Kinetics of the formation of symmetrical wax esters from the corresponding alcohols with the use of hydrobromic acid and hydrogen peroxide

Article abstract of DOI:10.1007/s11746-001-0332-x

The primary aliphatic alcohols n-octanol, n-decanol, and n-dodecanol have been converted to their corresponding symmetrical esters by using HBr and H₂O₂ in the absence of a solvent. The reaction was carried out at 30, 40, and 50°C and at mole ratios of alcohol to HBr of 1:0.1, 1:0.2, 1:0.3, and 1:0.5. The rate of the reaction was found to increase with increase in the reaction temperature and concentration of HBr. The maximal conversion of n-octanol was 72% at 40°C and a mole ratio of n-octanol to HBr of 1:0.5. The kinetics of the reaction have been established, and the reaction was found to be first-order with respect to alcohol and bromine concentration in the organic phase, and second-order with respect to both. The second-order rate constants for n-octanol, n-decanol, and n-dodecanol are 27.08, 32.58, and 37.42 mL mol^-1 min^-1, respectively, at 40°C. The activation energy for the esterification reaction of n-octanol was found to be 16 32 kcal mol^-1.

Full text of DOI:10.1007/s11746-001-0332-x

Kinetics of the Formation of Symmetrical Wax Esters
from the Corresponding Alcohols with the Use
of Hydrobromic Acid and Hydrogen Peroxide
Mangesh G. Kulkarni and Sudhirprakash B. Sawant*
Chemical Engineering Division, Department of Chemical Technology,
University of Mumbai, Mumbai-400 019, India
ABSTRACT: The primary aliphatic alcohols n-octanol, n-dec- vent-free esterification reaction between stearic acid and
anol, and n-dodecanol have been converted to their corre-
sponding symmetrical esters by using HBr and H2O2 in the ab-
sence of a solvent. The reaction was carried out at 30, 40, and
50°C and at mole ratios of alcohol to HBr of 1:0.1, 1:0.2, 1:0.3,
and 1:0.5. The rate of the reaction was found to increase with
increase in the reaction temperature and concentration of HBr.
The maximal conversion of n-octanol was 72% at 40°C and a
mole ratio of n-octanol to HBr of 1:0.5. The kinetics of the re-
action have been established, and the reaction was found to be
first-order with respect to alcohol and bromine concentration in
the organic phase, and second-order with respect to both. The
stearyl alcohol has been examined with a montmorillonite
clay as a catalyst (6). The production of symmetrical wax es-
ters by one-step oxidation of a fatty alcohol by hydrogen per-
oxide using Re2O7 as a catalyst has been reported (7).
In the recent past, wax esters have been synthesized by en-
zyme-catalyzed reactions (8–13). The high catalytic activity
and substrate specificity of enzymes make their use a rapidly
growing field in organic synthesis. Though the enzyme-cat-
alyzed reactions have advantages of lower reaction tempera-
tures and eco-friendliness, these reactions are not yet com-
second-order rate constants for n-octanol, n-decanol, and n-do- mercially viable for wax ester synthesis.
decanol are 27.08, 32.58, and 37.42 mL mol−1 min−1, respec-
tively, at 40°C. The activation energy for the esterification reac-
tion of n-octanol was found to be 16.32 kcal mol−1.
Paper no. J9720 in JAOCS 78, 719–723 (July 2001).
Primary aliphatic alcohols, R–CH2OH, react with H2O2
under mild conditions in a two-phase liquid–liquid system
with ethylene dichloride as a solvent in the presence of Br2 or
HBr to yield selectively the corresponding esters. The follow-
ing mechanistic steps have been suggested by Alessandre
and Fabriele (14): (i) oxidation of HBr by H2O2, liberating
bromine in the aqueous phase; (ii) extraction of bromine into
the organic phase; (iii) formation of the acyl bromide by reac-
KEY WORDS: n-Decanol, n-dodecanol, HBr, H2O2, n-oc-
tanol, wax esters.
The high-molecular-weight esters generally known as wax es- tion of the alcohol with a stoichiometric amount of Br2;
ters have a number of applications such as their use as lubri- (iv) alcoholysis of the acyl bromide to give the ester; and
cants for high-speed machinery and in pharmaceuticals, cos- (v) extraction of liberated HBr from the organic phase to the
metics, and food additives. This has increased the importance aqueous phase. The large excess of HBr can lead to substitu-
of the synthesis of analogs of natural esters of these kinds tion bromination of the alcohol, thus reducing the overall se-
such as sperm whale oil, carnauba wax, and jojoba oil (1).
