Esterification equilibrium constants of arsonic and arsinic acids

Article abstract of DOI:10.1007/s00706-012-0867-5

The esterification equilibrium constants between CD₃OD and five arsonic acids (methyl, pentyl, phenyl, 2,3- dipalmitoyloxypropyl, and 2,3,4-tripalmitoyloxybutyl) and one arsinic acid (cacodylic acid) were measured in the solvent system CDCl₃/CD₃OD at 25.0 °C. Equilibrium concentrations of the diester, monoester, and free acid for each compound were calculated from the integrations of the corresponding ¹HNMRpeaks. It was found that the nature of the organic part of the arsonic acids essentially did not affect the measured equilibrium constants which were around 0.114 and 0.014 for K₁ and K₂, respectively. However, the equilibrium constant K₁ of cacodylic acid was 0.053. Diesters and monoesters are very easily hydrolyzed, and in particular the diesters are extensively hydrolyzed even in the presence of tiny amounts of water. Thus, esters of arsonic or arsinic acids with monohydric alcohols are expected to be in very low concentrations inside cells, a conclusion that may be of interest to the arsenic bacterium debate . Springer-Verlag 2012.

Full text of DOI:10.1007/s00706-012-0867-5

Monatsh Chem (2012) 143:1603–1608
DOI 10.1007/s00706-012-0867-5
ORIGINAL PAPER
Esterification equilibrium constants of arsonic and arsinic acids
•
Gerasimos M. Tsivgoulis Panayiotis V. Ioannou
Received: 17 July 2012 / Accepted: 24 September 2012 / Published online: 18 October 2012
Ó Springer-Verlag Wien 2012
Abstract The esterification equilibrium constants between
CD3OD and five arsonic acids (methyl, pentyl, phenyl, 2,3-
dipalmitoyloxypropyl, and 2,3,4-tripalmitoyloxybutyl) and
one arsinic acid (cacodylic acid) were measured in the
solvent system CDCl3/CD3OD at 25.0 °C. Equilibrium
concentrations of the diester, monoester, and free acid for
each compound were calculated from the integrations of the
corresponding 1H NMR peaks. It was found that the nature of
the organic part of the arsonic acids essentially did not affect
the measured equilibrium constants which were around
0.114 and 0.014 for K1 and K2, respectively. However, the
equilibrium constant K1 of cacodylic acid was 0.053. Dies-
ters and monoesters are very easily hydrolyzed, and in
particular the diesters are extensively hydrolyzed even in the
presence of tiny amounts of water. Thus, esters of arsonic or
arsinic acids with monohydric alcohols are expected to be in
very low concentrations inside cells, a conclusion that may
be of interest to the ‘‘arsenic bacterium debate’’.
react with free thiols or enzymes bearing –SH groups is
important for their biochemical reactions [3]. There are
marine organisms capable of incorporating arsenic in
water-soluble [4, 5] or lipid-soluble [5, 6] molecules hav-
ing an As–C bond without apparent toxicity or, as recently
claimed, to their DNA through an As–O–C bond [7].
Moreover, certain arsenic compounds, e.g., arsenic triox-
ide, show anticancer properties [8]. In order to better
understand the role of arsenic compounds in living
organisms, information on esterifications of arsenous,
arsonic, and arsinic acids are necessary.
The reaction between an arsonic or arsinic acid with an
alcohol leads to esterification in a way similar to the
esterification of a carboxylic acid (Scheme 1).
However, the esterification of arsonic and arsinic acids
proceeds much faster, because the bigger size of the arsenic
atom compared to the carbon atom permits an easier entry
of the alcohol [9]. The same applies for the hydrolysis of
their esters [9].
Keywords Bioinorganic chemistry ꢀ NMR spectroscopy ꢀ
Main group compounds ꢀ Arsenic compounds ꢀ
Equilibrium constants
Although a number of articles have been published by
our group on the synthesis and properties of aliphatic
O
C
O
C
H2O
R
OR'
+
R
OH
+
R'OH
Introduction
Carboxylic acid
Arsenic compounds are known for their toxicity [1] and
they are also pollutants from certain industrial processes
[2]. Without doubt, the ability of arsenic oxo-compounds to
O
O
O
R'OH
H2O
R'OH
H2O
OH
OH
OR'
OR'
R
As
R
As
R
As
OH
OR'
Arsonic acid
O
O
R
R
H2O
R'OH
+
+
OR'
As
As OH
R
R
G. M. Tsivgoulis (&) ꢀ P. V. Ioannou
Department of Chemistry, University of Patras,
26500 Patras, Greece
Arsinic acid
e-mail: tsivgoulis@chemistry.upatras.gr
Scheme 1
123

