Thermal decomposition of 4-hydroxy-2-butanone in m-xylene solution: Experimental and computational study

Article abstract of DOI:10.1002/kin.20591

The mechanism of thermal decomposition of 4-hydroxy-2-butanone in m-xylene solution was studied experimentally and theoretically at the M05-2X/6-31G(d, p) level of theory. It follows first-order kinetics and appears to be homogeneous and unimolecular. The proposed mechanism is via a six-membered cyclic transition state to give a mixture of formaldehyde and acetone. Rate constant values were experimentally determined at three temperatures: 483.15, 493.15, and 503.15 K. Calculated rate constants are of the same order of magnitude than the experimental ones. Calculated Gibbs energies of activation agree very well with the experimental values. Computationally, the progress of the reactions was followed by means of the Wiberg bond index. The results indicate that the transition state has an intermediate character between reactants and products, and the calculated synchronicity shows that the reaction is slightly asynchronous. The bond-breaking processes are more advanced than the bond-forming ones, indicating a bond deficiency in the transition state.

Full text of DOI:10.1002/kin.20591

Thermal Decomposition of
4-Hydroxy-2-butanone in
m-Xylene Solution:
Experimental and
Computational Study
1
1
1
JULIANA MURILLO,1,2 DIANA HENAO, EDERLEY VELEZ, CAROLINA CASTANO,
´
˜
JAIRO QUIJANO,1 JAIR GAVIRIA,1 EDILMA ZAPATA1
1Laboratorio de Fisicoqu´ımica Orga´nica, Facultad de Ciencias, Universidad Nacional de Colombia, Sede Medell´ın,
AP 3840, Medell´ın, Colombia
2Departamento de Ciencias Qu´ımicas, Universidad Andre´s Bello, Repu´blica 275, Santiago, Chile
Received 30 January 2011; revised 28 April 2011, 20 June 2011; accepted 20 June 2011
DOI 10.1002/kin.20591
Published online 20 April 2012 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: The mechanism of thermal decomposition of 4-hydroxy-2-butanone in m-xylene
solution was studied experimentally and theoretically at the M05-2X/6-31G(d,p) level of theory.
It follows first-order kinetics and appears to be homogeneous and unimolecular. The proposed
mechanism is via a six-membered cyclic transition state to give a mixture of formaldehyde and
acetone. Rate constant values were experimentally determined at three temperatures: 483.15,
493.15, and 503.15 K. Calculated rate constants are of the same order of magnitude than the
experimental ones. Calculated Gibbs energies of activation agree very well with the experi-
mental values. Computationally, the progress of the reactions was followed by means of the
Wiberg bond index. The results indicate that the transition state has an intermediate character
between reactants and products, and the calculated synchronicity shows that the reaction is
slightly asynchronous. The bond-breaking processes are moꢀre advanced than the bond-forming
C
ones, indicating a bond deficiency in the transition state. 2012 Wiley Periodicals, Inc. Int J
Chem Kinet 44: 407–413, 2012
INTRODUCTION
The mechanism of thermal decomposition of β-
hydroxy ketones has been studied in many previ-
Correspondence to: J. Murillo; e-mail: jamurilll@unal.edu.co.
ous works both in the gas phase and in solution
[1–3]. These decompose thermally to give a mixture
of aldehydes and/or ketones in a reaction, which is
the reverse of the aldol condensation [1]. 4-Hydroxy-
4-methyl-2-pentanone was determined in the true
Contract grant sponsor: Vicerrector´ıa de Investigacio´n, Univer-
sidad Nacional de Colombia.
Tables S1 and S2 with detailed information about the theoretical
calculations are available as Supporting Information in the online
issue at www.wileyonlinelibrary.com.
c
ꢀ
2012 Wiley Periodicals, Inc.

