Determination of steric effect on the esterification of different alcohols with propanoic acid over cation-exchange resin catalyst Dowex 50Wx4

Article abstract of DOI:10.1524/zpch.2011.6153

This paper describes the application of LFER in the form of Taft correlation to understand the mechanism of the esterification of propanoic acid with different alcohols over heterogeneous catalyst, Dowex 50Wx4. The rate constant (k₁) in the rate equation decreases with change of alcohols in the order methanol, ethanol, n-propyl, n-butyl, n-pentyl, iso-butyl, iso-propyl and sec-butyl (2-buthanol). The reaction of propanoic acid with alcohols fits the Taft equation, log k₁.k_CH3 = 1.0061 E_s K 0.0012, which implies that the steric effect of the substituent governs that reaction and the mechanism is similar between the different alcohols. The experimental results were modelled according to a simple second-order model. It was found that the equilibrium constant of this reaction does not depend on the structure of the organic alcohols, and has for 333 K the value 4.04. by Oldenbourg Wissenschaftsverlag, Munchen.

Full text of DOI:10.1524/zpch.2011.6153

Z. Phys. Chem. 225(2011) 125-136. DOI 10.1524.zpch.2011.6153
© by Oldenbourg Wissenschaftsverlag, München
Determination of Steric Effect on the
Esterification of Different Alcohols with
Propanoic Acid over Cation-exchange Resin
Catalyst Dowex 50Wx4
*
By Beyhan Erdem and Mustafa Cebe
Department of Chemistry, Faculty of Science and Arts, Uludag University, 16059 Bursa,
Turkey
(Received March 30, 2010; accepted May 4, 2010)
Propanoic Acid . Esterification . Taft Relationship . Steric Effect .
Heterogeneous Catalyst
This paper describes the application of LFER in the form of Taft correlation to understand the
mechanism of the esterification of propanoic acid with different alcohols over heterogeneous
catalyst, Dowex 50Wx4. The rate constant (k1) in the rate equation decreases with change of
alcohols in the order methanol, ethanol, n-propyl, n-butyl, n-pentyl, iso-butyl, iso-propyl and
sec-butyl (2-buthanol). The reaction of propanoic acid with alcohols fits the Taft equation,
log k1.kCH3
= 1.0061 Es K 0.0012, which implies that the steric effect of the substituent
governs that reaction and the mechanism is similar between the different alcohols. The experi-
mental results were modelled according to a simple second-order model. It was found that the
equilibrium constant of this reaction does not depend on the structure of the organic alcohols,
and has for 333 K the value 4.04.
1. Introduction
The correlation between the structure of chemical compounds and their chemical
reactivity is used to obtain information on both reaction mechanism and factors
affecting the reactivity [1]. Dating back to the 1950’s, researchers have conceptu-
alized and quantified some of the possible effects of substituents on the chemical
reactivity of alkyl esters in hydrolysis reactions [2]. The pioneering research on
this topic related to physical organic chemistry was done by Hammett, who
quantified the effects of substituents in m- and p- position in benzene structure
*
Corresponding author. E-mail: gbeyhan@uludag.edu.tr
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126
B. Erdem and M. Cebe
on ester hydrolysis. Taft [3–4] and Charton [5] expanded this approach to ali-
phatic series. Two kinds of equations of the form log k = rσ, in which σ is the
characteristic for the polarity of the substituent, but is normally independent of
the nature of the reaction and r the characteristic for the reaction and measures
the susceptibility of the reaction to polar effects, are generally used: the Hammett
equation, involving polar, resonance and steric effects in aromatic systems and
the Taft equation involving polar and steric effects in aliphatic systems. The
basic assumption of the LFER is the existence of a relation between rate and
equilibrium constant, k = cons
!
Kα. This equation is equivalent to a linear
relation between Gibbs energy of activation (ΔG≠) and Gibbs energy of the reac-
tion (ΔGR); (ΔG≠) = α(ΔGR)+const. This approach can be established only for
congeneric series of compounds, i. e., sets of compounds that share the same
‘‘functional group or reaction center’’ Y (e. g., -SH, -OH, -COOH, -CO) and
only have variations in the substituents R attached to this functional group. Thus,
this concept implies that there is a quantitative relationship between the structural
features of a molecule and its reactivity. This relation can be explored such as
understanding of the mechanism, prediction of the rate and equilibrium constant
[6–7]. Linear free energy relationships (LFER) are widely used in homogene-
ously catalyzed reactions and these relationships correlate rate constants k with
equilibrium constants K in a series of analogous elementary reactions [8]. Al-
though most studies trying to establish that kind of relationships have been lim-
ited to the use of homogeneous catalyst [9], the qualitative conception and quan-
titative correlations in them provide a rational basis for understanding probable
activity of heterogeneous catalysts. However, application of such relationships
to heterogeneously catalysed reactions is of particular interest, since it is possible
to obtain information on the properties of both organic molecules and solids
[1–2].