1604
G. M. Tsivgoulis, P. V. Ioannou
Fig. 1 Structures of the acids
dealt with in this study
O
OH
As
O
O
OH
OH
OH
OH
As
CH3 As
OH
3
1
2
C15H31COO
C15H31COO
C15H31COO
C15H31COO
C15H31COO
O
CH3
O
As
OH
OH
OH
O
CH3
As
OH
OH
As
6
4
5
arsonic acids [10–13], no detailed studies were done on
their esterification. The kinetics for the hydrolysis of
arsenous triesters, As(OR)3 [14], the equilibrium constants
for the transesterification of arsenous triesters [3], and the
rates of esterification of aromatic arsonic acids with certain
alcohols [15] have been published but no quantitative data
are available for the esterification equilibrium constants of
arsonic or arsinic acids and this absence of relevant
information has been noticed recently [16–18].
R
O
O
O
O
O
OR'
OR'
As
R
As
I
II
Fig. 2 General structures of produced esters
character of the alcohol. Monohydric alcohols give the
expected diesters I [15], whereas in the case of 1,2-diols
the diesters react further and give spiro-products of the
general structure II [23] (Fig. 2).
Various techniques are available for the estimation of
esterification equilibrium constants, i.e., titrations [19],
NMR measurements of various nuclei [20, 21], UV–Vis
measurements [22], etc. These techniques are based on the
direct or indirect determination of the concentration change
To push the equilibrium towards the diester, either
azeotropic distillation of water [24, 25] or large excess [15]
of the monohydric alcohol under anhydrous conditions was
used. Thus, the electrospray ionization mass spectrometry
(ESI–MS) spectrum of ferrocenyl arsonic acid in methanol
revealed only the presence of the monoester [26]. It is
worth noting that under similar conditions the ESI–MS
spectrum of ferrocenyl phosphonic acid showed no sign of
esterification, because the free acid instead of the ester was
observed. These results indicate that esterification of ar-
sonic acids is very fast compared to the esterification of
phosphonic acids. Interestingly, the spiropyran-type com-
pounds II produced from 1,2-diols and arsonic acids are
much less sensitive to hydrolysis [23]. The equations
describing the reaction of deuterated arsonic and arsinic
acids with CD3OD are shown in Scheme 2.
1
of the reactants and/or products. Among them H NMR is
particularly suited because the change in concentrations of
both reactants and products can usually be followed and the
existence of by-products or stable intermediates can be
revealed.
In this paper, the esterification equilibrium constants
between deuterated methanol and five arsonic acids
[methyl (1), pentyl (2), phenyl (3), 2,3-dipalmitoyloxy-
propyl (4), and 2,3,4-tripalmitoyloxybutyl (5)] and one
arsinic acid [cacodylic acid (6)] are reported (Fig. 1). The
organic parts on these acids were chosen in such a way as
to permit the evaluation of the effect of parameters such as
the alkyl group length, the aromatic versus aliphatic nature,
and the arsonic versus arsinic group on the esterification
equilibrium constants.
Under the experimental conditions used in this work
(mixtures of CDCl3/CD3OD plus small quantities of added
D2O at 25.0 °C) it was found that both the esterification
and the hydrolysis reactions were fast, similar to the results
obtained in the case of arsenous acid [3]. Thus, addition of
either CD3OD (esterification) or D2O (hydrolysis) to the
solution of the arsonic acid in CDCl3/CD3OD resulted in a
Results and discussion
It is known that arsenous acid, H3AsO3, reacts reversibly
with alcohols. The reaction rates were found to be very fast
with simple alcohols but became slower only with bulky
ones like tert-butyl alcohol [3]. Similarly, arsonic and
arsinic acids react readily with alcohols [15]. The structure
of the produced esters depends on the mono- or dihydric
1
fast equilibrium, as was revealed by the identical H NMR
spectra obtained after 2 min, 30 min, and 24 h.
It should be emphasized that accurate measurements
of equilibrium constants are not an easy task. Many fac-
tors like pH, ionic strength, temperature, solvent system,
123

