Thermal decomposition of methyl β-hydroxyesters in m-xylene solution

Article abstract of DOI:10.1002/kin.20209

The products and kinetics of the thermal decomposition of several methyl-β-hydroxyesters in m-xylene solution have been studied. It has been shown that all β-hydroxyesters studied pyrolyze to form a mixture of methyl acetate and the corresponding aldehyde or ketone and that the decomposition follows first-order kinetics and appears to be homogeneous and unimolecular. The rate pyrolysis of methyl-3-hydroxypropanoate, methyl-3-hydroxybutanoate, and methyl-3-hydroxy-3-methylbutanoate has been measured between 250 and 320°C. The relative rates of primary, secondary, and tertiary alcohols at 553 K are 1.0. 8.5 and 54.1. respectively. The absence of large substituent effects indicates that little charge separation occurs during the breaking of carbon-carbon single bond. The activation entropy is compatible with a semipolar six-membered cyclic transition state postulated for other β-hydroxy compounds.

Full text of DOI:10.1002/kin.20209

Thermal Decomposition
of Methyl ␤-Hydroxyesters
in m-Xylene Solution
EDILMA ZAPATA, JAIR GAVIRIA, JAIRO QUIJANO
Laboratorio de Fisicoqu ´ı mica Org a´ nica, Facultad de Ciencias, Universidad Nacional de Colombia, Sede Medell ´ı n,
Apartado A e´ reo 3840, Medell ´ı n, Colombia
Received 2 January 2006; revised 7 April 2006, 23 May 2006; accepted 26 May 2006
DOI 10.1002/kin.20209
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The products and kinetics of the thermal decomposition of several methyl-
β-hydroxyesters in m-xylene solution have been studied. It has been shown that all β-
hydroxyesters studied pyrolyze to form a mixture of methyl acetate and the corresponding
aldehyde or ketone and that the decomposition follows first-order kinetics and appears to be
homogeneous and unimolecular. The rate pyrolysis of methyl-3-hydroxypropanoate, methyl-
3
2
1
-hydroxybutanoate, and methyl-3-hydroxy-3-methylbutanoate has been measured between
50 and 320 C. The relative rates of primary, secondary, and tertiary alcohols at 553 K are
.0, 8.5 and 54.1, respectively. The absence of large substituent effects indicates that little
◦
charge separation occurs during the breaking of carbon–carbon single bond. The activation
entropy is compatible with a semipolar six-membered cyclic transition state postulated for
other β-hydroxy compounds. ꢀC 2006 Wiley Periodicals, Inc. Int J Chem Kinet 39: 92–96, 2007
INTRODUCTION
3-hydroxy-3-methylbutanoate (tertiary alcohol), was
presented recently [5]. In that work, we studied the
principal and side reactions and compared these re-
sults with the available experimental data (see Figs. 1
and 2, Table I).
It was proposed that the reaction involves a
cyclic six-membered transition state similar to
that thought to be involved in other thermal
decompositions such as the thermolysis of β-
hydroxyketones [6,7] (R1R2C(OH)CH2COCH3), β-
hydroxyolefins [8] (R1R2C(OH)CH2CH CH2), and
β-hydroxyacetylenes (R1R2C(OH)CH2C CH) [9].
The experimental results show that these reactions
are homogeneous and unimolecular and follow a first-
order rate law.
Ethyl β-hydroxy carboxylates, R1R2C(OH)CH2-
COOEt, decompose thermally in the gas phase and
in m-xylene solution to a mixture of aldehydes and/or
ketones and ethyl acetate [1–4]. Small amounts of ethy-
lene and carboxylic acid (<5%) were detected in a tem-
perature range of 296–330 C and in a pressure range
of 29–108 Torr [4].
A theoretical study of the thermal decomposition
in the gas phase of three ethyl β-hydroxycarboylates,
ethyl 3-hydroxypropionate (primary alcohol), ethyl
◦
3-hydroxybutanoate (secondary alcohol), and ethyl
Entropies of activation are typical of those reactions
that are thought to involve a cyclic transition state [10].
The experimental results [2,4] (Table I) show that
the tertiary alcohol decomposes more readily than
the secondary alcohol (the experimental thermolysis
Correspondence to: J. Quijano; e-mail: jquijano@unal.edu.
co.
Contract grant sponsor: COLCIENCIAS (Instituto Colombiano
para el Desarrollo de la Ciencia y la Tecnologia).
Contract grant number: 1118-0511481.
c 2006 Wiley Periodicals, Inc.
ꢀ