Esterification of carboxylic acids with alcohols in the presence of acid cata-
lysts has been the subject of investigation by many research workers [10–15].
Since the conversions for esterification reactions have long been known to be
limited by a slow reaction rate and the existence of reversible reactions, both
homogeneous and heterogeneous catalysts have been used to accelerate the reac-
tion rate. Despite a strong catalytic effect, the use of homogeneous catalyst, e.
g., sulphuric acid, suffers from drawbacks, such as the existence of side reactions,
equipment corrosion. In many reactions ion exchangers have been found to offer
better selectivity towards the desired product(s) compared to homogeneous cata-
lysts [16] and there is a strong economic driving force to use solid catalysts
instead of liquid ones. By using a solid catalyst, the number of processing op-
tions, such as a gas flow reactor and a fixed bed is increased [17].
Gel type Dowex 50Wx8 as a solid catalyst was used to study the effects of
the particle size, the amounts of catalyst on the esterification reaction by Sharma
et al. They found that the internal mass-transfer resistance could be neglected
and the conversion could be enhanced in a smaller particle size of the resin [18].
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Determination of Steric Effect on the Propanoic Acid Esterifications
127
Mazotti and co-workers carried out a quasi-homogeneous kinetics model for
the esterification of acetic acid to ethyl acetate in presence of Amberlyst-15 ion
exchange resin catalyst. They assumed that the reaction occurs only in the poly-
mer phase, and that the bulk liquid and polymer phases are in constant equilib-
rium conditions [19].
In this work, experimental reaction rates were correlated by pseudo-homoge-
neous model for the esterification reactions systems catalyzed by Dowex 50Wx4
ion-exchange resin. A comparison of three acid ion-exchange resins was made
for synthesis of methyl propionate. The effect of nature of alkyl group in the
alcohol molecule has been investigated using the following alcohols: methanol,
ethanol, n-propyl alcohol, n-butyl alcohol, n-pentyl alcohol and iso-propyl (sec-),
iso-butyl and 2-butyl (sec-) alcohols in order to reveal the structural relationships
in kinetics. There are only few published papers in the open literature that ad-
dressed the effect of alcohol type on the kinetics of the esterification reaction
with propanoic acid. Awad et. al. investigated the reaction kinetics of the reaction
of propanoic acid with different alcohols using SBPF resin catalyst and fitted the
data to a simple model. Also, Aboul-Magd et. al. studied the esterification of
propyl and 1-butyl propionates using inorganic cation exchange catalyst [20–22].
2. Experimental
2.1. Apparatus and experimental procedures
Esterifications of propanoic acid with methyl, ethyl, n-propyl, iso-propyl, n-bu-
tyl, sec-butyl, iso-butyl and n-pentyl alcohols, all of which were purchased from
Merck, were carried out in an isothermal glass reactor equipped with a heating
jacket. The stirring speed was 700 rpm and temperature was kept within
60ϯ0.1 °C. A reflux condenser was placed on top of the reactor in order to
prevent the escape of the volatile compounds. The reactions were realized in the
solution of dioxane of 99.8 % purity (Merck). Because it is quite inert and
dissolves all components of the reaction system to a homogeneous solution [9].
Ion-exchange resins were purchased from Rohm and Haas Co and Fluka.
Ion-exchange resin and 1,4-dioxane of known amount were charged in to the
reactor and also in predetermined amount of alcohols was added in to the reactor.
After reaching desired temperature, propanoic acid was added to the reactor.
This was considered as the starting time of the reaction. Samples were withdrawn
and the amount of unreacted acid was analysed by titration with 0.1 M NaOH.
Typically the catalyst was 1 g and the total liquid volume in the experiments
was 100 dm3. The esterifications of different alcohols with propanoic acid were
performed at 333K with the initial molar ratio (1:1) between the acid and alcohol.
2.2 Catalyst properties
The properties of Dowex 50Wx4, Amberlite IR-120 and Amberlyst-15 are sum-
marized in Table.1. These are cation exchange resins with styrene-divinyl ben-
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128
B. Erdem and M. Cebe
Table 1. Some physical and chemical properties of ion-exchange resins used as catalysts for
the model reaction.
