reducible
forms (at pH > 4 and < 0). It is presumed that in going from the basic to strongly acidic medium and
backwards occur consecutive equilibrium transitions of four species: cis- -formylacrylic acid
+
anion (A ), its neutral molecule (HA) existing as linear and cyclic tautomers, and protonated forms (H A
2
+
and H A ). The formation mechanism thereof is considered.
3
The -formylacrylic acid is among the main
products in furan compounds oxidation by aqueous
hydrogen peroxide in the presence of compounds of
V and VI groups elements (V, Se, Cr) [2, 3]. This
compound is known to exist as tautomeric and iso-
meric forms: cyclic substance [4-hydroxy-2-buteno-
lide (I)], and acyclic cis- and trans-isomers II and III
meric and hydrolytic transitions of compound I in
water media in a wide range of pH values by means
of polarography.
We report in the present communication on results
of transformation study and the structure of 4-hydr-
oxy-2-butenolide (I) forms in water solution with pH
in the range from 10 to 0.5. The discussion of
results takes into account the assumption of [7, 8] on
the polarographic activity of I III forms.
[4 8] (Scheme 1).
Scheme 1.
We used in the study free of impurities butenolide
I prepared by acid hydrolysis of 4-acetoxy-2-buteno-
lide under mild conditions. The measurement of
current intensity and of half-wave potential of
reduction for different forms of compound I arising
at its dissolution in water media at various pH values
was carried out with a dropping mercury electrode at
2
l
5 0.2 C, the solution ionic strength 1.0 or 3.0 mol
1
(HClO + NaClO ), and initial concentration of
4
4
We reported formerly on isolation of cis- -formyl-
3
4
1
compound I 4.0 10 4.0 10 mol l As reference
was used normal calomel electrode. The butanolide I
solutions were subjected to measurement 1 5 min
after preparation. The acidity of solutions was
measured by titration with HClO4 or NaOH.The
acrylic acid as lactone I from the mixture of furfural
oxidation products [2]. However no published data
was found on the behavior of its isomers I III at
different pH which were important for the control of
butenolide preparation and for extending its applica-
tion as a synthon. Only reports appeared on polaro-
graphic reduction of 5-ethoxy-2-(5H)-furanone and
trans- -formylacrylic acid in water solutions [7, 8].
In this connection we carried out a study of tauto-
initial reduction potential E was 0.2 V.
in
On adding butenolide I into weakly acidic, neutral,
or basic solutions the reduction waves on polaro-
grams depended on the pH value (Fig. 1). In the
neutral solution were observed two waves. Therewith
the half-wave potential of the first one (E , V)
*
For communication XIV see [1].
1
/2
1
070-4280/01/3802-251$27.00 2002 MAIK Nauka/Interperiodica

2
52
STRIZHOV et al.
previously described in [7] depended on pH of the
medium according to equation E‘1/2 0.386 V +
.059 pH (curves 1 4), and the potential of the second
=
0
wave E 1/2 equal to 1.12 V was independent of
proton concentration (curves 3 5).
As NaOH was added to the butenolide I solution
the first wave decreased and the second wave grew
(
curve 4). On addition of 0.70 0.75 mol of the base
per 1 mol of compound I the first wave totally dis-
appeared, and the height of the second remained
constant at further alkalization (curve 5). At succes-
sive addition of HClO to the initial butenolide I
4
solution till pH 1 the first wave started to grow
(
curve 2) and then decreased (curve 1). This result
suggesting the kinetic character of the wave was
already reported in [7] at polarographic measurements
on the solutions of 5-ethoxy-2(5H)-furanone. The
latter is known to provide isomers of the -formyl-
acrylic acid on hydrolysis [5].
3
Fig. 1. Reduction waves of butenolide I (4 10 M) in
These data and also those presented in the table
and on Fig. 2 show that at pH > 1 the current
solutions of HClO4 and NaOH of ionic strength 1.0 M
3
(
NaClO + HClO or NaOH), E 0.2 V: (1) 4 10
M
M
4
4
in
intensity (i ) depends on pH as has been observed in
4
3
eff
HClO , (2)4 10 HClO , (3)no additives, (4)0.9 10
NaOH, (5) 2.7 10 M NaOH.
4
4
[
4].
3
We demonstrated that behavior of butenolide I in
more acidic media (pH < 1) in the range from pH 1
to 0.5 is characterized by the dependence of ieff on
pH that grew with rising acidity and did not attain any
limiting value (Fig. 2). This fact evidences formation
in the strongly acidic solutions of previously
unknown reducible species of compound I.
Kuzovnikova et al. [7, 8] suggested for the polaro-
graphic activity of 5-ethoxy-2(5H)-furanone that a
reduction mechanism of forming therefrom at pH 4 6
cis- -formylacrylic acid (II) involved C=C bond.