408
MURILLO ET AL.
homogeneous gas-phase state by Smith and Yates [1].
Additional kinetic studies of two other β-hydroxy
ketones, i.e., 4-hydroxy-3-methyl-2-pentanone and 4-
hydroxy-2-pentanone, were studied in a xylene solu-
tion to examine the influence of the substituted methyl
group on the mechanism and velocity [2]. In another
work, Chuchani and coworkers [3] probed, under sim-
ilar conditions, the homogeneous gas-phase pyroly-
ses of 4-hydroxy-2-butanone, 4-hydroxy-2-pentanone,
and 4-hydroxy-4-methyl-2-pentanone, which follow a
first-order rate law, and the data were fitted to the
Arrhenius equations [3]:
of the primary isotope effect supported the above
mechanism, characterized by partial bonds between
the hydroxyl hydrogen and the carbonyl oxygen, and
that hydrogen transfer had not a linear path.
This work is another contribution to the kinetic
study of β-hydroxy ketones in solution. To our knowl-
edge, kinetic measurements have not been carried
out for the thermal decomposition of 4-hydroxy-2-
butanone in the m-xylene solution and there are no
theoretical studies on this reaction. Also we present
an experimental and computational study of the py-
rolysis kinetics of this primary β-hydroxy ketone in a
temperature range of 483.15–503.15 K.
log k (s−1) = (12.18 0.39)
− (150.0 3.9)kJ · mol−1(2.303RT )−1 (1)
EXPERIMENTAL PROCEDURE
log k (s−1) = (11.64 0.28)
4-hydroxy-2-butanone was obtained commercially
(Aldrich, St. Louis, MO, USA), distilled carefully
before use, and its purity was checked by gas
chromatography–mass spectrometry (GC-MS) using
a DB-WAX column (30 m × 0.25 mm i.d., film thick-
ness 0.25 μm), which is a polyethylene glycol (PEG)
column with high polarity.
− (142.1 2.7)kJ · mol−1(2.303RT )−1 (2)
log k(s−1) = (11.36 0.52)
− (133.4 4.9)kJ · mol−1(2.303RT )−1 (3)
Thermolysis was carried out in a heated alu-
minum block, 60 mm length × 60 mm diameter,
insulated by glass wool. The block was heated by
resistance coil, and its temperature was controlled
to 0.2◦C by a Barber–Colman PID (proportional–
integral–derivative) temperature controller. The abso-
lute temperature was checked by a chromel–alumel
thermocouple. This thermolysis was carried out in
a sealed glass ampoule made from carefully washed
glass tubes, having 2-mm i.d. and 500-mm length; ap-
proximately 50 μL of a solution of β-hydroxyketone
(∼4% v/v) and toluene as an internal standard (∼1%
v/v) in m-xylene was injected in the tube. Toluene
also served as a free radical inhibitor. The ampoule
was then sealed. Twenty capillary tubes were placed
in holes drilled in the block, the diameter of holes was
such that the tubes fitted precisely, and at determined
respectively.
It was proposed that the reaction involves
a cyclic six-membered transition state (TS) [4],
as shown in Fig. 1, and is similar to that
thought to be involved in other thermal de-
compositions such as the thermolysis of β-
hydroxyesters [4–6] (R1R2C(OH)CH2COOR3), β-
hydroxyolefins [7,8] (R1R2C(OH)CH2CH CH2), and
β-hydroxyacetylenes [9,10] (R1R2C(OH)CH2C CH).
The experimental results show that these reactions are
homogeneous and unimolecular and follow a first-
order rate law.
An additional mechanistic study of β-
hydroxyketone decomposition was the primary
kinetic deuterium effect of 4-deuteroxy-4-methyl-2-
pentanone in dilute xylene solutions [11]. The result
R1
R1
H2C
HO
R1
R2
CH2
H
CH3
CH2
H
CH3
R2
C
O
C
O
R2
C
O
C
O
C
O
C
CH3
+
R1, R2 = H, Me
Figure 1 Mechanism of decomposition of β-hydroxy ketones.
International Journal of Chemical Kinetics DOI 10.1002/kin.20591

