Kinetics and Mechanism of the Hydrolysis of Benzyl Ether Bonds in Aqueous–Organic Media

Article abstract of DOI:10.1134/S0036024420020302

Abstract: Kinetic parameters (rate constant, energy of activation, and entropy of activation) of the acid-catalyzed hydrolysis of the benzyl ethers (3,4-dimethoxyphenyl)-(2-methoxyphenoxy)methane and (4-hydroxy-3-methoxyphenyl)methoxymethane are determined for a wide range of compositions for mixtures of water and organic solvents (dioxane, DMSO, and acetic acid). It is shown that the acid-catalyzed hydrolysis of these benzyl ethers in mixtures of water and aprotic solvents occurs as a reaction of bimolecular nucleophilic substitution. In aqueous acetic acid solutions, the mechanism of hydrolysis can be bimolecular or unimolecular, depending on the structure of the ether and the content of the organic solvent. The effect the solvents have on the rate and mechanism of the studied reaction is discussed in terms of solvation concepts.

Full text of DOI:10.1134/S0036024420020302

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2020, Vol. 94, No. 2, pp. 310–316. © Pleiades Publishing, Ltd., 2020.
Russian Text © The Author(s), 2020, published in Zhurnal Fizicheskoi Khimii, 2020, Vol. 94, No. 2, pp. 230–237.
CHEMICAL KINETICS
AND CATALYSIS
Kinetics and Mechanism of the Hydrolysis of Benzyl Ether Bonds
in Aqueous–Organic Media
T. S. Seliverstovaa,*, M. A. Kushnera, and L. G. Matusevicha
aBelarusian State Technological University, Minsk, 220006 Belarus
*e-mail: tamara.tsel@yandex.ru
Received April 5, 2019; revised April 5, 2019; accepted April 9, 2019
Abstract—Kinetic parameters (rate constant, energy of activation, and entropy of activation) of the acid-cat-
alyzed hydrolysis of the benzyl ethers (3,4-dimethoxyphenyl)-(2-methoxyphenoxy)methane and (4-
hydroxy-3-methoxyphenyl)methoxymethane are determined for a wide range of compositions for mixtures
of water and organic solvents (dioxane, DMSO, and acetic acid). It is shown that the acid-catalyzed hydro-
lysis of these benzyl ethers in mixtures of water and aprotic solvents occurs as a reaction of bimolecular nuc-
leophilic substitution. In aqueous acetic acid solutions, the mechanism of hydrolysis can be bimolecular or
unimolecular, depending on the structure of the ether and the content of the organic solvent. The effect the
solvents have on the rate and mechanism of the studied reaction is discussed in terms of solvation concepts.
Keywords: benzyl ethers, acid hydrolysis, solvent effect
DOI: 10.1134/S0036024420020302
INTRODUCTION
4-alkoxybenzyl ethers, (3,4-dimethoxyphenyl)-(2-
methoxyphenoxy)methane (1), and (4-hydroxy-3-
methoxyphenyl)methoxymethane (2), used to model
the benzyl ether structural fragments of lignin:
Investigating reactions of the hydrolysis of benzyl
ether bonds is important, since these bonds, along
with other types of ether bonds and C–C bonds, are
present in the natural polymer lignin, which is charac-
terized by an irregular three-dimensional network
structure. The cleavage of these bonds largely deter-
mines the delignification of wood and other plant
materials to obtain such cellulosic fibrous materials as
paper. Among the many alternative ways of producing
cellulose, technological processes that employ orga-
nosolvent wood digestion remain important. These
meet increasing environmental requirements, reduce
power consumption, and tend to ensure more com-
plete recycling of reagents and higher yields of cellu-
lose with suitable consumer performance [1–3].
Investigating the transformation of lignin on com-
pounds that model the main types of chemical bonds
between its structural fragments gives insight into the
processes of wood digestion, allowing us to create the
optimum conditions for delignification and predict
the properties of its products, since data from studying
the reactions of model compounds were found to cor-
relate well with the results of the experiments on orga-
nosolvent wood digestion [4].
