KINETIC STUDIES ON THE EPOXIDATION
OF 4-METHYL-4-OCTENE
Valéria Dutra Ramos*, Daniel Derouet** & Leila Léa Yuan Visconte*
*Instituto de Macromoléculas Professora Eloisa Mano,
Universidade Federal do Rio de Janeiro, 21945 - 970 - Rio de Janeiro, RJ - Brazil
**Laboratoire de Chimie Organique Macromoléculaire (UMR du CNRS),
Université du Maine, Avenue Olivier Messiaen 72085, Le Mans, France
Abstract - The kinetics concerning the performic acid epoxidation of 4-methyl-4-octene, a
model compound for 1,4-polyisoprene, was studied by the High Performance Liquid
Chromatography (HPLC) technique. The samples were characterized by Nuclear Magnetic
Resonance (13C and 1H-NMR) and Fourier Transform Infrared Spectroscopy (FTIR) analysis.
This investigation allowed an insight on the influence of the different reaction parameters
(temperature and
concentration of hydrogen peroxide) on the conversion
rate.
The determining step of the epoxidation was the formation of performic acid. The activation
enthalpy of the reaction was found to be ∆H* = 87 kJ/mol and the entropy of activation
∆S* = -44 J/mol K.
Key words: Kinetics, Model compound, Epoxidation, 1,4-Polyisoprene
INTRODUCTION
Polydienes constitute a family of polymers suitable to several applications. Among
them, natural rubber is, probably, the most employed material, in reason of its cost and the
mechanical properties it offers. The chemical modification of natural rubber, or of synthetic
polydienes with similar structure, has been the object of several research works, including
chemical modification, in such a way as to provide the products with the characteristics of
sensitivity to light. Recent studies in this area are being directed particularly to the chemical
modification of epoxidized polydienes. Actually, epoxides constitute a class of synthesis
intermediates very reactive towards most of nucleophiles and they offer multiple possibilities
of chemical modification for 1,4-polyisoprene. This reaction has been used in the fixation of
naphtylacetic acid, stimulant of plants growth1-3, of acrylic acid, for obtaining
photocrosslinkable varnishes4, of antioxidants5 and of reticulation agents.6
Reactions involving polymers are hindered by the simultaneous production of lateral
products and also by restrictions due to the molecular size. To obtain more detailed
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information on the mechanism of certain reactions, low molecular weight compounds are
submitted to the same reaction conditions in order to provide a better understanding of the
system.
This work is devoted to the kinetic study concerning the performic acid epoxidation of
4-methyl-4-octene, a model compound for 1,4-polyisoprene, by the High Performance Liquid
Chromatography (HPLC) technique. The investigation allowed an insight on the influence of
the different reaction parameters (temperature and concentration of hydrogen peroxide) on the
conversion rate.
EXPERIMENTAL
Reagents
Formic acid (98%) was obtained from Merck and hydrogen peroxide (35 wt %) from
Reagen. Phenanthrene was obtained from Jansen Chimica (98-99%) and used as the internal
standard. All reagents were used without further purification. Solvents used in High
Performance Liquid Chromatography (HPLC) were chromatography solvents for HPLC.
Deuterated chloroform was used in N.M.R analysis.
Procedure
The model compound (4-methyl-4-octene) was prepared through the reaction of WittigSchölkopf7-9 between n-butylidenetriphenylphosphorane and 2-pentanone. This reaction
requires the previous preparation of the joint-stock n-butylidenetriphenylphosphorane from
sodium t-amylate and bromide of n-butyltriphenylphosphonium. The main spectroscopics
characteristics of the model compound are: Boiling point (760 mmHg) = 141-142°C, Yield =
60%.
1
H-NMR (CDCl3): δ = 0.85-1.00 (m , CH3-CH2-, 6H), 1.30-1.50 (m, CH3-CH2-CH2-, 4H),
1.58 (trans) and 1.68 (cis) (s, CH3-C=C-, 3H), 1.92-2.02 (m, CH2-C=C-, 4H), 5.10-5.20
(t, -C=CH-, 1H).