The ester production methods are based on the reaction be-
tween an alcohol and acid to form the ester and water. The
lectivity of the oxidation:
R–CH2OH + HBr → R–CH2Br + H2O
[1]
classical method uses a mineral acid such as sulfuric acid as a In the catalytic process, the oxidation of HBr by H2O2 in the
catalyst. The use of sulfuric acid as a catalyst has various aqueous phase and extraction of HBr formed in the organic
problems, including formation of undesirable products (2). phase into the aqueous phase are very fast. These reduce the
Often, the product obtained is colored because of the high concentration of HBr in the organic phase substantially and
temperature of the reaction. The catalytic system based on hence no formation of alkyl bromide, as indicated by the
cobalt salts avoids the above-mentioned problems and re- above reaction, is detected in the organic phase. The interme-
duces production costs. A kinetic study of the synthesis of n- diate aldehyde, R–CHO, is more reactive toward bromine
octyl octanoate using cobalt chloride as a catalyst has been than the starting alcohol (14).
reported (3). The direct formation of esters by the oxidation
The present work deals with direct conversion of primary
of primary alcohols by hemiacetal oxidation has also been aliphatic alcohols to their corresponding esters with the use
studied (4). Attempts were made to synthesize wax esters of HBr and H2O2. A model overall reaction for wax ester syn-
from lard oil as possible sperm oil replacements (5). The sol- thesis is
2 R–CH2OH + 2 H2O2 → R–COO–CH2R + 4 H2O
[2]
*To whom correspondence should be addressed at Chemical Engineering Di-
vision, Department of Chemical Technology, University of Mumbai, Natha-
lal Parekh Marg, Matunga, Mumbai-400 019, India.
where R is CnH2n+1; in the present work, n = 7, 9, and 11. The
reaction was carried out in the absence of a solvent. This has a
E-mail: sbs@udct.ernet.in
Copyright © 2001 by AOCS Press
719
JAOCS, Vol. 78, no. 7 (2001)

720
M.G. KULKARNI AND S.B. SAWANT
number of advantages. First, it eliminates the need for the sol- rately weighed quantity of the organic phase and a measured
vent recovery step. Second, when ethylene dichloride is used quantity of aqueous NaOH were added to a conical flask con-
as a solvent, substantial loss of the solvent takes place during taining methanol. The mixture was refluxed for 2 h in order
the processing and separation owing to the volatility of ethyl- to hydrolyze the ester completely. The excess of NaOH in the
ene dichloride. Finally, elimination of the solvent from the mixture was then estimated by titrating against standardized
process reduces the reactor volume considerably. The informa- HCl (reading A). The same quantity of aqueous NaOH added
tion about this reaction in the literature is limited to a single set in the above step was titrated separately with standardized
of reaction conditions (14). Thus, the effects of different pa- HCl (reading B). The organic phase also contains unreacted
rameters such as the reaction temperature and the mole ratio of Br2/HBr. The difference between B and A corresponds to the
the reactants are not available in the literature. In view of this, amount of NaOH consumed by the hydrolysis of the ester and
the present paper presents an investigation of the effects of var- the reaction with Br2/HBr in the organic phase. The amount
ious operating parameters on the progress of the reaction and of Br2/HBr in the organic phase was determined by titrating a
the kinetics of the direct esterification reaction of alcohols.
weighed sample of the organic phase dissolved in chilled
methanol (~10°C) with NaOH. Using these results, the
amount of ester present in the organic phase was estimated.
Gas chromatography (GC) analysis. Qualitative analysis
MATERIALS AND METHODS
Materials. n-Octanol (99% purity), hydrogen peroxide of the organic phase was carried out by using a gas chromato-
(H2O2) (30% wt/vol), and n-decanol (99% purity) were pur- graph (Chemito; Mumbai, India) equipped with a flame-ion-
chased from S.D. Fine-Chem Ltd. (Mumbai, India). n-Dodec- ization detector. A 2-m long and 1/8″ diameter OV-17 column
anol (lauryl alcohol) (98% purity) was supplied by Godrej was employed. The injector and the detector were maintain at
Soaps Ltd. (Mumbai, India). Hydrobromic acid (HBr) (49%) 325°C, and the oven temperature was maintained at 120°C
was purchased from Merck India Ltd. (Mumbai, India).