Properties
Amberlyst-15
Sulphonic acid
Macroreticular
20–50
Dowe50Wx4
Amberlite IR-120
Sulphonic acid
Functional group
Type
Sulphonic acid
Gel(microporous) Gel type
Mesh size
20–50
570
16–45
525
Mean particle size (μm)
Thermal stability (°C)
Water content (%)
Capacity (meq.gram)
Crosslinking (%)
560
120
150
120
2.00
66.41
4.5502
4
53.50
4.3289
8
4.9003
20
zene matrix bearing sulfonic acid groups, which were used in the form of beads.
The catalysts were first pre-treated. Dowex 50Wx4 was dried under vacuum at
348 K over night after washing with deionised water to eliminate poisoning or
fouling. Amberlite IR-120 was dried at 363 K over 48 h to remove moisture
completely after being washed with deionised water. Similarly, Amberlyst-15
was dried at 363 K over night after being washed with deionised water.
3. Results and discussion
3.1. The effect of different catalysts on reaction rate
The esterification of propanoic acid with methanol was carried out at 333 K in
the presence of acidic ion-exchange resin catalysts, Dowex 50Wx4, Amberlite
IR-120 and Amberlyst-15, with the initial molar ratio of (1:1) between the reac-
tants. Fig. 1 shows the conversion versus time plots for propanoic acid esterifica-
tions. The activity of these catalysts was evaluated on the basis of rate constants
k1 which were obtained with respect to pseudo-homogeneous model. This second
order model for heterogeneous catalyst has been used also by other authors
[23–24]. Table 2 shows the values of k1 for various catalysts. According to
the rate constants catalyst activity order was determined as: Dowex 50Wx4 >
Amberlyst-15 > Amberlite IR-120. As the particle sizes and the cross-linkages
of these catalysts are different, it is essential to reveal the role of internal mass
transfer resistance inside the catalyst particles. The observations thus suggest
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Determination of Steric Effect on the Propanoic Acid Esterifications
129
Fig. 1. Effect of different catalysts on the conversions of propanoic acid versus time.
Table 2. According to Eq.(3), the calculated rate constants of the model reaction carried out
with different types catalysts in the same conditions by volumetric method.
Catalyst Type
Dowex 50Wx4
Amberlyst-15
k1 (!103 dm3molK1minK1
)
7.2730
3.5227
1.9856
Amberlite IR-120
that Dowex 50Wx4 is the most chemically active catalyst among these three
different catalysts.
3.2 Kinetic model
Esterification reactions are known to be equilibrium-limited chemical reactions.
The reaction of a carboxylic acid with an alcohol results in the formation of
the corresponding ester and water molecules, where the overall reaction can be
expressed as
Acid + Alcohol # Ester + Water
(1)
When using an acidic type of catalyst containing sulfonic acid groups such as
the strongly acidic cation-exchange resin, it is proposed that the reaction is initi-
ated by donating a proton to the carboxylic acid molecule. After that, the hy-
droxyl group from the alcohol attacks the accessible carboxylic acid, and the
reaction continues with water elimination and ester formation. Through the last
step, the catalyst is recovered. The detailed mechanism with a schematic illustra-
tion is given in our previous study [25] following that proposed by Ali et. al.
[26].
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130
B. Erdem and M. Cebe
Fig. 2. The suitability of pseudo second order kinetic model for representation of the rate data.
The general reaction rate expression can be written as
dCA
dt
CE.CW
–rA = –
= k1 CA .CB –
(2)
( )
K
where subscripts A, B, E and W refer to acid, alcohol, ester and water respec-
tively, k1 is forward reaction rate constant, K is the equilibrium constant of the
reaction.
Eq. (2) can be integrated to give the following equation [27–28] while keep-
ing the acid:alcohol molar ratio at (1:1), the catalyst weight at 10 g dry resin .
LK1
.
XAe–(2XAe–1)XA
1
ln
= 2k1
– 1 CA0 t
(3)
( )
XAe–XA
XAe
where XA, fractional conversion of reactant A; XAe, fraction of the equilibrium
conversion of reactant A. Thus, the plot was made of the left-hand side versus
time for Eq. (3) to determine the slope of the line from which the values of k1
were calculated. Fig. 2 shows the suitability of this model (Eq. (3)) for represen-
tation of the rate data. The data fall on straight line, for different catalyst confirm-
ing the applicability of the model for this reaction.
The apparent equilibrium constant of the esterification reaction can be ob-
tained from the equilibrium conversion determined experimentally and using Eq.