THERMAL DECOMPOSITION OF 4-HYDROXY-2-BUTANONE IN m-XYLENE SOLUTION
409
O
O
HO CH2 CH2
C
CH3
CH3
(a)
(b)
H3C
C
CH3
H2C
+
O
+
O
C
O
C
H2O
H2C CH
CH3
HO CH2 CH2
Figure 2 (a) Elimination reaction of 4-hydroxy-2-butanone and (b) dehydration reaction of 4-hydroxy-2-butanone.
intervals the tubes were withdrawn and the amount
of acetone produced was determined by gas–liquid
chromatography (GLC) as the same of purity check.
The rate of decomposition of 4-hydroxy-2-butanone
was determined by observing the disappearance rate
of 4-hydroxy-2-butanone again by GLC using a DB-
WAX column (30 m × 0.25 mm i.d., film thickness
0.25 μm).
The bonding characteristics of the reactant, TS, and
products were investigated using a population parti-
tion technique, the natural bond orbital (NBO) analysis
of Reed and Weinhold [22,23]. The analysis was per-
formed using the NBO program [24] implemented in
the Gaussian03 package that also provides values for
the atomic natural total charges and the Wiberg bond
index [25] used to follow the progress of the reaction.
We have selected the classical TS theory) [26,27] to
calculate the kinetic parameters.
COMPUTATIONAL DETAILS
All calculations were carried out using the Gaussian03
computational package [12]. The geometric parame-
ters for all the reactant, TS, and products of the studied
reactions were fully optimized using density functional
theory [13] with M05-2X [14] functional with the 6-
31G(d,p) basis set [15]. A scaling factor [16] of 0.9631
for the zero-point vibrational energies (ZPE) was used.
Thermal corrections to enthalpy (TCH) and entropy
values were evaluated at the experimental tempera-
tures of 483–503 K. To calculate enthalpy and entropy
values at a temperature T , the difference between the
values at that temperature and 0 K was evaluated ac-
cording to standard thermodynamics [17]. M05-2X is
a high nonlocally functional with double the amount
of nonlocal exchange (2X) that is parameterized only
for nonmetals, and it is based on simultaneously opti-
mized exchange and correlation functionals, both in-
cluding kinetic energy density. It has shown very good
performance for applications involving thermochem-
istry, kinetics, and noncovalent interactions [18], and
we have used it with good results in a previous theo-
retical study [19].
Energies and vibrational frequencies in solution
were computed using the integral equation formalism
variant of the polarizable continuum model. [20] To
simulate m-xylene as the solvent interacting with the
substrate, a dielectric constant of 2.3478 was used [21].
Full geometry optimizations were followed by
analytical calculation of frequencies to determine the
nature of the stationary point, i.e. each structure was
characterized as a minimum or a saddle point of first
order by these calculations.
RESULTS AND DISCUSSION
Experimental
The elimination products of 4-hydroxy-2-butanone
(Fig. 2a) in m-xylene solution were mainly (∼95%)
acetone and formaldehyde. The side reaction (∼5%)
(Fig. 2b) was not studied in this work. For this reason,
and to follow only the main reaction, the amount of
acetone as a function of time was obtained by chro-
matographic analysis.
Good first-order kinetics was observed for the ther-
molysis of 4-hydroxy-2-butanone, the reaction being
followed both by the rate of appearance of the ketone
as by the rate of disappearance of the β-hydroxyketone.
The plot of log10C/C0 was found to be linear to at
least three half-lives. No rate constant variation was ob-
tained by using different concentrations of 4-hydroxy-
2-butanone in m-xylene (in the range of 1%–10% of
ester by volume); the reaction was shown to be ho-
mogeneous by increasing the surface area of the reac-
tion vessel. Thus, in the case of 4-hydroxy-2-butanone,
a rate constant of 0.54 × 10−4 s−1 was obtained at
483.15 K in capillary tubes packed with glass wool,
with surface area increased by a factor of 4, compared
with 0.51 × 10−4
s
−1 in unpacked tubes.
The Arrhenius plot showed excellent linearity, and
the equation is
log k (s−1) = (13.3 0.01)
− (162.3 0.1) kJ · mol−1 (2.303RT )−1
International Journal of Chemical Kinetics DOI 10.1002/kin.20591

