Kinetics of the Liquid-Phase Hydrochlorination of Ethanol

Article abstract of DOI:10.1134/S0023158418050099

The results of a study on the kinetics of the liquid-phase hydrochlorination of ethanol with hydrogen chloride are presented. The form of the rate equation, the preexponential factor, the activation energy, and the empirical coefficients that characterize the effect of chloride anion hydration on the reaction rate of ethanol hydrochlorination were determined. The rates of hydrochlorination of monohydric alcohols and polyols were compared based on the examples of methanol, ethanol, 1,2-propylene glycol, and glycerol.

Full text of DOI:10.1134/S0023158418050099

ISSN 0023-1584, Kinetics and Catalysis, 2018, Vol. 59, No. 5, pp. 553–556. © Pleiades Publishing, Ltd., 2018.
Original Russian Text © M.N. Makhin, G.S. Dmitriev, L.N. Zanaveskin, 2018, published in Kinetika i Kataliz, 2018, Vol. 59, No. 5, pp. 539–542.
Kinetics of the Liquid-Phase Hydrochlorination of Ethanol
M. N. Makhina, *, G. S. Dmitrieva, and L. N. Zanaveskina
aTopchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Moscow, 119991 Russia
*e-mail: Makhin.Maxim@gmail.com
Received February 19, 2018
Abstract—The results of a study on the kinetics of the liquid-phase hydrochlorination of ethanol with hydro-
gen chloride are presented. The form of the rate equation, the preexponential factor, the activation energy,
and the empirical coefficients that characterize the effect of chloride anion hydration on the reaction rate of
ethanol hydrochlorination were determined. The rates of hydrochlorination of monohydric alcohols and
polyols were compared based on the examples of methanol, ethanol, 1,2-propylene glycol, and glycerol.
Keywords: hydrochlorination, ethanol, ethyl chloride, kinetics, solvation
DOI: 10.1134/S0023158418050099
INTRODUCTION
reactants (primarily, the chloride anion) by water mol-
ecules on the rates of reactions. Because of the forma-
tion of hydrated shells around the molecules of reac-
tants, the solvent should also be considered as the
agent that directly participates in the reaction. The
higher the concentration of water in the reaction mix-
ture, the greater the number of water molecules that
interact with the chloride anion and the stronger its
hydrated shell; therefore, the activation energy
increases and the rate of reaction decreases with the
concentration of water.
For describing the rates of processes of this kind,
we proposed to introduce the coefficient γ into rate
equation (1) to characterize the degree of hydration of
the chloride anion. This coefficient is a ratio of the
concentration of water to the concentration of hydro-
gen chloride:
The following reactions occur in the reaction zone
of the liquid-phase hydrochlorination of ethanol: (I)
the formation of ethyl chloride, (II) the dehydration of
ethanol to diethyl ether, and (III) the subsequent
interaction of this latter with hydrogen chloride to
form ethyl chloride and ethanol.
C2H5OH + HCl ꢀ C2H5Cl + H2O,
(I)
H+
⎯⎯⎯→
2C H OH
C H OC H + H O,
(II)
←⎯⎯⎯
2
5
2
5
2
5
2
C2H5OC2H5 + HCl ꢀ C2H5Cl + C2H5OH. (III)
The hydrochlorination of primary alcohols is the
SN2 reaction of bimolecular nucleophilic substitution
[1], and its kinetics is described by the equation
E
RT
w = k0e−
C
CHCl.
(2)
(1)
γ = CH O CHCl.
ROH
2
However, in many experimental works [2–7] dedi-
cated to the hydrochlorination of monohydric and
polyhydric alcohols, it was noted that the rate constant
calculated according to Eq. (1) changed with the con-
version of alcohols. This fact was explained by changes
in the concentration of water, the presence of which
even in small quantities led to a noticeable decrease in
the rate of reaction. Until recently, there was no a uni-
fied approach to explain the observed effect based on
series of experiments performed in wide ranges of
reactant concentrations and temperatures.
At the same time, it is well known that the reaction
medium (solvent) has an essential effect on the rates of
reactions [8–11]. Previously, we studied the reaction ethanol hydrochlorination was carried out in the con-
kinetics of the hydrochlorination of glycerol [12], 1,2- tinuous mode in a 45-mL glass reactor equipped with
propylene glycol [13], and methanol [14] and experi- a stirrer, a bubbler, a reflux condenser, a thermocou-
mentally demonstrated the effect of the solvation of ple, and a heating jacket. The speed of rotation of the
The empirical equations thus obtained make it pos-
sible to adequately describe the kinetics of the reac-
tions, as demonstrated with both monohydric alcohols
[14] and polyols [12, 13].
The aim of this work was to study the reaction
kinetics of the liquid-phase hydrochlorination of eth-
anol and to compare the experimental results with the
already known data on the relative reactivity of differ-
ent alcohols.
EXPERIMENTAL
The experimental study of the reaction kinetics of
553

