Pyrolysis of β-hydroxyketones and α-ketoesters: Gas-phase elimination kinetics of 3-hydroxy-3-methyl-2butanone and methyl benzoylformate

Article abstract of DOI:10.1002/(SICI)1097-4601(1997)29:4<295::AID-KIN8>3.0.CO;2-P

The rates of gas-phase elimination reactions of methyl benzoylformate (1) and 3-hydroxy-3-methyl-2-butanone (2) were obtained at T = 600 K. The two substrates undergo unimolecular first-order elimination for which the Arrhenius equations are, respectively, log k = 13.2 - 53270/(4.574 × 600) for (1) and log k= 12.4 - 53060/(4.574 × 600) for (2). The products of pyrolysis of (I) are benzaldehyde, formaldehyde and CO, while those of (2) are acetaldehyde and acetone. The kinetics of the elimination reactions show benzoylformic acid to be 10₆-fold more reactive than (1), and pyruvic acid ca. 10₅-fold more reactive relative to (2); an indication of the rate-controlling part played by the acidity of the hydrogen atom involved in the elimination process of the present compounds in this particular type of reaction.

Full text of DOI:10.1002/(SICI)1097-4601(1997)29:4<295::AID-KIN8>3.0.CO;2-P

Pyrolysis of
␤-Hydroxyketones and
␣-Ketoesters: Gas-Phase
Elimination Kinetics of
3-Hydroxy-3-Methyl-2-
Butanone and Methyl
Benzoylformate
NOURIA A. AL-AWADI, OSMAN M. E. EL-DUSOUQUI
Department of Chemistry, University of Kuwait, P. O. Box 5969, Safat 13060, Kuwait
Received 26 March 1996; accepted 29 August 1996
ABSTRACT: The rates of gas-phase elimination reactions of methyl benzoylformate (1) and 3-
hydroxy-3-methyl-2-butanone (2) were obtained at T ϭ 600 K. The two substrates undergo
unimolecular first-order elimination for which the Arrhenius equations are, respectively, log
k ϭ 13.2 Ϫ 53270/(4.574 ϫ 600) for (1) and log k ϭ 12.4 Ϫ 53060/(4.574 ϫ 600) for (2). The
products of pyrolysis of (1) are benzaldehyde, formaldehyde and CO, while those of (2) are
acetaldehyde and acetone. The kinetics of the elimination reactions show benzoylformic acid
to be 106-fold more reactive than (1), and pyruvic acid ca. 105-fold more reactive relative to
(2); an indication of the rate-controlling part played by the acidity of the hydrogen atom in-
volved in the elimination process of the present compounds in this particular type of reac-
tion. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 295–298, 1997.
INTRODUCTION
cyclic transition state with partial negative charge be-
ing developed at the ␣-carbonyl carbon, eq. (1).
Earlier work on gas-phase elimination kinetics of
pyruvic acid showed that the acid decomposes into
carbon dioxide and acetaldehyde [1]. The mechanism
proposed for this reaction involves a four-membered
O
C
H
O
C
heat
␣
Me
C
O
O
Me
H ϩ CO2 (1)
Correspondence to: N. A. Al-Awadi
Contract grant Sponsor: Kuwait University
Contract grant number: SC 067
The thermal elimination of benzoylformic acid has
also been described and was found to be ca. 18 times
faster than for pyruvic acid [2]. This was considered
© 1997 John Wiley & Sons, Inc.
CCC 0538-8066/97/040295-04