Esterification equilibrium constants of arsonic and arsinic acids
1605
K2
numerator and denominator have been replaced by
amounts nAA, nAB, and nAC in moles (because at each
equilibrium point the total volume was the same).
K1
O
O
O
CD3OD
D2O
CD3OD
D2O
R
As OCD3
OCD3
R
As OCD3
OD
R
As OD
OD
AA
In order to calculate the constants K1 and K2, at any
equilibrium point, all the variables IAA, IAB, IAC, [D2O],
and [CD3OD] should be estimated. Although this is rela-
tively easy for the variables IAA and IAB by integration of
AC
AB
Arsonic acid
K1
1
O
O
the respective peaks in the H NMR spectra, it cannot be
CD3OD
R
As OCD3
R
R
As
R
OD
applied for the determination of [D2O] and [CD3OD].
Estimation of IAA, IAB, and IAC under experimentally
fixed [D2O]/[CD3OD] concentration ratios has been used as
a solution to this problem [21]. Thus, in a typical experi-
ment known volumes of D2O and CD3OD were added into
the NMR tube and, after equilibration, the IAA, IAB, and IAC
relative peak intensities were measured. The signals cor-
responding to the diester, monoester, and free acid were
identified by their behavior during the stepwise addition of
D2O. Thus, the signal of the diester was reduced continu-
ously, that of the monoester initially increased and then
decreased (see Fig. 3), whereas the signal of the free acid
continuously increased. In this case, the value of [D2O]/
[CD3OD] at each ratio can be easily calculated as follows:
D2O
A
B
Arsinic acid
Scheme 2
parallel equilibria present, solubility limitations, etc. can
affect the measured variables. In the present study, the
temperature was constant at 25.0 °C and the initial con-
centration of the arsonic and arsinic acids was kept
identical in all experiments permitting comparisons
between the various acids despite the fact that not all of the
above factors were precisely fixed.
Ionization of the arsonic and arsinic acids will result in
the appearance of additional equations in Scheme 2. Ar-
sonic acids are weak acids [27] and calculations for the
concentration range used in our experiments gave
approximately 20 % deprotonation in water. However, in
the CDCl3/CD3OD mixture (volume ratio 5:4) used in this
work deprotonation should be less than 1 %, because it is
known that dissociation is reduced by several orders of
magnitude on going from water to organic solvents [28].
The solvent system can influence the equilibria in
Scheme 2 not only because of ionization but also because
of solubility changes. The choice of the solvent system of
CDCl3/CD3OD was based mainly on solubility consider-
ations because all the studied acids, lipophilic or not, were
soluble in this mixture.
½D2Oꢁ
n
V
d
D2O D2O
=M
D2O
D2O
¼
¼
ð3Þ
½CD3ODꢁ
where VCD OD is the volume of CD3OD added initially,
n
V
d
CD3OD CD3OD
=M
CD3OD
CD3OD
3
VD O corresponds to the total volumes of D2O added each
time, and d and M represent density and molecular weight,
2
1
respectively. H NMR spectra recorded for pentylarsonic
acid (2) under these conditions are reproduced in Fig. 3.
1H NMR signals for the acids 1–6 are shown in Fig. 4
and the related data are presented in Table 1.
Plots of [D2O]/[CD3OD] versus IAA/IAB (or IAB/IAC) for
different [D2O]/[CD3OD] ratios gave linear relationships
as shown in Fig. 5 for the case of methylarsonic acid (1).
The lines drawn by the linear least-squares method gave
R2 values close to unity and from the slopes of these lines
the equilibrium constants K1 or K2 were calculated. Results
for all the studied acids are summarized in Table 2.
It should be noted that in the case of 2,3,4-tripalmitoy-
loxybutylarsonic acid (5) integration of the 1H NMR peaks
is difficult (see Fig. 4) as a result of diastereoisomers
present, and so the values obtained for K1 and K2 are
subject to significant errors. Also, in the case of pheny-
larsonic acid (3) the order of chemical shifts of the signals
AA, AB, and AC is reversed (see Fig. 4) and, as a result of
the peak overlapping, additional experimental points were
required to estimate the equilibrium constants. In particu-
lar, addition of D2O to a total amount of at least 50 mm3
was necessary to ensure that no diester was present and the
1H NMR signals at around 7.50–7.65 were caused by the
monoester and the free acid only.
The expressions for the esterification equilibrium con-
stants shown in Scheme 2 can be written as
½ABꢁ½D2Oꢁ
nAB ½D2Oꢁ
K1 ¼
¼
½AAꢁ½CD3ODꢁ nAA ½CD3ODꢁ
IABnt ½D2Oꢁ
IAB½D2Oꢁ
¼
¼
ð1Þ
ð2Þ
IAAnt ½CD3ODꢁ IAA½CD3ODꢁ
IAC½D2Oꢁ
K2 ¼
IAB½CD3ODꢁ
1
where IAA, IAB, and IAC represent the H NMR relative
peak intensities for the peaks of R–CH2–As(O)(OD)2,
R–CH2–As(O)(OD)(OCD3), and R–CH2–As(O)(OCD3)2,
respectively, and nt represents the total amount, in moles,
of the arseno compound added initially. Concentrations of
free acid (AA), monoester (AB), and diester (AC) in the
123