C-NMR (CDCl3): δ = 13.1 (trans) and 13.3 (cis) (CH3-CH2-CH2-C=C-), 13.3 (trans) and
13
13.5 (cis) (CH3-CH2- CH2-C(CH3)=C-), 15.2 (trans) and 22.8 (cis) (CH3-C=C-), 20.5 (trans)
and 20.6 (cis) (CH3-CH2-CH2-C=C-), 22.5 (trans) and 22.7 (cis) (CH3-CH2-CH2-C(CH3)=C-),
22.7 (trans) and 29.4 (cis) (CH2-C=C-), 33.2 (cis) and 41.3 (trans) (CH2-C(CH3)=C-), 124.0
(trans) and 124.8 (cis) (-C=CH-), 134.5 (trans) and 134.7 (cis) (CH3-C=C-).
FTIR: 1655 cm-1 : C=C, 1460 cm-1 : -CH3 (asym), 1380 cm-1 : -CH3 (sym).
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All the kinetics studies of the epoxidation of the model compound by performic acid
were carried out starting from a " mother " solution containing the model compound
(1.5869 x 10-2 moles), the internal standard (phenantrene = 2.781 x 10-3 moles) and the
solvent (benzene = 25 ml).
The reaction was carried out with different molar ratios of hydrogen peroxide (1 and 2
in relation to the model concentration) at room temperature, and a molar ratio
[HCOOH]/[model] = 1, and at various temperatures for a molar ratio [H2O2]/[model] = 1. The
rate of reaction was then estimated at various regular time intervals and the reaction was
interrupted when the entire model compound was consumed.
Analysis
13
C and 1H-NMR spectra of the products were taken at room temperature on a Bruker
DPX 200 (50.32 and 200.13 MHz) spectrometer using deuterated chloroform as the solvent.
Tetramethylsilane (TMS) was used as the internal reference and chemical shifts are given in
parts per million (ppm).
Infrared spectra were recorded using a ATI Mattson Genesis Fourier Transform
spectrometer, Series FTIRTM. Liquid film samples were analyzed on NaCl plates.
Analyses in HPLC were done with the help of a modular WATERS equipped with a
duplicate detection system (UV and differential refractometry). The used device comprises
two pumps MODEL 510, an injector U6K. The different module control, the acquirement and
the treatment of data are assured with the help of a microcomputer NEC IV (operating system:
Baseline 810 Waters). Analyses were achieved in isocratic mode.
RESULTS AND DISCUSSION
Identification of reaction products
In this work, the epoxidation of the model compound by performic acid was carried out
at room temperature with [H2O2]/[model] = 2.0. The molar ratio [HCOOH]/[model] = 1 was
kept constant.
The 1H-NMR spectrum of the complex mixture obtained after 20 hours of reaction
shows the presence of four main products. Among these, one comes from the epoxidation of
the model compound: 4,5-epoxy-4-methyloctane 1 with a 58% yield. The three other
products, 5-methyl-4-octanone 2, 4-methyl-4,5-octanediol 3 2-hydroxy-2-methyl-1-(n-
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propyl)pentyl formate 4, are the result of epoxide rearrangements with the respective yields of
1%, 20% and 2%.
The epoxide 1 was characterized by chemical shifts at 2.70 ppm for proton of the
oxirane cycle.
The ketone 2 is identified in the reaction mixture by the presence of the triplet at 2.35 2.45 ppm (-CH2-CO-; 3J = 7.3 Hz) and the multiplet at 2.50-2.60 ppm (-(CH3)CH-CO-; 3J =
6.9 Hz), characteristics of protons in α and α'-position of the carbonyl function.
The presence of diol 3 in the reaction mixture was confirmed by the chemical shifts of
proton in α-position of the secondary alcohol function (CH-OH, triplet) at 3.40 - 3.55 ppm
and of the two singlets at 0.96 and 1.28 ppm attributed to the methyl group bound to the αcarbon of the alcohol function (-C(CH3)-OH).
The glycol ester 4 was characterized by the triplet at 4.90 - 5.10 ppm attributed to the
proton in α-position of the ester function (CH-OCOH), by the singlet at 8.15 ppm attributed
to the proton bound to the carbon of the carbonyl function (CH – OCOH) and of the two
singlets at 0.96 and 1.28 ppm attributed to the methyl group bound to the α-carbon of the
alcohol function (-C(CH3)-OH).
In 13C-NMR spectrum, the chemical shifts characteristics of the products are due to the
carbons of the double bond of the ketone function.