O
HO
O
O
O
O
(2)
O
(1)
The ethers were hydrolyzed in organic solvent–
water mixtures of varying compositions in the tem-
perature range of 50–90°C using HCl or H2SO4 as a
catalyst.
(3,4-Dimethoxyphenyl)-(2-methoxyphenoxy)meth-
ane (1) was synthesized from vanillin, methylated with
dimethyl sulfate to obtain veratraldehyde. The vera-
traldehyde was transformed into veratric alcohol via
the Cannizzaro reaction, after which the alcohol was
treated with thionyl chloride. The resulting veratryl
chloride was treated with sodium guaiacolate (Wil-
liamson reaction) giving the desired product; Tmelt
72°C, which agrees with the data in [5].
=
(4-Hydroxy-3-methoxyphenyl)methoxymethane (2)
was synthesized from vanillin according to the proce-
dure in [6], via reduction with sodium borohydride
with the subsequent methylation of vanillyl alcohol.
Tmelt = 34–35°C, which agrees with published data.
EXPERIMENTAL
In this work, we studied the effect organic solvents
(dioxane, DMSO, and acetic acid) had on the rate and
mechanism of the acid hydrolysis of 4-hydroxy- and
310

KINETICS AND MECHANISM OF THE HYDROLYSIS
311
Dioxane was purified as described in [7], DMSO carbinols to form colored compounds with quinone
was distilled under vacuum, and glacial acetic acid was monochlorimide in an alkaline medium.
used without additional purification.
Our kinetic studies were performed using ampules.
Samples of a reaction mixture were kept in sealed
ampules inside a thermostat. At certain time intervals,
the ampules were opened, and the concentrations of
the required substances were measured. The kinetics
of the hydrolysis of ethers 1 and 2 were determined
from the concentrations of guaiacol and vanillyl alco-
hol, respectively. The concentration of phenol com-
pounds was determined spectrophotometrically at a
wavelength of 612 nm [8], based on the ability of phe-
RESULTS AND DISCUSSION
Hydrolysis Mechanism
Hydrolysis of the benzyl ether bonds of lignin can
be attributed to one of the most numerous types of
organic reactions, nucleophilic substitution (SN).
There are two limiting cases of the mechanism of nuc-
leophilic substitution on a saturated carbon atom: uni-
molecular substitution (SN1) and bimolecular substi-
tution (SN2). There are two mechanisms for the acid
nols with an unsubstituted para position and guaiacyl hydrolysis of benzyl ether bonds:
CH2
+H2O
Ar
Ar
OH
−H
CH2
R
H3O
H2O
CH2
R
Ar
O
Ar
O
H
N
R
H
H
H
+
+
δ
δ
O
C
O
H
H
Ar
Unlike other SN reactions, however, the mecha- Xs = 0.2 (30–40% acetic acid), and then accelerates
nism of the hydrolysis of benzyl ethers remains poorly sharply. This suggests that the mechanism of the
studied, since dialkyl and alkyl aryl ethers typically hydrolysis of compound 2 changes as the concentra-
have low activity. Among the organic solvents used in tion of acetic acid grows.
organosolvent delignification, such aprotic solvents as
Mathematical processing of the experimental
dioxane and dimethyl sulfoxide (DMSO) are of spe-
results revealed a linear correlation between the loga-
cial importance, as are such protic solvents as acetic
rithm of rate constants k1 and k2 of the acid hydrolysis
acid.
of ethers 1 and 2, respectively, in water–dioxane and
Figure 1 shows the dependence of the rate of the
water–DMSO mixtures at different contents of the
hydrolysis of benzyl ethers 1 and 2 on the composition
organic solvent: logk1 = a + blogk2.
of the solvent. Adding aprotic solvents to water results
This correlation (Fig. 2) indicates there is only one
mechanism of the hydrolysis of these ethers in water–
aprotic solvent media. The absence of such a correla-
tion for water–acetic acid mixtures shows there are
different mechanisms of the hydrolysis of ethers 1 and
2 in this solvent system.
in a substantial (by almost two orders of magnitude)
drop in the rate of cleavage of the ether bonds. As
functions of mole fraction Xs of the solvent, the shapes
of the curves of kexp are virtually identical for the two
ethers in both the aqueous dioxane medium and the
aqueous DMSO medium. The hydrolysis of the
4-hydroxy- and 4-alkoxybenzyl ethers consequently
occurs in these solvents according to the same mecha-
nism.