1606
G. M. Tsivgoulis, P. V. Ioannou
2.5 9 10-6 mol, which is 100 times smaller. Therefore, the
amountofwaterliberatedbytheesterificationreactionshould
not be important. Indeed, as shown in Table 1, when the K1
and K2 values are recalculated taking into account this addi-
tional factor (presented as K1cor and K2cor, respectively) they
are essentially identical to the values obtained before cor-
rections. For the calculation of K1cor and K2cor the
expression 1 was rewritten as follows:
IAB½D2Oꢁ
IAB n
D2O
K1 ¼
¼
¼
IAA½CD3ODꢁ IAA
n
CD3OD
IAB
ðVD O
d
D O=MD O þ nAB þ 2nAC
Þ
2
2
2
IAA ðVCD ODdCD OD=MCD OD ꢂ nAB ꢂ 2nAC
Þ
3
3
3
IAB
ðVD O
d
D O=MD O þ IABnt þ 2IACntÞ
2
2
2
¼
ð4Þ
IAA ðVCD ODdCD OD=MCD OD ꢂ IABnt ꢂ 2IACntÞ
3
3
3
where the additional factors correspond to water and
methanol produced and consumed, respectively, during the
two-step esterification.
The values of K1 and K2 (Table 2) show that the equilibria
presented in Scheme 2 are shifted towards the monoesters
(AB) and the free acids (AA) even in the presence of small
amounts of water. Indeed, as is evident from Figs. 3 and 4, for
the arsonic acids 1–5, the presence of only 15 mm3 of D2O in
400 mm3 of CD3OD is enough to significantly diminish the
concentration of the corresponding diesters (AC). Previous
results [29], based on 1H NMR data, showed that when longer
alcohols like CD3CD2OD were used, sensitivity to water was
further enhanced.
The esterification equilibrium constants are not signifi-
cantly affected by the nature of the R group in arsonic acids.
Therefore, aromatic and short or long aliphatic groups gave
similar values for K1 and K2 of around 0.114 and 0.014,
respectively. One exception seems to be the triester arsonic
acid 5 but as mentioned above no safe conclusions can be
drawninthiscasebecausesignificanterrorsarepresent. When
the arsonic acids are compared to the arsinic acid 6, differ-
ences in K1 were found. Thus, in the case of dimethylarsinic
acid (6), the value obtained for K1 was 0.053 which is about
half of the values observed for the arsonic acids. A second
arsinic acid (diphenyl arsinic acid) was synthesized and
measured as well (results not shown) but correct integrations
were not possible due to the extensive overlapping of the 1H
NMR signals in the aromatic region observed in this case.
Fig. 3 Part of the 1H NMR spectra of pentylarsonic acid (2,
2.5 nmol) in CDCl3/CD3OD (500/400 mm3) in the presence of added
D2O equal to 0, 5, 10, 15, and 20 mm3. AA, AB, and AC correspond
to C4H9–CH2–As(O)(OD)2, C4H9–CH2–As(O)(OD)(OCD3), and
C4H9–CH2–As(O)(OCD3)2 protons, respectively
In the experiments described above, there was concern for
the very small amount of D2O (5–40 mm3) found to be nec-
essary to shift the equilibrium towards the monoester and the
free acid. Besides the relatively large experimental errors
involved in the addition of such tiny amounts of D2O, ques-
tions can be raised about the validity of the assumption that
the [CD3OD]/[D2O] ratio is fixed and, for example, it is not
affected by the amount of water or methanol in the ester-
ification/hydrolysis reactions. However, calculations showed
that even 5 mm3 of D2O corresponds to 2.5 9 10-4 mol,
whereas the total amount of arseno compound added was
Conclusions
We have reported herein that the esterification between
arsonic and arsinic acids with CD3OD and the hydrolysis of
1
their esters with D2O were both very fast reactions. H
NMR measurements revealed that the equilibria shown in
Scheme 2 were shifted towards the monoesters and the free
123