The FTIR spectrum confirms the presence of the reaction products through the
following absorption bands: 4,5-epoxy-4-methyloctane 1 : 1255 cm-1 (sym) and 889 cm-1
(asym) : C-O-C, 1467 cm-1 (asym) and 1381 cm-1 (sym) : C-H of -CH3 and -CH2-.
5-methyl-4-octanone 2 : 1713 cm-1 : C=O, 1460 cm-1 (asym) and 1379 cm-1 (sym) : CH3-CH-.
4-methyl-4,5-octanediol 3 : 3450 cm-1 : (CH3-C-OH), 3370 cm-1 : (CH-OH), 1465 cm-1
(asym) and 1379 cm-1 (sym) : CH3-CH-.
2-hydroxy-2-methyl -1-(n-propyl)pentyl formate 4 : 3450 cm-1 : (CH3-C-OH), 1730 cm-1 :
C=O, 1210 - 1163 cm-1 : -C(=O)-O, 1460 cm-1 (asym) and 1379 cm-1 (sym) : CH3-CH-.
Kinetics
The performic acid used for the epoxidation of the 4-methyl-4-octene was generated "in
situ" (equations 1 and 2).
k1
H-COOH + H2O2
H-COOOH +
k -1
H-COOOH + H2O
O
(1)
+
4-methyl-4-octene
4,5-epoxy-4-methyloctane 1
(2)
H-COOH
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Taking the rate determining step as the production of the performic acid, the following
rate law applies.
d[1] = k1([H2O2]0 - [1]) [HCOOH]
dt
= k1([H2O2]0 - [1]) [HCOOH]0
(3)
where the subscript 0 denotes the initial concentration.
In equation (3), the concentration of the formic acid is assumed to remain constant
throughout the reaction and is approximated to [HCOOH]0 for the following reasons : (1) both
the equilibrium constant for the performic acid formation (equation 1), k ≅ 1 and the
dissociation constant of the formic acid, k ≅ 1,78 x 10-4
10, 11
are small; (2) formic acid is
regenerated in the epoxidation process (equation 2).
Integration of equation (3) yields
Ln ([H2O2]0 - [1]) = - k1[HCOOH]0t + Ln [H2O2]0.
(4)
The analysis method used in the present study was the High Performance Liquid
Chromatography (HPLC). The residual mass of the model compound and of the formed
epoxide 1 were determined at regular intervals of time and the reaction was stopped when the
model compound had been totally consumed. The reaction was done at room temperature
with estequiometric amounts of model, formic acid and hydrogen peroxide, and the yield of
residual model was 48.8%, after 25 hours of reaction, and the yield in epoxide 1 was only
36.9% (Fig. 1). This indicates the consumption of the epoxide by the secondary processes
previously described, from which an improvement of the yield in epoxides with the use of an
excess of hydrogen peroxide can be expected.
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100
80
Yield (%)
% residual model
60
% epoxides
40
% secondary products
20
0
0
500
1000
1500
time (mn)
Figure 1: Yields in residual model, epoxides and
secondary products at 25°C versus reaction time.
Influence of [H2O2]/[model] molar ratio
The epoxidation of 4-methyl-4-octene was carried out at different molar ratios 1 and 2,
of hydrogen peroxide/model compound, at 25°C. The results (Fig. 2) show an increase in the
reaction rate with increasing [H2O2]/[model] molar ratios. The reaction becomes very slow
when the molar ratio is 1 leading to an incomplete consumption of the model compound
(conversion of 50%). However, 100% conversion is reached (Fig. 2.a) when an excess of
hydrogen peroxide is used, but the yield in epoxides, under these conditions, is only of 58.5%.
This result indicates the existence of secondary processes. As can be seen in Figure 2.b, the
yield in epoxides is inferior when the molar ratio is 1. On the other hand, happen an increase
of the yield in secondary products with increase [H2O2]/[model] molar ratios (Fig. 2.c).