Thermodynamic Parameters of the Reaction
The effects acetic acid has on the studied ethers are
Using the results from kinetic measurements at dif-
different. For compound 1 in aqueous acetic acid ferent compositions of mixed solvents and different
solutions, the rate of the reaction grows monotonically temperatures, we calculated the thermodynamic
along with the concentration of acetic acid (Fig. 1). As parameters of the hydrolysis of the studied ethers (i.e.,
acetic acid is added to the water, the rate of the hydro- the free energy of activation, the enthalpy of activa-
lysis of ether 2 initially slows, reaches a minimum at tion, and the entropy of activation) (Table 1).
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SELIVERSTOVA et al.
logk2
kexp × 104, s−1
10
9
8
7
6
5
4
6
20
15
10
5
7
3
3
4
2
8
9
1
10
11
2
1
Fig. 2. Dependences logk = f(logk ) for the acid hydroly-
1
2
sis of benzyl ethers 1 and 2 in aqueous–organic media of
varying compositions: (1) water–dioxane, (2) water–
DMSO, and (3) water–acetic acid.
0
0.2
0.4
0.6
0.8
Xs, mole fraction of organic solvent
values and positive ΔS≠ values. Adding a protic solvent
during the hydrolysis of ether 1 results in the reaction
mechanism transitioning from bimolecular to unimo-
lecular, as is implied by the behavior of ΔG≠ = f(Xs)
Fig. 1. Dependences of the rate constant of the hydrolysis
of benzyl ethers on the composition of a mixed solvent: (1)
ether 1, water–dioxane, and water–DMSO; (2) ether 2,
water–dioxane, and water–DMSO; (3) ether 1 and
water–acetic acid; and (4) ether 2 and water–acetic acid.
and the change in ΔS≠ from negative to positive values.
There is also a linear correlation between ΔH≠ and
ΔS≠ that is described by the equation ΔH≠ = a + bΔS≠
(Fig. 3) for the hydrolysis of benzyl ethers 1 and 2.
Our data show that an increase in the fraction of the
aprotic solvent in the binary mixture raises the free
energy of activation of the hydrolysis of the benzyl
ethers. This also lowers the enthalpy of activation and
shifts the entropy of activation toward higher negative
values. During the hydrolysis of ether 2 in water–
protic solvent media, an increase in the fraction of
acetic acid lowers the free energy of activation and the
enthalpy of activation. The entropy of activation of the
hydrolysis of this ether is positive throughout the stud-
ied range of compositions of the mixed solvent.
During the hydrolysis of ether 1, the free energy of
activation initially grows along with the fraction of
acetic acid in the binary solvent, and then (starting at
Xs = 0.22) falls. The enthalpy of activation grows
The effects of the aprotic solvents (dioxane and
DMSO) are described by the same linear dependence
for both ethers. This coincidence of the dependences
for the water–aprotic solvent mixtures is a sign of a
single mechanism of the hydrolysis of these ethers. At
the same time, such a linear correlation is observed for
each ether in the water–acetic acid mixtures.
Role of the Polarity of the Medium
The rates of many acid-catalyzed hydrolysis reac-
monotonically, and the entropy of activation shifts tions are described by electrostatic theory, which con-
from high negative values to positive values.