(4),
CEeCWe
CAeCBe
XA2e
(1–XAe)2
K =
=
(4)
The calculated equilibrium constants of propanoic acid (as model acid) with the
related organic alcohols at 333K according to Eq. (4), are almost the same for
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Determination of Steric Effect on the Propanoic Acid Esterifications
131
propanoic acid esterification with little exception of the methanol esterification
and we may state that the most probable value of the equilibrium constant for
esterification of alcohols with propanoic acid at 333K, is 4.04. Because the stand-
ard error in equilibrium constant K calculated for the esterification reactions
was in order 10K2 and the changing of K is in experimental error range. The
reproducibility of the titration results was found to be within ϯ1.5 %. This was
determined by repeating the titration procedure at least three times.
3.3 Effect of nature of alcohols
Reactivity is greatly complicated in reaction series in which substituents are
introduced close to the reaction center. This situation prevails in reactions involv-
ing ortho-substituted benzene derivatives and most aliphatic derivatives. For this
reason, Taft developed a different equation to explain the behaviour of those
compounds. This equation is often employed in biological chemistry and medici-
nal chemistry for the development of quantitative structure activity relationships.
In a recent example, Sandri and co-workers [29] have used Taft plots in studies
of polar effects in the aminolysis of ꢀ-lactams. In the esterifications of propanoic
acid with methanol, ethanol, n-propyl, n-butyl, n-pentyl, iso-propyl, iso-butyl and
sec-butyl alcohols, propanoic acid conversions and the reaction velocity constant
decreases with increase in the molecular weight of the alcohol (see Fig. 3 and
Fig. 4), i. e. with increase in size of the alkyl group. As can be seen from Fig. 4,
the magnitude of decrease in the rate gradually diminishes with increase in the
size of the alkyl group. That is: a large difference is observed in the behaviour
of methanol and ethanol, while the difference is only marginal between ethanol
and n-propyl alcohol. This is due to the fact that methanol has no alkyl group,
whose presence may restrict the alcohol molecule to approach the acid molecule
or the active sites of the catalyst. Thus, the alkyl group in ethanol causes a
decrease in the reactivity of ethanol in comparison to that of methanol. This
decrease in the reaction velocity constant may be attributed to (i) the operation
of steric hindrance and inductive effect of the alkyl group attached to the alco-
hol molecule, and (ii) decrease in pore diffusion with increase in the molecular
weight of the alcohol. But in the literature, it was pointed out that the external
diffusion doesn’t generally control the overall rate in ion exchange resin cata-
lyzed process unless the viscosity of the reactant mixture is very high or the
speed of agitation is very low. Theoretical analysis showed that internal resist-
ance exists for particle sizes larger than 0.6 mm and the resins having between
500–600 μm particle size were chosen as catalysts in this study. The first sugges-
tion may be tested using Taft equation. It is known in the literature, [9, 30–32]
that rate constants of aliphatic organic compounds could be in general, correlated
using Taft equations, having for compound without a mesomeric effect, the com-
mon form e. g.:
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132
B. Erdem and M. Cebe
Fig. 3. Effect of different alcohols on the conversions of propanoic acid esterifications versus
time.
Fig. 4. Effect of different alcohols on the velocity constants of propanoic acid esterifications.
k1
log
= r*σ*+∂Es
(5)
k
CH3
k1; rate constant of the individual aliphatic members of the series, kCH ; rate
3
constant of the standard reaction in this series, σ*; induction effect level of the
substituents in aliphatic compounds, r*; sensitivity of the reactions to these ef-
fects, Es; steric effect level of the substituents in aliphatic compounds, ∂; sensitiv-
ity to steric effects. Es is measure of the total steric effect associated with a given
substituent relative to the standard of comparison. The standard of comparison
in each case is the CH3 group, that is: the reaction between ethanol and propanoic
acid was chosen as standard reaction and its rate constant was shown as kCH the
3
name of which came from the alkyl group (-CH3) in ethanol. The other esterifica-
tion rate constants were shown as k1. If the inductive effect is equal to zero, Eq.