554
k × 106, L mol–1 s–1
MAKHIN et al.
lnk [L mol–1 s–1]
2.5
2.0
1.5
1.0
0.5
–13.0
–13.5
–14.0
–14.5
80°C
γ = 1
75°C
70°C
65°C
60°C
2.82
2.87
2.92
2.92
3.02
1/T × 103, K–1
0
0.5
1.0
1.5
2.0
γ
Fig. 2. Linearization of the Arrhenius equation (γ = 1).
Fig. 1. Dependence of the reaction rate constant of ethyl
chloride formation on the coefficient γ in a temperature
range 60 to 80°C.
this allowed us to consider it as an irreversible reaction.
In the process performed with an excess of gaseous
hydrogen chloride, its maximally possible concentra-
tion in the reaction mass was reached. In this case, the
formation of diethyl ether in the reaction products was
reduced to a minimum, but its selectivity did not
exceed 0.2% in all of the experiments. Thus, it was
possible to neglect reactions (II) and (III) under the
conditions of the above experiments.
With the aid of the found rates of formation of ethyl
chloride and the concentrations of reactants in the
reaction mass, we calculated the reaction rate con-
stants of ethyl chloride formation by Eq. (1) and deter-
mined their dependences on the coefficient γ (Fig. 1).
These dependences make it possible to determine the
reaction rate constants at different temperatures for
any value of the coefficient γ. At a constant value of γ,
the rate constants obtained were described by the
Arrhenius equation (Fig. 2) with γ = 1.
stirrer was 1500 rpm, which ensured the absence of
reactant concentration and temperature gradients in
the entire reaction volume. The temperature in the
reactor was measured with an accuracy of 0.1°C
using the thermocouple. Ethanol (or its mixture with
water) was continuously supplied to the reactor with a
pump at a rate of 15–20 g/h. The concentration of
water in ethanol was varied from 4.4 to 30 wt %.
Hydrogen chloride was continuously supplied to the
reactor through the bubbler in a quantity of 10–20 g/h.
The liquid reaction mass was removed from the reac-
tor by level through a lateral outlet with a water lock.
Excess hydrogen chloride together with the resultant
ethyl chloride (Tb = 12°C) was removed from the reac-
tor through the reflux condenser into a watered
absorption column. The amount of absorbed HCl in
the solution obtained was analyzed by titration. The
gas flow rate after the column was determined with the
aid of a flow meter. The composition of all of the con-
tinuously outgoing liquid and gas flows was deter-
mined by gas–liquid chromatography at regular inter-
vals of 30 min with the use of a flame-ionization
detector. We assumed that the reaction system reached
a steady-state regime when the compositions of the
flows ceased to change with time.
With the aid of the mathematical treatment of data
shown in Fig. 2, we determined the activation energy
and the preexponential factor. The reaction rate equa-
tion of the formation of ethyl chloride at γ = 1 takes the
following form:
wC H Cl = 5.7 ×103e−63800 100
RT
(3)
2
5
×CC H OH CHCl (mol L–1s–1).
2
5
An analogous treatment of the experimental data in a
range of γ from 0.5 to 2 showed that, as expected, an
increase in the degree of hydrogen chloride hydration led
to an increase in the apparent activation energy (Fig. 3).
The dependence shown in Fig. 3 is described by the
following equation:
RESULTS AND DISCUSSION
The kinetics of the liquid-phase hydrochlorination
of ethanol was studied in a temperature range from 60
to 80°C with changes in water concentration from 2 to
13 mol/L. In this case, the coefficient γ varied from 0.3
to 2.0. The analysis of the liquid reaction mixture out-
going from the reactor showed the absence of ethyl
chloride from it. This was due to the immediate
removal of the resulting ethyl chloride from the reac-
tion zone with a gas flow. In that case, the equilibrium
of reaction (I) of the hydrochlorination of ethanol was
completely shifted to the side of product formation;
(4)
E = (63800 + 6120lnγ) 100(J/mol).
In turn, a change in the preexponential factor is
described by the equation
k0 = 5.7 ×103γ1.76(L mol−1s−1).