siders the relationship between the reaction rate con-
stant and the permittivity of the medium. The main
conclusion is that we observe linear dependence
logk = f(1/ε) when there is no specific solvation in
solution. The deviation of linearity or the absence of
such a dependence suggests there is a considerable
contribution from specific solvation. This dependence
was used to describe the electrostatic interaction
during reactions of the acid-catalyzed hydrolysis of
The thermodynamic parameters obtained for the
acid hydrolysis of benzyl ethers indicate the mecha-
nism of the studied reaction in the mixed solvents. A
bimolecular mechanism of nucleophilic substitution is
most likely for both ethers in the water–aprotic solvent
mixtures, judging from the higher ΔH≠ values. A uni-
molecular mechanism is preferred for ether 2 in the
water–acetic acid media, as is shown by the high ΔH≠ esters and amides [9]. The observed negative slope of
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KINETICS AND MECHANISM OF THE HYDROLYSIS
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Table 1. Thermodynamic parameters of the acid-catalyzed hydrolysis of benzyl ethers
ΔG≠, kJ/mol
ΔH≠, kJ/mol
ΔS≠, J/(mol K)
Xs
Organic solvent
1
Dioxane (0.01 M HCl)
0
102.7
103.5
107.3
111.0
103.7
108.5
114.4
101.5
102.6
102.4
102.2
100.5
2
95.8
90.8
87.0
76.2
91.3
91.3
73.2
74.6
76.6
78.4
94.1
100.9
−16.7
−30.1
−50.2
−104.7
−37.8
−49.0
−123.5
−83.5
−78.7
−72.8
−24.7
2.84
0.02
0.12
0.32
0.03
0.15
0.37
0
0.12
0.22
0.31
0.44
DMSO (0.01 M HCl)
Acetic acid (0.01 M H2SO4)
Dioxane (0.01 M HCl)
0
108.0
109.7
113.1
115.6
109.8
113.1
114.4
102.6
101.5
99.7
108.9
104.9
99.1
3.7
−10.5
−35.5
−55.9
−7.0
−32.8
−57.0
35.2
33.7
32.9
31.5
30.9
0.02
0.12
0.32
0.03
0.15
0.37
0
0.03
0.12
0.22
0.44
94.7
DMSO (0.01 M HCl)
106.3
100.1
95.2
111.1
109.5
107.4
105.3
103.1
Acetic acid (0.01 M H2SO4)
98.0
96.0
T is temperature, and r≠ is the maximum distance of
approach (the radius of the activated complex).
the resulting plot was interpreted as proof of the high
polarity of the transitional state.
In this work, a similar dependence of the rate con-
stant on the polarity of the medium was detected
during the acid hydrolysis of benzyl ethers in the pres-
ence of aprotic solvents (Fig. 4). We may therefore
assume that the transitional state observed during acid
hydrolysis of the studied ethers is highly polar.
This dependence is virtually linear at high concen-
trations of water, testifying to the dominance of elec-
trostatic interactions in the solution with no apprecia-
ble contribution from specific solvation.
Empirical dependences of the rate constant on the
permittivity of the medium are used to consider the
electrostatic interactions in a solution [10]. For the
ion–dipole interaction by which the hydrolysis of ben-
zyl ethers occurs, this dependence is described by the
equation logkε = logkε=∞ + [Z]eμ/(KTr≠ε), where kε=∞
is the rate constant at infinitely high ε, Z is the charge strongly with water. We may assume that the stronger
number of the ion, е is the electron charge, μ is the the interaction between an aprotic solvent and water,
dipole moment of water, K is the Boltzmann constant, the greater the weakening of the solvation of the acti-
This equation allows us to find maximum approach
distance r≠ between the reacting particles in the con-
trolling step of the reaction; r≠ can also be used to
determine the specific effects of a medium. The r≠ val-
ues of compounds 1 and 2 in our water–dioxane
medium were 0.93 and 0.216 nm, respectively. The
order of magnitude of these values suggests the per-
mittivity of the medium substantially affects the acid
hydrolysis of benzyl ethers; in any case, the role of the
permittivity of the medium is comparable to that of
specific solvation. The r≠ values are surprisingly low in
the DMSO–water system (0.034 and 0.036 nm,
respectively), testifying to the overwhelming influence
of specific effects of the medium.
In the DMSO–water system, specific effects con-
siderably surpass that of permittivity. DMSO interacts
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SELIVERSTOVA et al.