for 4 min. The temperature was then elevated to 290°C using
Experimental procedures. All experiments were carried a ramp rate of 30°C/min.
out batchwise in a borosilicate glass reactor of 65 mm i.d. and
The GC analysis showed that the organic phase of the re-
400 mL capacity, with four baffles and a six-blade turbine- action mixture did not contain carboxylic acid, aldehyde, or
type impeller (22 mm diameter). The reaction temperature alkyl bromide at detectable levels. Only two peaks were ob-
was maintained constant by immersing the reactor in a con- served, one for the unreacted alcohol and the other for the
stant-temperature water bath.
ester. The identification of these peaks was verified by com-
A predetermined quantity of aqueous HBr (60 mmol, un- parison with authentic samples.
less otherwise stated) was added (as a 49% aqueous solution)
Bromine partitioning. The oxidation of HBr by H2O2 in
to the reactor placed in the constant-temperature water bath. the aqueous phase liberates bromine, which is then extracted
A measured quantity of H2O2 (300 mmol) was then added, into the organic phase. Br2 reacts with alcohol to generate
followed by addition of the alcohol (200 mmol). All the reac- HBr, which is extracted back into the aqueous phase. The par-
tants were preheated to the reaction temperature before being titioning of bromine between the organic and aqueous phases
added to the reactor.
was determined at three different temperatures, viz., 30, 40,
Samples were withdrawn from the reaction mixture at pre- and 50°C, as follows.
determined time intervals (30 min, 60 min, etc.) and the
The reaction mixture was stirred for 15 min at a constant
progress of the reaction was monitored by analyzing the or- temperature. This procedure was the same as described above.
ganic phase by a titrimetric method. After 10 h, the stirring The organic and aqueous phases were rapidly separated. A mea-
was stopped, and the organic and aqueous phases were al- sured quantity of organic phase (5 mL) was added to a conical
lowed to separate in a separatory funnel.
flask containing a known quantity of methanol at ~10°C. A
The samples withdrawn from the reaction mixture were al- known quantity of aqueous NaOH was then added to the coni-
lowed to separate into two phases (organic and aqueous) in an cal flask; the quantity added was in excess of that required for
ordinary glass tube. The separated phases were carefully drained. complete reaction. The unreacted NaOH was titrated against
The organic phase was immediately analyzed by the titrimetric standardized 0.1 N HCl. From the titration reading, the amount
method. The phase separation with subsequent draining took of Br2/HBr (combined) in the organic phase was calculated.
about 3 min. To check whether an appreciable amount of reac-
Material balance. In a typical batch experiment, n-octanol
tion would take place during this step, the two separated phases (26 g, 200 mmol) was reacted at 40°C. The mole ratio of al-
were retained in the tube for 15 min prior to draining and analy- cohol to HBr was 1:0.5. At the end of the batch reaction time
sis of the organic phase. The results of the analysis were practi- of 10 h, the organic phase was separated from the reaction
cally the same as those obtained when the phases were separated mixture and washed with water to remove traces of acid. The
in less than 3 min. Thus, the phase separation was sufficient to organic phase was dried and weighed (25.1 g). Analysis of
ensure that practically no reaction occurred during this stage.
the organic phase showed the ester content to be 72%. Thus
Analysis. Conversion of the alcohol to the ester was deter- the material balance to the extent of 97% based on the initial
mined by a titrimetric method. This titration method is a mod- n-octanol was obtained. No attempt was made to separate the
ification of the official Ester Value method (15). An accu- alcohol–ester mixture.
JAOCS, Vol. 78, no. 7 (2001)

WAX ESTERS FROM ALCOHOLS USING HBr AND H2O2
721
Effect of mole ratio of alcohol to HBr. The effect of the
n-octanol/HBr mole ratio (1:0.1, 1:0.2, 1:0.3, and 1:0.5) was
investigated at 40°C. As this mole ratio decreases, the rate of
the overall reaction increases (Table 1). As the concentration
of HBr in the aqueous phase increases, more bromine is
formed. This effectively increases the concentration of bromine
in the organic phase, and the rate of the overall reaction in-
creases. It is noted that after 10 h the amount of n-octyl oc-
tanoate formed is more than that corresponding to the stoichio-
metric quantity of HBr consumed. It can be seen from the
mechanistic steps that as reaction proceeds, HBr is generated
in the organic phase. The HBr produced is transferred back to
the aqueous phase, where it is reoxidized by H2O2 to generate
bromine, which drives the reaction forward. The identity of the
products was confirmed by comparison with authentic samples.