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100
100
B
80
80
Yield in epoxides (%)
[H2O2]/[model] = 1
60
40
[H2O2]/[model] = 2
[H2O2]/[model] = 2
60
40
20
20
[H2O2]/[model] = 1
0
0
0
500
1000
0
1500
500
1000
1500
time (mn)
time (mn)
100
C
Yield in secondary products (%)
Yield in residual model (%)
A
80
[H2O2]/[model] = 1
60
[H2O2]/[model] = 2
40
20
0
0
500
1000
1500
time (mn)
Figure 2 : Yields of the reaction products versus reaction time at various molar ratios
[H2O2]/[modelo] at 25°C. A : yield in residual model, B : yield in epoxides and
C : yield in secondary products. [model]0 = 1.9039 x 10-3 moles,
[HCOOH]0 = 1.9231 x 10-3 moles.
Equation (4) allows the confirmation of the reaction second order (Fig. 3).
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-5,0
[H2O2]/[model] = 2
K1 = 1.4534 x 10-3 l.mol-1.mn-1
ln ([H2O2]0 - [1])
-5,5
[H2O2]/[model] = 1
K1 = 9.8776 x 10-4 l.mol-1.mn-1
-6,0
-6,5
-7,0
0
100
200
300
400
tim e (m n)
Figure 3 : Plots of ln ([H2O2]0 - [1]) versus reaction time at various
molar ratios [H2O2]/[model] at 25°C. [model]0 = 1.9039 x 10-3 moles,
[HCOOH]0 = 1.9231 x 10-3 moles.
Influence of the temperature
The epoxidation of 4-methyl-4-octene was carried out at 25, 35 and 45°C. The molar
ratio [H2O2]/[model] = 1 was kept constant. The results show that an increase in the reaction
rate is found at higher temperatures (Fig. 4.a). The conversion of the model compound is
100% at 350C, but the yield in epoxides is only 65.1%. This result indicates a partial
consumption of the formed epoxides as a secondary process. The yield in epoxides increases
up to a maximum value with the temperature (Fig. 4.b). However, happens a fall of the yield
and it is fall becomes so more important when higher it goes to temperature. This again
indicates the occurrence of secondary reactions. The influence of the temperature was also
observed for the formation of the secondary products (Fig. 4.c) since an increase in the
reaction rate with the temperature was noticed.
These results indicate that the yields in reaction products are dependent of the
temperature and, consequently dependent on the structure of the activated complex in the
transition state.
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A
Yield in residual model (%)
100
25°C
80
60
35°C
40
45°C
20
0
0
500
1000
1500
time (mn)
100
80
Yield in epoxides (%)
B
45°C
60
25°C
40
35°C
20
0
0
500
1000
1500
time (mn)
100
Yield in secondary products (%)
C
80
60
45°C
40
25°C
20
35°C
0
0
500
1000
1500
tim e (m n )
Figure 4 : Yields of the reaction products versus reaction time at various temperatures.
A : yield in residual model, B : yield in epoxides and C : yield in secondary products.
[model]0 = 1.9039 x 10-3 moles, [HCOOH]0 = 2.1577 x 10-3 moles
and [H2O2]0 = 1.9039 x 10-3 moles.
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The use of the equation (4) allows the determination of the rate constant of epoxide
formation at each used temperature (Fig. 5).
-6,0
T = 250C
K1 = 9.8776 x 10-4 l.mol-1.mn-1
ln ([H2O2]0 - [1])
-6,5
T = 350C
K1 = 5.1032 x 10-3 l.mol-1.mn-1
-7,0
-7,5
T = 450C
K1 = 9.4328 x 10-3 l.mol-1.mn-1
-8,0
0
100
200
300
400
tim e (m n)
Figure 5 : Plots of ln ([H2O2]0 - [1]) versus reaction time at various temperatures.
[model]0 = 1.9039 x 10-3 moles, [HCOOH]0 = 2.1577 x 10-3 moles
and [H2O2]0 = 1.9039 x 10-3 moles.
Table 1 : Rate constants k1 at various temperatures.
Product
1
T (0C)
k1 (l.mol-1.min-1)
25
9.8776.10-4
35
5.1032.10-3
45
9.4328.10-3
Rate constants k1 obtained from the plots of ln ([H2O2]0 - [1]) vs time are shown in
Table 1. The Arrhenius plot yields the enthalpy of activation of the reaction which was found
to be ∆H* = 87 kJ/mol and the entropy of activation ∆S* = -44 J/mol.K. The rate determining
step of the epoxidation was found to be the formation of performic acid.
ACKNOWLEDGEMENTS
The authors are indebted to UMR du CNRS, Université du Maine, Le Mans - France
where this research was carried out.
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