410
MURILLO ET AL.
The rate constants obtained at 483, 493, and 503 K
are (0.54 0.02) × 10−4, (1.23 0.01) × 10−4, and
(2.70 0.01) × 10−4
s
−1, respectively.
The differences between the Arrhenius equations
obtained in the gas phase and in solution can be ob-
served as increments of both the activation entropy and
the activation energy when the system is in the solu-
tion; the former is due to the fact that the solvated TS is
more rigid than the gas-phase TS because of the solvent
interaction, and the latter because the reactant is sta-
bilized or the TS is destabilized due to the interaction
with the solvent molecules.
Computational
Electronic energies, Eel, zero-point vibrational ener-
gies, TCHs, and entropies (S), at three temperatures
(483, 493, and 503 K), for the reactant, TS, and prod-
ucts involved in the reaction were studied (see Fig. 3)
and were calculated at the M05-2X/6-31G(d,p) level
of theory; the results are presented in Table S1 in
the Supporting Information. The Cartesian coordinates
for all the structures, optimized at the M05-2X/6-
31G(d,p) level, are collected in Table S2 in the Sup-
porting Information.
Calculated values of the activation parameters and
experimental and calculated rate constants for the ther-
mal decomposition of 4-hydroxy-2-butanone are col-
lected in Table I.
As can be observed from Table I, the calculated
rate constants using M05-2X functional are of the
same order of magnitude than the experimental ones.
With increasing the temperature by 10 K, the rate
constant doubles; this fact is in agreement with the
Arrhenius equation linearity. The current study con-
firms the good performance of the M05-2X func-
tional to determinate rate constants. The activation en-
tropy is compatible with a semipolar six-membered
cyclic TS postulated for other β-hydroxy compounds;
it is negative in all cases, indicating that the TS has
less degrees of freedom. Calculated Gibbs energies
of activation agree very well with the experimental
values.
By comparing the experimental results with the cal-
culated ones, it can be seen that an increment in the
calculated rate constant is related to lower activation
enthalpies. The differences in these results could be due
to the theoretical method applied; probably the contin-
uum model used is not as adequate as other methods
like the Multiple Minimum Hypersurface methodol-
ogy used by some authors to study chemical organic
reactions [28,29]. Furthermore, it should be noted that
the product obtained from TS is the ketone, both in
enol and keto forms. It is known that the keto form
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THERMAL DECOMPOSITION OF 4-HYDROXY-2-BUTANONE IN m-XYLENE SOLUTION
411
H
H
H
5
4
O
C
6
6
O1
O
5 O
O 1
C 2
6
5 O
1
+
2
C 2
4 CH2
3
4 CH2
3
C
3
H
H
H3C
CH2
H3C
CH2
H3C
CH2
TS
R
P2
P1
Figure 3 Proposed mechanism of decomposition of 4-hydroxy-2-butanone.
is more stable than the enol form, and therefore it is
the experimentally observed. Andre´s et al. [30] have
suggested that the interconversion of the two forms in-
volves the movement of a proton and the shifting of
bonding electrons through a four-membered ring tran-
sition structure. The 1,3-intramolecular hydrogen mi-
gration is much more advanced than the hybridization
changes on donor and acceptor centers at the transi-
tion structure. The corresponding barrier heights can
be related to the change in bond orders and acid/base
properties of these centers. A comparison of the re-
sults obtained with different methods made by the au-
thors renders that the nature of the transition struc-
ture seems to be a rather robust entity. This may be
the reason why the rate constants do not fit quite
well.
bond indices [21], Bi. The bond index between two
atoms is a measure or the bond order, and hence of
the bond strength between these two atoms; thus if
the evolution of the bond index corresponding to the
bonds being made or broken in a chemical reaction is
analyzed along the reaction path, a very precise im-
age of the timing and extent of the bond-breaking and
bond-forming processes at every point can be achieved
[35].
The Wiberg bond indices corresponding to the
bonds involved in the reaction center of the mecha-
nism proposed are collected in Table II.
As it can be seen in Table II, the transformation of
the O1 C2 double bond into a single one (64.7%) is
the most advanced process, followed by the breaking
of bonds O5 H6 (61.6%) and C3 C4 (54.6%), and the
formation of the H6 O1 single bond (51.3%). The least
advanced processes are the formation of double bonds
Because NBO is a powerful tool for understand-
ing and interpreting results from mechanistic studies,
the progress of the reaction has been followed, like
in other theoretical studies on reaction mechanisms
carried out by us [31–34] by means of the Wiberg
C2 C3 (44.2%) and C4 O5 (38.5%). The δBav
0.52 value indicates that the TS has a character interme-
diate between reactants and products. Synchronicities
=
200
TS
150
100
167,4
P
50
0
56,6
R
Reaction coordinate
Figure 4 Gibbs energy profile at 483.15 K, evaluated at the M05-2X/6-31G(d,p) level, for the thermal decomposition of
4-hydroxy-2-butanone (first step).
International Journal of Chemical Kinetics DOI 10.1002/kin.20591

412
MURILLO ET AL.
Table II Wiberg bond indices, Bi , of reactant (R), transition state (TS), and products (P)
O1 C2
C2 C3
C3 C4
C4 O5
O5 H6
H6 O1
BiR
1.744
1.249
0.979
64.73
0.996
1.380
1.866
44.19
0.980
0.445
0.000
54.62
0.904
1.245
1.789
38.48
0.710
0.273
0.000
61.55
0.015
0.376
0.720
51.28
BiTS
BiP
%EVa
δBbav
0.52
Syc
0.91
Values are calculated at the M05-2X/6-31G(d,p) level. See Fig. 3 for atom labeling.
aPercentage of evolution through the chemical process of the bond rates at the TS, %EV = [(BTS − BR)/(BP− BR)] × 100.
bDegree of advancement of the TS, δ Bav = ((1/n) × %EV)/100, where n is the number of bonds involved in the reaction.
ꢀ
cAbsolute synchronicity for the thermolysis of 4-hydroxy-2-butanone in the m-xylene solution (first step), Sy = 1 − [1/(2n− 2) × (δBav−
%EV/100)/δBav], where n is the number of bonds involved in the reaction.
vary between 0 and 1, which is the case when all of
the bonds implicated in the reaction center have bro-
ken or formed at exactly the same extent in the TS.
The Sy value obtained in this way is, in principle, inde-
pendent of the degree of advancement of the TS. The
synchronicity value is 0.91, indicating that the mecha-
nism corresponds to slightly asynchronous.
The Gibbs energy profile for the thermal decompo-
sition process of 4-hydroxy-2-butanone at 483.15 K is
presented in Fig. 4.
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reaction studied; and Table S2 with the M05-2X/6-
31G(d,p)-optimized Cartesian coordinates for all the
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International Journal of Chemical Kinetics DOI 10.1002/kin.20591