(5) could be expressed in its reduced form
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Determination of Steric Effect on the Propanoic Acid Esterifications
133
Fig. 5. Testing of Taft equation on the esterification of propanoic acid with different alcohols.
k1
log
= ∂Es
(6)
k
CH3
involving only the steric effect of substituents. The plot of logk1.kCH versus Es
3
is shown in Figure 5. Taking in to account that alkyl substituents of organic
alcohols have a constant inductive effect (with a little exception in the case of
methanol), Eq. (6) was applied for correlating the found results of esterification
(Fig. 5). Thus, using the method of linear regression, the following equation for
the esterification rate constants was obtained:
k1
log
= 1.0061Es–0.0012
(7)
k
CH3
It can be seen from Fig. 5. that the slope of equation (7) could be considered
with high probability as ∂ = 1 and the intercept on y-axis as ‘0’. According to
Eq. (7), esterification of propanoic acid with different alcohols, catalyzed by an
acid catalyst, follows the Taft equation by considering only the steric part, since
the inductive part is equal ‘0’. It can be concluded that the mechanism of forma-
tion of esterification is the same, i. e., there is no difference between the forma-
tion of propyl propionate and propyl butrate and steric effect of the substituent
governs these esterification reactions.
The velocity constants of propanoic acid esterifications with different alco-
hols were given in Table 3 and according to the rate constants, the activity order
was determined as: methanol > ethanol > n-propyl alcohol > n-butyl alcohol >
n-pentyl alcohol > iso-butyl alcohol > iso-propyl alcohol (sec-) > 2-buthanol
(sec-). It was noticed that, this ordering was fallowed by prim- > sec- > ter-. The
formation of the transition state will be inhibited by an increase in steric interac-
tions within the molecule and by the repulsive interaction between the entering
nucleophile and the molecule itself [32]. Because the equilibrium constants of
esterification reaction of this type are equal, it is true that steric effects considera-
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134
B. Erdem and M. Cebe
Table 3. According to Eq. (3), the calculated rate constants of the propanoic acid esterification
reactions carried out in the same conditions and steric substituent constants.
Alcohol Type
k1 (!103
logk1.kCH
Es
3
dm3molK1minK1
)
Methanol
7.273
0.368
0.258
0.184
0.153
0.092
0.048
0.030
1.2958
0
+1.24
0
Ethanol
n-Propyl Alcohol
n-Butyl Alcohol
n-Pentyl Alcohol
so-Butyl Alcohol
iso-Propyl Alcohol
2-Butyl Alcohol
K0.1551
K0.3
K0.07
K0.36
K0.39
K0,93
K0,47
K1.13
K0.38
K0.6
K0.8817
K1.0922
bly affect the transition rate from the substrate through the active complex to the
final product retarding it (the transition), but from the thermodynamic point of
view (i. e. at the time t = ∞), this has no influence on the equilibrium constant.
4. Conclusion
The esterifications of different alcohols with propanoic acid were treated in order
to establish a relationship between the structure of alcohols and their reactivity.
To accomplish this goal Linear Free-Energy relation was used in the form of
Taft correlation.
Based on the experiments, the apparent rate constants for different alcohols
decreased in the following order: kapp(methanol) > kapp(ethanol) > kapp(n-propyl
alcohol) > kapp(n-butyl alcohol) > kapp(n-pentyl alcohol) > kapp(iso-butyl alcohol)
> kapp(iso-propyl alcohol) > kapp(sec-butyl alcohol) which demonstrates the im-
portance of steric hindrance and the suitability of the ordering followed by prim-
> sec- > ter-. This is because as the alkyl chain in the alcohol and branching of
it increase in size, the steric effect also increases and steric effect is one of the
main factors directly affecting the rate of reaction. The substituent effects of
different alcohols followed the Taft relationship with exactly the same slope. A
comparison among the different catalysts (Amberlyst-15 and Amberlite IR-120
and Dowex 50Wx4) was done by using them and the highest catalytic activity
was observed in case of Dowex 50Wx4.
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Determination of Steric Effect on the Propanoic Acid Esterifications
135
Appendix
Nomenclature
A, B, E, W propanoic acid, alcohol, ester and water, respectively
rA
reaction rate of propanoic acid
k1
rate constant of the individual aliphatic members of the series
rate constant of the standard reaction in the series
nduction effect level of the substituents in aliphatic compounds
sensitivity of the reactions to induction effect
kCH
σ*i 3
r*
Es
∂
steric effect level of the substituents in aliphatic compounds
sensitivity to steric effect
CA
CA0
CB
CE
CW
Ke
XA
XAe
t
concentration of propanoic acid (mol. LK1
)
initial concentration of propanoic acid (mol. LK1
concentration of alcohol (mol. LK1
concentration of ester (mol. LK1
concentration of water (mol. LK1
)
)
)
)
apparent equilibrium constant of the reaction
fractional conversion of propanoic acid
fraction of the equilibrium conversion of propanoic acid
time (min)
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