296
AL-AWADI AND EL-DUSOUQUI
to be consistent with the 4-center pathway previously
reported for pyruvic acid. The present work seeks to
examine the influence of the hydrogen atom involved
in the transition state of the elimination process, and
to assess its contribution to the rate-controlling step
of the elimination reaction vis-a-vis the contribution
from the polarization of the bond between the two ad-
jacent carbonyl groups:
proposed for the elimination pathway of the ␣-keto
acids in which a partial negative charge is located
on the ␣-carbonyl carbon. (ii) The ␣-carbonyl car-
bon of the ␣-keto acids (eq. (1)) and the equivalent
carbon of the ␤-keto alcohol (1), eq. (3), are both
expected to exhibit partial negative charge in their
transition states. If the bond polarization leading to
this charge development is considered the major rate-
controlling factor, the acid and the alcohol would
differ in reactivity but only moderately. This, how-
ever, is found not to be the case as evidenced by the
present rate profiles and the consequent relative reac-
tivities.
O
C
O
C
␣
␦
Ϫ
␦
ϩ
Hence, we have sought to reduce the acidity of the
hydrogen in question: in benzoyl formic acid through
replacing the hydroxyl group of this ␣-keto acid by
methoxy group to obtain the ␣-keto ester, methyl
benzoylformate, and by replacing the carbonyl moi-
ety of the carboxyl function in pyruvic acid, also an
␣-keto acid, with CMe2 group to obtain 3-hydroxy-3-
methyl-2-butanone (a ␤-hydroxyketone).
O
␦
ϩ
heat
Me ␦ϪC CMe2
H
O
O
C
Me
H ϩ O CMe2 (3)
The kinetic data (Table I and Scheme I), the reac-
tion products, and the suggested elimination path-
ways allow the following comparisons and attendant
conclusions to be drawn: (a) Benzoylformic acid
eliminates only ca. 18 times faster than pyruvic acid,
a result which can be rationalized in terms of the elec-
tronic effect of the phenyl group neutralizing the par-
tial negative charge credited to the ␣-carbonyl carbon
[2]. It is to be noted that we have reassigned this rate
factor a value of ca.46, a result one obtains from com-
RESULTS AND DISCUSSION
Each of the substrates gave excellent and repro-
ducible first-order kinetics, with linearity up to
Ͼ 95% (Figs 1 and 2). The homogeneity of the reac-
tion was tested by comparing the kinetic rate using an
empty reacting tube with that of similar vessel packed
with glass helices. The results show that the change
in the kinetic rate was within experimental error.
Thus an increase in the surface area of more than
50% caused a negligible increase in the reaction rate.
1
The data summarized in Table I shows log A(sϪ ) val-
ues to be well within the range expected to apply to
this type of process. The kinetic data together with
the analysis of the products of reaction reveal the fol-
lowing: (i) The gas-phase elimination reaction of
methyl benzoylformate (1) involves a 5-membered
cyclic transition state, eq. (2).
O
␣
heat
Ph
C
O
Ph
C
H
O ϩ
C
O
H
C
O
O
H2C O ϩ C
O (2)
This is consistent with the present results and
analogous to the 4-center transition state previously
Figure
butanone.
1
Arrhenius plot for 3-hydroxy-3-methyl-2-

␤-HYDROXYKETONES AND ␣-KETOESTERS
297
O
C
H
O
C
H
Ph
C
O
O
Me
C
O
O
krel.
(3)
(4)
6.31 ϫ 10Ϫ1
1.38 ϫ 10Ϫ2
4.57 ϫ 10
ca. 46a
O
O
Me
C
H
C
O
O
Me
C
H
CMe2
O
(4)
(2)
1.38 ϫ 10Ϫ2
1.26 ϫ 10Ϫ7
1.1 ϫ 105
Figure 2 Arrhenius plot for methyl benzoylformate.
O
O
O
Ph
C
H
C
O
O
Ph
C
C
O
H
puting the data reported in [2]; (b) However, pyruvic
acid (␣-keto acid) eliminates some 105 times faster
than 3-hydroxy-3-methyl-2-butanone (␤-hydroxy ke-
tone). This very large rate factor follows a change in
the magnitude of acidity of the incipient hydrogen
from that of a hydroxyl group influenced by the acid
CRO moiety to that of a hydroxyl function affected
by the CMe2 group of the alcohol; and (c) Besides,
benzoylformic acid (␣-keto acid) eliminates 106 times
faster than methyl benzoylformate (␣-keto ester).
This million-fold drop in reactivity reflects the de-
crease in acidity of the incipient hydrogen on replac-
CH2
(3)
6.31 ϫ 10Ϫ1
(1)
6.31 ϫ 10Ϫ1
1 ϫ 106
1
Scheme I Rate Coefficients (k/sϪ ) at 600 K and Rela-
tive Rates (krel.) of Methyl Benzoylformate (1), 3-hy-
droxy-3-methyl-2-butanone (2), benzoylformicacid
(3), and pyruvic acid (4)
a From recalculation of data in ref. [2].
Table I Coefficients and Arrhenius Parameters for the Thermolysis of Methyl
Benzoylformate (1) and 3-Hydroxy-3-Methyl-2-Butanone (2)
104k(sϪ
)
log A(sϪ
)
Ea(kJ molϪ
)
k600K(sϪ
)
1
1
1
1
Compound
T(K)
7
(1)
683.0
693.1
703.1
713.2
718.1
723.0
733.1
723.0
738.1
748.2
758.2
768.2
778.1
1.50
2.64
4.77
13.2 Ϯ 0.2
222.9 Ϯ 2.7
6.3 ϫ 10Ϫ
7.14
10.13
13.30
22.06
2.00
4.21
6.97
11.55
16.59
27.62
7
(2)
12.4 Ϯ 0.2
222.0 Ϯ 3.1
1.3 ϫ 10Ϫ