120
110
100
90
(1/ε) × 102
1
2
3
4
5
2.5
6
5
3.0
4
3
3.5
4.0
4
1
2
80
1
3
2
−150
−100
−50
ΔS≠, J/(mol К)
0
50
4.5
−logkexp
≠
≠
Fig. 3. Dependences ΔH = f(ΔS ) for the acid hydrolysis
of benzyl ethers in aqueous–organic media of varying
compositions: (1) ether 1, water–dioxane, and water–
DMSO; (2) ether 1 and water–acetic acid; (3) ether 2,
water–dioxane, and water–DMSO; and (4) ether 2 and
water–acetic acid.
Fig. 4. Dependences of logk on 1/ε for the hydrolysis of
benzyl ethers: (1) ether 1 and water–dioxane, (2) ether 1
and water–DMSO, (3) ether 2 and water–dioxane, (4)
ether 2 and water–DMSO, (5) ether 1 and water–acetic
acid, and (6) ether 2 and water–acetic acid.
vated complex. The results from physicochemical
investigations of mixed solvents [11] indicated weak-
ening of the interaction between an aprotic solvent and
water in the order DMSO > dioxane.
tion, two processes with the participation of specifi-
cally solvated particles M…Si and unsolvated particles
M occur in the system concurrently and independently
of each other. The total process rate is V = k°[M] +
ki[M…Si]. In a solvent with a low content of a specifi-
cally solvating component, the total reaction rate is
consequently determined by product k°[M]. Solvation
equilibria shift as the content of component Si rises,
and the contribution from the concentration of M…Si
to the total process rate grows and can dominate if k°
and ki are comparable. In accordance with this condi-
tion, we may assume that the reaction in a water–ace-
tic acid solvent with a low content of the specifically
solvating component (acetic acid) proceeds mainly
with the participation of specifically unsolvated parti-
cles through a bimolecular mechanism (SN2). The rate
of the reaction is in this case strongly affected by the
polarity of the medium. The polarity of the medium
falls as the mole fraction of acetic acid in a mixed sol-
vent rises, and this lowers the rate of the reaction. In
concentrated acetic acid solutions, the role of particles
solvated specifically by acetic acid increases. As the
fraction of acetic acid in a mixed solvent rises, their
concentration grows because of a shift in the equilibria
In water–acetic acid media, dependence logk =
f(1/ε) (Fig. 4) for ether 1 is virtually linear at low acetic
acid contents and has a positive slope, suggesting that
the controlling step requires a positive ion, and that
the mechanism of transformation is carbocationic
(SN1). The calculated r≠ value (0.117 nm) shows that
along with specific solvation, there are electrostatic
interactions that considerably affect the rate of the
reaction. As the content of acetic acid rises, we observe
an inflection in curve logk = f(1/ε), which could be
due to specific solvation of the reacting particles and
the transitional state.
With the hydrolysis of ether 2, two virtually linear
portions can be distinguished in the curve of logk as a
function of 1/ε. This curve has a positive slope for the
mixed solvent with a high content of acetic acid (Xs >
0.32), confirming that the reaction requires a carboca-
tion (SN1 mechanism). The slope is negative at high
concentrations of water, testifying to a change in the
reaction mechanism.
The effect acetic acid has on the rate of the hydro-
lysis of the benzyl–aryl ether bond can be interpreted
in terms of solvation concepts. If one of the compo-
nents of a binary mixture is capable of specific solva- of solvation, and the rate of hydrolysis also increases.
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KINETICS AND MECHANISM OF THE HYDROLYSIS
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Table 2. Constants A and n in Eq. (3) for the acid-catalyzed
Comparing the numerical values of the constant A
(Table 2), we may conclude that the changes in the
degree of solvation of the initial and transitional states
of both ethers in two solvents of different polarities
(dioxane and DMSO) differ little; there is only a slight
drop in A upon moving from DMSO to dioxane. These
results confirm that the acid-catalyzed hydrolysis of
the benzene ethers occurs in these solvents through
the same mechanism with a highly polar transition
state similar in properties to the initial state [12].
hydrolysis of benzyl ethers in aqueous–organic media
Ether
Solvent
n
−A
1
Dioxane–water
DMSO–water
Dioxane–water
DMSO–water
3.5
3.2
3.0
2.1
9.3
8.9
8.2
6.7
2
Constant n is interpreted as the number of water
molecules that participate in solvation upon moving
from the initial to the transitional state. For both
ethers, there is a certain tendency for n to fall when
using the DMSO–water system, testifying to the
stronger interaction between DMSO and water.