THERMAL DECOMPOSITION OF METHYL β-HYDROXYESTERS IN m-XYLENE SOLUTION
93
Figure 1 Mechanism of decomposition of ethyl β-hydroxycarboxylates suggested by the experiments.
priate aldehyde and/or ketone and α-bromoester.
Methyl-3-hydroxypropanoate (VII) was prepared by
esterification of β-hydroxypropionic acid previously
obtained by the hydrolysis of 3-hydroxy propionitrile
[12]. All these β-hydroxyesters were distilled care-
fully before their use, and their purity was checked
by MSGC using a Agilent 6890N gas chromatograph,
equipped with a HP-5 column (30 m × 0.32 mm i.d.,
film thickness 0.25 µm) and coupled to Agilent 5973
1
13
mass-selective detector, and by H and C NMR and
infrared spectroscopy.
Kinetic Procedure
Figure 2 Side reactions.
Thermolysis was carried out in a heated aluminum
block, 60 mm of length by 60 mm in diameter insulated
by glass wool. The block was heated by a resistance
of the primary alcohol has not been studied). The
experimental relative rates of secondary/tertiary al-
cohols are 1.0:4.4 at 320 C [2]. This behavior is
◦
◦
coil, and its temperature was controlled to ± 0.2 C by
similar to that observed in β-hydroxy ketones [6,7]
a Barber-Colman PIP temperature controller. The ab-
solute temperature was checked by a chromel–alumel
thermocouple.
◦
(
relative rates 1.0:4.2 at 230 C) and although less pro-
◦
nounced in β-hydroxyolefins (1.0:1.9, at 377 C [8])
and β-hydroxyacetylenes (1.0:1.3, at 350 C [9]).
◦
In order to eliminate the side reactions (Fig. 2) and
compare these results with the thermolysis in the gas
phase, we present in this paper the pyrolysis kinet-
ics of methyl-3-hydroxypropionate (primary alcohol
IV), methyl-3-hydroxybutanoate (secondary alcohol
V), and methyl-3-hydroxy-3-methylbutanoate (tertiary
alcohol VI) in the m-xylene solution (∼4% v/v).
Furthermore, no kinetic measurements have been
carried out for the thermal decomposition of these com-
pounds in the solution of xylene.
Thermolysis of ␤-Hydroxyesters
Thermolysis of esters was carried out in a sealed glass
ampoule made from carefully washed glass tubes, hav-
ing 2 mm i.d. and 50 mm length, approximately, 20 mL
of a solution of β-hydroxyester (∼4% v/v) and ben-
zene as an internal standard (∼2% v/v) in m-xylene
was injected in the tube. The ampoule was then sealed.
Twenty capillaries were placed in holes drilled in the
block, the diameter of holes was such that the tubes fit-
ted precisely, and at determined intervals the tubes were
withdrawn and the amount of methylacetate and alde-
hyde or ketone produced was determined by gas chro-
matography using a HP-5 column (30 m × 0.32 mm
i.d., film thickness 0.25 µm). The rate of decompo-
sition of each of β-hydroxyesters was determined by
observing the rate disappearance of β-hydroxyester
again by GLC using a HP-5 column (30 m × 0.32 mm
EXPERIMENTAL
Synthesis of ␤-Hydroxyesters
Methyl-3-hydroxybutanoate (V) and methyl-3-
hydroxy-3-methylbutanoate (VI) were prepared by
the Reformatsky reaction [11] between the appro-
International Journal of Chemical Kinetics DOI 10.1002/kin