However nonlinear dependence of ieff on pH and the
direction of the E‘1/2 shift at the change in the acidity
that we have observed in the study of compound I
correspond to the behavior of the majority of the
previously investigated aldehydes and ketones [9].
Thus it is presumable that the reduction of butenolide
I occurs at the aldehyde C=O groups of its open-
chain forms arising in water solutions. This assump-
tion is supported by the fact of one-electron reduction
of these forms as we have established along the
known methods [10], whereas the C=C bond (e.g.,
in ethylene, maleic and fumaric acids, etc.) is reduced
along the two-electron mechanism [10].
Fig. 2. Effective limiting current intensity for the first
reduction wave of the molecular form II of butenolide I
(
i/c)as a function of pH ( log cHClO4).
Scheme 2.
On Scheme 2 and in the table are presented the
main forms of butenolide I that according to this study
and [7, 8] arise in its water solutions at various pH.
Their presumable structures are considered below.
n = 1 or 2.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 38 No. 2 2002

INVESTIGATION IN THE SERIES OF SUBSTITUTED BUTAN- AND BUTENOLIDES: XV.
253
Main existing forms of butenolide I and hydrolysis products of 5-ethoxy-2(5H)-furanone at various pH values
Polaro-
graphic
activity
Registration conditions and polarographic characteristics of the first wave
base pH
Presumed
structure
E1/2, V
a
a
a
a
a
a
1
2
1
2
1
2
IV
II
+
+
NaClO + NaOH
Universal
buffer
0.1 M HCl
+ 1 M KCl
1 M HCl
> 6
6 8
3 6
< 3
0.7 1.0
0.6 0.7
0.7 0.8
0.5 0.7
0.35 0.5
4
NaClO or
4 6
4
NaClO + HClO
4
4
4
I
V
NaClO + HClO
0 4
<0.5
0.4 0.6
<0.4
4
+
same
a
1
, data of this study; 2, data from [7, 8].
The dependence of ieff on pH (Figs. 1 3)
However as seen from Fig. 2 at further acidification
of the solution in question, namely at pH < 0.5,
occurs a sharp increase in the current intensity of
the first wave evidencing the transformation of
irreducible species I into polarographically active
suggests that in the electrode process at different pH
take part at least four forms of compound I; therewith
three of them, namely II, IV, and V and/or VI are
polarographically active. The behavior features we
revealed permit stating the following: (a) the reduction product of presumable structure V and/or VI
of species I, IV, and V, VI involves an aldehyde
group C=O; (b) compound I in a water solution with
no addition of acids or bases exists mainly as acyclic
tautomer II that on variation of the medium acidity
undergoes a number of transitions detected by polaro-
graphy.
(Scheme 2).
Thus we found a definite range of solution pH
0 < pH < 4) where occurred significant accumula-
(
tion of the cyclic form of cis- -formylacrylic acid (I):
up to 30 60% to the overall amount of its speciations
(depending on its initial concentration and the ionic
The decrease in the current intensity at growing
pH was ascribed in [7] to the decrease in the con-
centration of the molecular form of cis- -formyl-
acrylic acid II due to its transformation into anion IV
characterized by another reduction potential. Our data
support this assumption. As shown above (Fig. 1)
the decrease in the current intensity of the first wave
is accompanied by appearance at more negative
potentials of the second wave. Its current intensity
grows with pH to attain finally the value of the
limiting current intensity of reduction of trans- -
formylacrylic acid III.
strength of the medium) (Figs. 2 and 3). Note that
the degree of transition of molecule II into irreducible
form I directly depends on the ionic strength of the
solution.
As the neutral solution of butenolide I existing
nearly totally in the form of acyclic tautomer II is
acidified till pH > 0.5 a transformation into
irreducible form occurs that has been previously
regarded as lactol I [7]. Thus our data on polaro-
graphic activity of butanolide I species in water
solutions at pH > 0.5 are in general consistent with
the results of [4, 5] (see the table). In both case in
the basic, neutral, and weakly acidic media were
observed the reducible forms IV and II, and at pH
from 3 4 to 0.5 was present irreducible form I.
Fig. 3. Effective limiting current intensity i/c of acid II as
a function of neutralization degree of acid II: cof acid II
1
1
4.0 mmol l ; cNaClO
HClO 1.0 mol l ; at X < 0 X =
4
+
4
c
HClO4/cof acid II; at X > 0 X = cNaOH/cof acid II.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 38 No. 2 2002

2
54
STRIZHOV et al.