Esterification equilibrium constants of arsonic and arsinic acids
1607
Fig. 4 1H NMR signals of the
acids 1–6 (2.5 nmol), in CDCl3/
CD3OD/D2O (500/400/15 mm3).
AA = RCH2As(O)(OD)2,
AB = RCH2As(O)(OD)(OCD3),
and AC = RCH2As(O)(OCD3)2.
In the case of phenylarsonic acid
(3) the signals correspond to the
ortho protons of the phenyl group
Table 1 1H NMR chemical shifts and multiplicities in parenthesis of the acids 1–6 (2.5 nmol) in CDCl3/CD3OD/D2O (500/400/15 mm3)
Acid
R–CH2–As(O) (OD)2
AA
R–CH2–As(O) (OD)(OCD3)
AB
R–CH2–As(O) (OCD3)2
AC
1
2
3
4
5
6
1.69 (s)
1.72 (s)
1.78 (s)
2.04 (t)
2.09 (t)
2.17 (t)
7.58 (apparent d)
2.40 (apparent dd)
2.35 (apparent dd)
1.56 (s)
7.56 (apparent d)
2.46 (apparent dd)
2.41 (apparent dd)
1.61 (s)
7.53 (apparent d)
2.55 (apparent dd)
2.50 (apparent dd)
–
The term apparent indicates that the multiplicity of the signal is lower than the expected one because of accidental equivalence of some protons
or values close to zero for some coupling constants. In the case of phenylarsonic acid (3) the signals refer to the ortho protons of the phenyl group
acids even in the presence of very small amounts of water.
Thus, the concentration of arsonic esters of the general type
I inside cells should be negligible. This result may provide
some additional insight into the recently raised ‘‘arsenic
bacterium debate’’ [16–18]. The nature of the R group in
the arsonic acids 1–5 does not seem to affect the esterifi-
cation equilibrium constants. However, for cacodylic acid
(6, an arsinic acid) the value of K1 is about half of the
values obtained for the arsonic acids 1–5. Future experi-
ments with other arsinic acids may disclose if this behavior
is general or not.
Experimental
Dimethylarsinic acid (cacodylic acid) was from Serva.
Fig. 5 Plot of [D2Oadded]/[CD3ODadded] versus IAA/IAB in the ester-
ification of methylarsonic acid (1) with CD3OD
Methylarsonic acid was prepared from alkaline ‘‘Na3AsO3’’
123

1608
G. M. Tsivgoulis, P. V. Ioannou
Table 2 Esterification equilibrium constants for arsonic and arsinic acids with CD3OD calculated from various [D2O]/[CD3OD] ratios
Acid
K1
Ka1cor
K2
Ka2cor
1
2
3
4
5
6
a
0.113 0.002
0.117 0.006
0.110 0.006
0.113 0.004
0.20b
0.113 0.002
0.116 0.006
0.111 0.006
0.113 0.004
0.20b
0.010 0.001
0.014 0.002
0.014 0.002
0.016 0.001
0.010b
0.010 0.001
0.013 0.003
0.014 0.002
0.015 0.001
0.010b
0.053 0.001
0.053 0.001
–
–
Values were corrected for the amount of water liberated and the amount of CD3OD consumed during the esterification reaction
1
Significant errors are involved in this measurement because the H NMR peaks are difficult to separate (see text)
b
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and methyl iodide as Me-AsO3Na2ꢀ6H2O [30] and converted
to the free acid via the strong cation-exchange resin Dower
AG50W-X8, eluting with water, evaporating (rotary 50 °C)
the fractions with pH 2–3 and drying in vacuo. Phenylarsonic
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essentially according to literature methods. CDCl3 (xD =
99.8 %) with silver foil (wAg = 0.5 %) as stabilizer, CD3OD
(xD = 99.8 %), and D2O (xD = 99.9 %) were from Aldrich.
1H NMR spectra at 400 MHz were obtained on a Bruker
Avance DPX spectrometer at 25.0 °C. All weighings were
done on a 5-digit Mettler-Toledo balance, model XS205.
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syringes of 500 mm3, whereas D2O was added with a gas-
tight Hamilton syringe of 10 mm3.
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General experimental procedure
The arsenic compound (2.5 nmol) was dissolved in 500 mm3
CDCl3 in an NMR tube. CD3OD (400 mm3) was added,
and the tube was tumbled until equilibrium was reached. In
the cases of methylarsonic acid (1), 2,3-dipalmitoyloxy-
propylarsonic acid (4), 2,2,4-tripalmitoyloxybutylarsonic
acid(5), and cacodylic acid(6), dissolutionwas completeonly
1
after the CD3OD addition. H NMR spectra showed that
equilibrium was practically reached immediately after the
CD3OD addition. Then, increasing amounts of D2O were
added, in the order of total 5, 10, 15, 20, and 40 mm3. After
each addition the tube was tumbled and the 1H NMR spectra
were recorded.
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