9
4
ZAPATA, GAVIRIA, AND QUIJANO
i.d., film thickness 0.25 µm) and benzene as an internal
standard.
RESULTS
Methyl-β-hydroxyesters listed in Table II were py-
rolyzed in evacuated sealed glass tubes. All β-
hydroxyesters were pyrolyzed to a mixture of ester
and aldehyde or ketone according to Eq. (1)
R1R2C(OH)CH2COOCH3 → R1R2CO +
CH3COOCH3
(1)
R1 = R2 = H (IV); R1 = H, R2 = CH3 (V);
R1 = R2 = CH3 (VI)
The products of the reactions were identified by
their GLC retention time and by their MS-spectrum.
No other major products of reaction were observed.
No peak due to the presence of water was observed.
GC-MS indicated that under condition of reaction no
dehydration occurred.
Good first-order kinetics was observed for the ther-
molysis of all β-hydroxyesters, the reaction being fol-
lowed by both the rate of appearance of the products
and the rate of disappearance of β-hydroxyesters. In all
these cases plots of log10 C/C0, were linear to at least
three half-lives. The rate constants obtained are listed in
Table II. No variation in the rate constant was obtained
by using different concentrations of β-hydroxyester in
m-xylene (in the range of 1–10% of ester by volume);
the reaction was shown to be homogeneous by increas-
ing the surface area of the reaction vessel. Thus, in the
case of methyl-3-hydroxy-3-methylbutanoate (VI), a
−5
−1
s was obtained at 553 K
rate constant of 3.18 × 10
Table II Rates Constants (s 1) for the Thermal
Decomposition of Methyl β-Hydroxyesters at Several
Temperatures
−
k (s 1)
−
Methyl β-Hydroxyester
T (K)
−
−
−
7
6
5
Methyl-3-hydroxy-
propanoate IV
553
573
(5.80 ± 0.13) × 10
(2.78 ± 0.02) × 10
(1.21 ± 0.25) × 10
593
−7
Methyl-3-hydroxy-
butanoate V
523
553
(5.13 ± 0.1) × 10
(4.91 ± 0.01) × 10
(3.70 ± 0.02) × 10
(3.14 ± 0.06) × 10
(6.29 ± 0.059 × 10
−
−
−
6
5
5
583
Methyl-3-hydroxy-3-
methyl-butanoate VI
553
563
−
4
5
−
583
(2.36 ± 0.35) × 10
International Journal of Chemical Kinetics DOI 10.1002/kin