Authentic samples of n-octyl octanoate were prepared by reac-
tion of n-octanoic acid with n-octanol using p-toluene sulfonic
acid (P-TSA) as a catalyst and toluene as a water entrainer (17).
Effect of temperature. The effect of temperature on the
progress of the reaction was investigated at 30, 40, and 50°C,
with all other parameters unaltered. The initial rate of reac-
tion increased substantially with temperature when the reac-
tion was carried out at 40 and 50°C (Table 1).
RESULTS AND DISCUSSION
The effects of the following variables on the conversion of al-
cohol were investigated: (i) stirring speed, (ii) mole ratio of
reactants, (iii) temperature, and (iv) substrates.
Effect of stirring speed. In liquid–liquid systems, a species
that is transferred from the aqueous phase to the organic phase
encounters resistance at two stages, viz., (i) upon passing from
the bulk aqueous phase to the interface, and (ii) upon passing
from the interface to the bulk organic phase. It is important to
eliminate these resistances to achieve maximal overall reac-
tion rates. This may be done by increasing the stirring speed.
For the reaction under investigation, Br2 formed in the aque-
ous phase is transferred to the organic phase, and HBr formed
in the organic phase is transferred back to the aqueous phase.
At a certain stirring speed, the concentration of the species in
the bulk of the phase equals the interphase concentration. At
this stage, the mass transfer resistances are eliminated.
To investigate the effect of the speed of stirring, n-octyl
octanoate was synthesized by reacting n-octanol with aque-
ous HBr and H2O2. The experiments were performed at 50°C.
Elimination of mass transfer resistance at 50°C also ensures
that reactions at 50°C and lower temperatures under similar
agitation conditions or speed of agitation are kinetically con-
trolled (16). There was no effect of speed of agitation in the
range of 750–2500 rpm on the progress of the reaction
(Table 1). Thus, beyond a speed of 750 rpm, the reaction be-
comes kinetically controlled. However, all the experiments
described below were performed above 1500 rpm to ensure
that the effects of various operating parameters were evalu-
ated under conditions of kinetic controlled.
Effect of substrate. Oxidation of n-decanol and n-dodec-
anol was also investigated at 40°C. The progress of the oxi-
dation reaction of n-decanol was identical to that of n-octanol.
Slightly lower conversions were observed in the oxidation of
n-dodecanol under otherwise identical conditions. Results are
given in Table 1.
Kinetics. For a second-order reaction, the following rate
equation applies in the present case:
TABLE 1
Effect of Stirring Speed, Molar Ratio of Alcohol to HBr, and Substrate Alcohol on the Progress of the Reaction
Conversion of alcohola
Substrate
Stirring speedb (rpm)
n-Octanolc
Time
(min)
750
1500
2500
I
II
III
IV
V
n-Decanold
n-Dodecanold
20
30
40
60
80
—
—
—
0.25
—
—
0.33
—
0.36
0.39
—
—
0.18
—
0.24
—
—
0.37
—
0.41
—
—
—
—
0.1
—
—
—
0.29
—
—
0.37
—
0.43
—
—
—
0.12
—
0.15
—
—
0.3
—
—
0.46
—
0.52
—
—
—
—
—
0.020
—
—
0.03
—
—
—
0.04
—
—
—
—
0.089
—
—
0.13
—
0.16
—
—
0.21
—
0.32
—
—
—
—
—
—
—
—
—
—
0.2
—
0.3
0.41
0.46
—
—
—
0.17
—
—
—
—
—
0.14
—
—
—
90
0.58
0.61
—
—
—
—
—
0.68
0.72
120
160
180
240
300
360
480
600
0.28
—
0.24
—
0.39
0.44
0.49
0.50
—
0.36
0.38
0.43
0.48
—
—
—
—
0.25
0.28
0.3
—
—
—
0.06
—
—
—
aWeight of alcohol converted to esters/weight of original alcohol.
bReaction conditions: 50°C; mole ratio of alcohol (n-octanol) to HBr, 1:0.3.
cReaction conditions: stirring speed, 1500 rpm; I: 40°C; mole ratio of alcohol to HBr, 1:0.3; II: 40°C; mole ratio of alcohol
to HBr, 1:0.1; III: 40°C; mole ratio of alcohol to HBr, 1:0.2; IV: 40°C; mole ratio of alcohol to HBr, 1:0.5; V: 30°C; mole
ratio of alcohol to HBr, 1:0.3.
dReaction conditions: stirring speed, 1500 rpm; 40°C; mole ratio of alcohol to HBr, 1:0.3.