(5)
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KINETICS OF THE LIQUID-PHASE HYDROCHLORINATION OF ETHANOL
555
Thus, the reaction of ethyl chloride formation in
the test ranges of temperatures and concentrations can
be described by the following kinetic equation:
E, kJ/mol
69
wC H Cl = 5.7 ×103 γ1.76 e−(63800 +6120 ln γ) 100
RT
(6)
2
5
×CC H OH CHCl (mol L−1s−1).
64
2
5
Table 1 summarizes numerical ratios between the
rate constants of alcohol hydrochlorination in a series
of methanol [14], ethanol, 1,2-propyleneglycol [13],
and glycerol [12]. The reaction constants of formation
for methyl chloride, propylene chlorohydrin isomers,
and glycerol monochlorohydrin were also calculated
from equations that took into account the degree of
hydration of hydrogen chloride.
The relative reactivity of alcohols is determined by
the inductive effects of substituents at the reactive car-
bon atom. Substituents with a negative inductive effect
stabilize the protonated transition complex and, thus,
increase the rate of reaction, whereas substituents with
a positive inductive effects slow down it. This fact
explains a relatively high rate in the bimolecular sub-
stitution reactions of ethanol. According to Eq. (IV),
hydrogen and a methyl group, which has the greatest
negative inductive effect in the series of normal-struc-
ture alkyl groups, are substituents in the molecule of
ethanol [15].
59
0.5
1.0
1.5
2.0
γ
Fig. 3. Dependence of the activation energy on the coeffi-
cient γ.
the rate constant of the hydrochlorination of ethanol
was found lower than that of methanol by a factor of
16. In this case, this can be explained by the high
polarity of reaction medium on the hydrochlorination
of methanol, whose dielectric constant is considerably
higher than those of other alcohols [15].
The negative inductive effect in the series of nor-
mal-structure alkyl groups decreases with alkyl group
length; therefore, the reaction rate of the hydrohalo-
genation of primary alcohols decreases with the num-
ber of carbon atoms, as confirmed by literature data.
Thus, it was found [3, 16] that the rate of n-propanol
interaction with hydrogen halides (HCl and HBr) is
lower by a factor of 1–1.15 than the rate of ethanol
reactions with them. In this case, the ratio between the
inductive effects of donor substituents in these alcohols is
1–1.1. Under the conditions specified in Table 1, it is
possible to make a forecast that the rate constant ofn-pro-
panol hydrochlorination is ~ 4.5 × 10–6 mol L–1 s–1.
Thus, the data obtained are consistent with the exper-
iments, and they can be used for evaluating the kinetic
R1
R2
R1
R2
〉CH−OH + HCl ꢀ 〉CH−Cl + H2O. (IV)
Thus, at the β-C atom of propylene glycol, one
substituent (–CH3) exerts a negative inductive effect,
and the other (–CH2OH) exerts a positive effect. At
the α-carbon atom, R1 = H and R2 = –CH(OH)–
CH3 (an acceptor group); therefore, the rate of reac-
tion at the β-atom is higher. On the contrary, the β-C
atom in the molecule of glycerol has two substituents
with positive inductive effects, whereas the α-C atom
has only one substituent. Therefore, the rate of forma-
tion of primary monochlorohydrin from glycerol is
higher than that of secondary one. However, note that parameters of other similar reactions.
Table 1. Comparison of the rate constants of hydrochlorination of different alcohols*
k,
Alcohol
Reaction product
kproduct
/
k
CH3Cl
L mol–1 s–1
8.43 × 10–5
5.18 × 10–6
Methanol
Ethanol
CH3Cl
1
C2H5Cl
0.0614
2.12 × 10–7
1.60 × 10–6
1.39 × 10–6
4.51 × 10–8
α-C3H7OCl
β-C3H7OCl
0.0025
0.0190
1,2-Propylene glycol
Glycerol
α-C3H7O2Cl
β-C3H7O2Cl
0.0165
0.0005
*At a temperature of 100°C and γ = 3.
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556
MAKHIN et al.
CONCLUSIONS
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contract at the Topchiev Institute of Petrochemical
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Translated by V. Makhlyarchuk
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