THERMAL DECOMPOSITION OF METHYL β-HYDROXYESTERS IN m-XYLENE SOLUTION
95
Table III Comparative Rates and Activation Parameters at 553 K of the Pyrolysis of Methyl β-Hydroxyesters in the
Gas Phase and m-Xylene Solution
ꢁ
ꢁ
ꢁ=
ꢀG
Ea
(kJ mol
ꢀS
ꢀH
Relative rates
6
−1
−1
−1
−1
−1
Substrate
IV
10 k (s
)
)
log A (J mol
)
(kJ mol
)
(kJ mol )
kg/ks
Reference
Gas
Solution
Gas
Solution
Gas
Solution
1.99
178.0
207.2
164.4
180.8
159.8
179.8
11.1
13.1
10.9
11.8
11.2
12.5
−37.3
5.2
−41.4
−33.1
−11.2
−19.5
173.4
202.6
159.8
176.2
155.2
175.2
194.0
199.7
182.7
189.9
175.2
186
3.43
4.73
3.79
[3]
This work
[3]
This work
[3]
0.58
23.2
4.91
119
31.4
V
VI
This work
in glass tubes packed with capillary tubing, which in-
creased the surface area by a factor of four, compared
Comparing the kinetics of decomposition of α- and β-
hydroxyesters, it can be observed that the order of ex-
perimental rate constants for methyl β-hydroxyesters
is tertiary alcohol > secondary alcohol, both in the gas
phase (k(sec)/k(tert) = 0.19 at 553 K) [3] and in the m-
xylene solution (k(sec)/k(tert) = 0.16 at 553 K), [this
work], whereas the order is reversed in the case of
methyl α-hydroxyesters (k(sec)/k(tert) = 1.3 at 553 K)
[13].
−6
−1
to 3.14 × 10
s in unpacked tubes.
All Arrhenius plots showed excellent linearity.
The calculated activation parameters are listed in
Table III.
DISCUSSION
It has previously been observed that [6] in
m-xylene solutions at 553 K, β-hydroxyketone and
4-hydroxy-4-methyl-2-pentanone pyrolyze 134 times
more rapidly than do the analogously substituted
β-hydroxy ester: ethyl-3-hydroxy-3-methylbutanoate
pyrolyzes 2170 times more rapidly than do methyl-
3-hydroxy-3-methylbutanoate. The proposed mecha-
nism is consistent with the greater difficulty in the
formation of enol from esters than from ketones [11]
(see Fig. 3).
As it would be expected [10] for an intramolecular
unimolecular reaction, the effect of a nonpolar solvent
on the velocity of pyrolysis of methyl-β-hydroxyesters
is small (Table III).
In changing the alkyl side from OCH2CH3
to OCH3 (ethyl and methyl ester, respectively)
(Table IV), the rates are quite different kMe/kEt ≈ 6.3
contrasted with the results in the gas phase, kMe/kEt ≈ 1
[2,3].
It has previously been shown [2] that β-hydroxy
ketones pyrolyze to mixtures of aldehydes and ke-
tones via a cyclic transition state. The results ob-
tained by the thermolysis of ethyl β-hydroxy es-
ters [2–4] suggest that the pyrolysis of β-hydroxy
ketones and ethyl β-hydroxyesters is closely re-
lated: both reactions appear to be homogeneous and
monomolecular with negative entropies of activation
with values typical of unimolecular reactions that are
thought to involve cyclic transition state [10]. On
these bases, it is proposed that methyl β-hydroxyesters
pyrolyze through a six-membered cyclic transition
state similar to that previously proposed for the py-
rolysis of ethyl β-hydroxyesters. Very recently [13],
an experimental and theoretical study on the mech-
anism of decomposition in the gas phase of two
methyl α-hydroxyesters, methyl 2-hydroxypropionate,
and methyl 2-hydroxyisobutyrate has been published.
Figure 3 Mechanism of thermolysis of β-hydroxymethylesters.
International Journal of Chemical Kinetics DOI 10.1002/kin

9
6
ZAPATA, GAVIRIA, AND QUIJANO
Table IV Comparative Rates and Activation Parameters for the Pyrolysis in m-Xylene Solution at 553 K
6
−1
−1
Ea (kJ mol )
Substrate
10 k (s
)
log A
Reference
Ethyl-3-hydroxy-3-methyl-butanoate III
Methyl-3-hydroxy-3-methyl-butanoate VI
5.0
31.4
155.9
179.8
11.4
12.5
[1]
This work
On comparing the relative rates of pyrolysis
of methyl β-hydroxyesters for primary, secondary,
and tertiary alcohols (IV, V, and VI), it was
found that methyl substitution at the hydroxyl car-
bon caused a small, but significant increase in
rates (tertiary: secondary: primary 54.1:8.5:1.0). This
means that in the case of bond breaking from
the primary to tertiary C(OH) in the sense
The authors thank Prof. Ederley V e´ lez (Unalmed,
Colombia) and Rafael Notario (CSIC, Spain) for their in-
terest and helpful discussions.
BIBLIOGRAPHY
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CONCLUSIONS
A experimental study of the thermal decomposition
in m-xylene solution of three methyl β-hydroxyesters
shows that the reaction takes place via a six-membered
cyclic transition state similar to that proposed for β-
hydroxy ketones and ethyl-β-hydroxyesters with a
small negative activation entropy. The solvent effect
and effects of methyl groups on the breaking of carbon-
carbon bond are similar to those previously observed in
the pyrolysis of β-hydroxy ketones, β-hydroxyesters,
β-hydroxyolefins, and β-hydroxyacetylenes. There is
a large positive charge developed on the hydroxylic car-
bon at the transitions state [14] which increases with
the methyl substitution on this carbon atom (incipient
carbonation carbon). This can explain the reactivity of
tertiary > secondary > primary alcohols observed.
8
9
49; (b) Smith, G. G.; Yates, B. J Chem Soc 1965, 7242.
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International Journal of Chemical Kinetics DOI 10.1002/kin