JAOCS, Vol. 78, no. 7 (2001)

722
M.G. KULKARNI AND S.B. SAWANT
FIG. 2. Plot of k′ (Eq. 4) vs. concentration of bromine in organic phase.
Initial conditions: 200 mmol n-octanol, 300 mmol H2O2, 150 mL H2O,
1500 rpm.
FIG. 1. Plot of –ln(1 − X) vs. time (min), where X is the fraction of alco-
hol converted at time t. Initial conditions: 200 n-octanol, 300 mmol
H2O2, 150 mL H2O, 40°C, 1500 rpm. Mole ratios of alcohol to HBr
shown in inset.
R = k [Br2] [alcohol]
[3]
where R = rate of consumption of alcohol, k = second-order rate
constant for the reaction in the organic phase, [Br2] = concen-
tration of bromine, and [alcohol] = concentration of alcohol.
The concentration of Br2 in the organic phase can be as-
sumed to be constant, because the HBr formed is immediately
extracted into the aqueous phase and bromine is liberated. Be-
cause the agitation rates are very high, one can assume that
Br2 gets partitioned rapidly in the organic phase. Thus, at any
stage, the concentration of Br2 remains constant for the given
reaction conditions. Thus,
R = k′ [alcohol]
[4]
where k′ = k[Br2].
The validity of the pseudo-first-order rate equation (Eq. 4)
can be confirmed by plotting –ln(1 − X) vs. reaction time, t,
where X is the fraction of alcohol converted at time t. Figure 1
shows that the reaction is first-order with respect to n-octanol.
The slopes (k′) obtained from Figure 1 were plotted against
the concentration of bromine in the organic phase (Table 2).
FIG. 3. Arrhenius plot for n-octanol. Data from Table 2.
The straight line obtained (Fig. 2) also indicates that the reac-
tion is first-order with respect to bromine concentration in the
organic phase. From the slope of this straight line, the sec-
ond-order rate constant k was evaluated. The values of the
second-order rate constant are given in Table 2. An Arrhenius
plot for n-octanol (Fig. 3) shows the activation energy for the
reaction to be 16.32 kcal mol−1. This high value also confirms
that mass transfer limitations are absent.
TABLE 2
Concentration of Bromine in the Organic Phase
and Second-Order Rate Constant (k) Valuesa
Concentration of bromine
k
in the organic phase (mol L−1
)
(mL mol−1min−1
)
Alcohol
30°C
40°C
50°C
30°C
40°C
50°C
n-Octanol
n-Decanol
n-Dodecanol
0.125 0.0985 0.0918
8.28
—
—
27.08
32.58
37.42
43.57
—
—
ACKNOWLEDGMENTS
—
—
0.0798
0.0652
—
—
The authors thank M/s. Godrej Soaps Ltd., Mumbai, India, for the
generous gifts of n-dodecanol (lauryl alcohol). M.G. Kulkarni would
like to thank the Director, Department of Chemical Technology,
aReaction conditions: 200 mmol alcohol, 60 mmol HBr, 300 mmol H2O2,
150 mL H2O; stirring speed, 1500 rpm.
JAOCS, Vol. 78, no. 7 (2001)

WAX ESTERS FROM ALCOHOLS USING HBr AND H2O2
723
University of Mumbai, India, for the award of the Prof. M.M.
Sharma Doctoral Fellowship.
9. Isono, Y., H. Nabetani, and M. Nakajima, Wax Ester Synthesis
in a Membrane Reactor with Lipase Surfactant Complex in
Hexane, Ibid. 72:887–890 (1995).
10. Ucciani, E., M. Schmitt-Rozieres, A. Debal, and L.C. Comeau,
Enzymatic Synthesis of Some Wax-Esters, Fett/Lipid 98:
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11. Wehtje, E., D. Costes, and P. Aldercreutz, Continuous Lipase-
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12. Basheer, S., U. Cogan, and M. Nakajima, Esterifaction Kinetics
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Alcoholysis of Crambe Oil and Camelina Oil for the Prepara-
tion of Long-Chain Esters, Ibid. 77:361–366 (2000).
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JAOCS, Vol. 78, no. 7 (2001)