For the acid hydrolysis of the benzyl ethers, the
logarithmic dependence of the rate constant on the
ratio between the contents of water and an aprotic sol-
vents is also linear in the Xs = 0.05–0.3 range of aprotic
solvent concentration (Fig. 5).
The slope m of this dependence is interpreted as the
true number of water molecules bound directly to the
substrate in the activated complex [12]. The slope cal-
culated from the linear portions of this plot is m = 1 for
both of the studied ethers, and it is independent of the
nature of the solvent. In water–aprotic solvent mix-
tures, one water molecule is thus bound strongly to the
benzyl ether in the activated complex and participates
directly in the rearrangement of the electronic config-
urations related to hydrolysis.
Role of Water in the Acid Hydrolysis of Benzyl Ethers
The role of water is particularly evident when
studying hydrolysis in media containing aprotic sol-
vents. Hydrolytic reactions in aqueous–organic media
are often found to be characterized by a linear loga-
rithmic dependence between the rate constant and the
concentration of water, or the activity of water aw:
logk = A + nlogaw, where A and n are constants. Inves-
tigating this dependence yields additional information
on the mechanism of transformation. This depen-
dence is linear if the coefficients of the activities of the
initial and transitional states vary synchronously with
the composition of the solvent [12].
In the hydrolysis of studied ethers 1 and 2, the
dependence of logk on logaw is linear only at low con-
centrations of the aprotic solvent (up to approximately
Xs = 0.2). There is strong deviation from linearity at
higher contents of dioxane or DMSO.
−log(Xs/(1 − Xs))
CONCLUSIONS
2.0
1.5
1.0
0.5
0
3.0
3.4
The acid-catalyzed hydrolysis of benzyl ethers in
mixtures of water with aprotic solvents dioxane and
DMSO proceeds as a bimolecular reaction of nucleo-
philic substitution (SN2). However, this obviously
does not mean that the formation of a new bond and
the cleavage of the old bond occur simultaneously.
The transitional state in this process is highly polar:
the bond between the benzyl atom and the departing
group is looser than the newly formed bond. A key role
is played by the effect an aprotic solvent has on the
change in the degree of solvation of the initial sub-
stances and the transitional state. The slower rate of
hydrolysis is due to the lower degree of solvation of the
transitional state, due to the bonding of water mole-
cules to those of DMSO and dioxane. Because water
differs from aprotic solvents mainly in the ability to be
a proton donor, we may assume that hydrogen bonds
play an important part in the solvation of the transi-
tional state, along with electrostatic forces.
2
3.8
4.2
4.6
1
In water–acetic acid media, the mechanism of the
hydrolysis of benzyl ethers differs according to the
structure of the ether and the composition of the
medium. In highly concentrated solutions of ionizing
solvent acetic acid, the reaction proceeds through the
Fig. 5. Dependences logk = f(log(X /(1 – X ))) in the
s
s
hydrolysis of benzyl ethers in mixtures of water with
aprotic solvents: (1) ether 1 and (2) ether 2.
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SELIVERSTOVA et al.
5. P. Larsson and B. Lindberg, Acta Chem. Scand. 16,
formation of a carbocation at the controlling step
(SN1). The same mechanism is also characteristic of
the hydrolysis of benzyl–alkyl ether bonds in diluted
acetic acid solutions, where the hydrolysis of benzyl–
aryl ether bonds proceeds through a bimolecular
mechanism.
1757 (1962).
6. N. Mikawa, Bull. Chem. Soc. Jpn. 27, 53 (1954).
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8. V. M. Reznikov and S. F. Yakubovskii, Khim. Drev.,
No. 11, 61 (1972).
9. K. Laidler, Suomen kem. A 33 (2), 44 (1960).
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