Additional information on butenolide I transform-
ations and on the forms arising therefrom we obtained
by titration of its water solutions in the polarographic
in general consistent with the previously reported
suggestion [7, 11, 12] that the irreducible form
existing at pH 0 4 is lactol I. At the same time the
unusual form of the curve on Fig. 2 and the
dependence of the current intensity of reduction on
the ionic strength of the medium and the reasoning
given further suggest that in the above mentioned pH
range may also exist the other polarographically
inactive forms of the cis- -formylacrylic acid
(Scheme 3).
cell with solutions of HClO or NaOH. The experi-
4
ment was carried out with no buffer; the solution
contained predominantly molecular form II, and the
acidity was adjusted by adding various amounts of
acid or base solutions. The results of the experiment
describing the variation of current intensity for the
first wave depending on the neutralization degree of
the open-chain molecular form II (X= cNaOH/cof acid II
)
and on the degree of its acidification ( X = cHClO4/
cof acid II) are given as a curve on Fig. 3. As
seen, the pattern of molecule II behavior at small pH
variations is different at acidification or neutraliza-
tion. Thus in the presence of a small amount of
Scheme 3.
HClO the content of acyclic form II reaches the
4
maximum (the current intensity of its reduction
attains that of reduction of trans-isomer III). At
further acidification to pH 1 its content starts to
decrease. At the same time on adding only 0.7 mol
of NaOH per 1 mol of acid II its concentration falls
to zero. The data on Fig. 3 suggest the following. In
the water solution without HClO4 or NaOH
butenolide I nearly completely is present as molecular
acyclic form II. As the acidity of the medium grows
HA molecule II goes through a maximum of its
content and then affords polarographically inert
species that on further acidification is transformed
+
+
into protonated forms H A (V) and/or H A (VI).
2
3
It was remarkable that the above interconversions
of butenolide I and its forms II, IV, and V, VI
within all the pH range under study occurred virtually
instantaneously (the time of equilibration tends to
zero) and reversibly (the curves obtained by direct
and reversed titration of the neutral compound I solu-
tion coincided). This fact indicates the absence of
significant energy barrier to the transitions. The
experimental value of the activation energy for the
reversible conversion of acid II into the irreducible
In principle on acidifying the weakly acidic solu-
tion containing mainly molecular species II may arise
several monoprotonated forms with presumable struc-
ture VIIa c. Polarographically inactive intermediates
VIIb and VIIc may contribute to the decrease in
current intensity of reduction in the pH range 0 4.
However their concentration as also that of lactone I
should be dependent on the medium acidity. Actually
it is not observed (at pH 0 2.5 ieff is virtually
independent of pH). Thus the accumulation of these
species is insignificant. Therefore it is presumable
that in this pH range alongside the cyclic structure I
exists one more irreducible form of the cis- -formyl-
acrylic acid that may be represented as acid VIId with
an intramolecular hydrogen bond.
1
form I amounts to 14 kcal mol . The value was
obtained by measuring the current intensity in the
minimum point on the curve (Fig. 3) as a function of
the temperature. The calculated value of the heat
evolution in this stage has a close figure ( H
1
2
3 kcal mol ) [10].
At pH < 0 the proton concentration becomes
apparently sufficient for decyclization of structures I
and VIId to afford reducible species V, VI (bi-
protonated molecule VI is polarographically in-
distinguishable from adduct V).
Low energy values characterizing this process and
the polarographic inertness of the form accumulating
at pH 0 4 are inconsistent with the structure of trans-
isomer III.
The results obtained and literature data on the
ring-chain tautomerism of -acylacrylic acids [6] are
In consideration of spatial models of cis- -acyl-
acrylic acids it was previously presumed that their
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 38 No. 2 2002

INVESTIGATION IN THE SERIES OF SUBSTITUTED BUTAN- AND BUTENOLIDES: XV.
255
open-chain structures could exist only in the
energetically unfavorable noncoplanar form prevent-
ing full-fledged conjugation of the -bonds [6]. We
carried out a formal computation by the software
PCModel of the structural and energy parameters of
molecular form II and of hypothetical biprotonated
structure VI. The calculation confirmed that
theoretically the most stable conformation of
molecular form II is nonplanar molecule of cis- -
formylacrylic acid that arises apparently as a result
of mutual repulsion of two C=O groups. The
simulation with the use of the above software of two
hydroxonium ions addition to molecule II resulted in
measured the dependence of current intensity of
reduction on the neutralization degree 20 ml of base
electrolyte with appropriate pH value was charged
into the polarographic cell then the temperature was
brought to the desired value, the cell was flushed with
nitrogen, the solution of compound I was charged
with a microsyringe, and the mixing was performed
by the nitrogen flow. The polarograms were recorded
at definite time intervals till the current no more grew
due to reduction of the polarographically active
forms.
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an analogous nonplanar structure VI, therewith
+
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3
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HCl, the mixture was stirred for 6 10 h at 40 50 C
till complete hydrolysis of the initial butenolide, the
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dried in air, the reaction product was extracted with
anhydrous ethyl ether, and dried with Na SO . On
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substance that slowly crystallized. The physico-
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3
,
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1
4
.0 10 4 mol l . The cell was maintained at
temperature 25 0.2 C and was flushed with nitrogen
to remove oxygen. In experiments where was
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 38 No. 2 2002