298
AL-AWADI AND EL-DUSOUQUI
ing the hydrogen of the hydroxyl (O—H) moiety of
the acid function by the methoxyl (OCH2 —H) hy-
drogen of the ester group.
On the basis of these observations, it seems evi-
dent that the acidity of the hydrogen atom involved in
the elimination pathway provides what could propor-
tionally be considered the rate-controlling factor be-
hind the elimination process.
9 ml. Coating of the tubes was not possible as a new
wall surface would have been exposed when the tubes
were sealed. The content of the tube was then ana-
lyzed using the HPLC set-up [3].
This process was repeated for every 5–10°C rise
in the temperature of the pyrolyser and for the same
time range until about 80–90% pyrolysis occurred.
The rate coefficients were obtained from the first-
order expression, kt ϭ ln a0 /a [4]. The Arrhenius pa-
rameters were obtained from a plot of log k vs.
1/T(K).
EXPERIMENTAL
Materials
Product Analysis
Methyl benzoylformate and 3-hydroxy-3-methyl-2-
butanone were samples from Aldrich. GLC analysis
of the substrates on a 9 ft. column packed with 50%
OV101 adsorbed on Chromosorb G (100–120 mesh)
and operated at 175°C indicated that the samples
were Ն 99% pure.
(i) Using on-line pyroprobe GC-MS: Pyroprobe CDS
Analytical Model 2000 which is a multiple step plat-
inum filament pyrolysis instrument interfaced to the
GC-MS system by means of a heated chamber which
houses the filament rod during pyrolysis. A minute
amount of the compound to be pyrolysed was placed
in a quartz tube inside the coil probe. The probe is
placed inside the interface and sealed into the inter-
face using a septum with 1/4ЈЈ aperture. The tempera-
ture programming of the interface and probe were so
adjusted to make an efficient pyrolysis of the com-
pound. The pyrolysates are swept into the GC-MS
system by the carrier gas. The conditions of the GC
and the MS were adjusted to affect a good separation
of the pyrolysates and to allow proper identification
and (ii) Using flow technique: This technique has
been described in an earlier communication [5].
Kinetic Studies
The experimental set-up consists of: (i) HPLC (Bio-
rad model 2700) with UV-VIS detector (Bio-rad
model 1740), HPLC column LC-8, 25 cm, 4.6 mm,
54 ␮m (Suppleco); mobile phase: 40% acetonitrile
solution in water, flow rate of 1 ml/min and (ii) CDS
(Chemical Data System) custom-made pyrolysis unit
in which the reactions are conducted. The pyrolysis
unit consists of an insulated aluminium block, a plat-
inum resistance thermometer and a thermocouple
connected to a Comark microprocessor thermometer.
Kinetic Procedure
The support of Kuwait University through research grant
SC 067 is gratefully acknowledged.
A standard solution of the compound and internal
standard (chlorobenzene) was prepared, such that the
compound peak height was about a third higher than
the internal standard peak. Analysis by liquid chro-
matography allowed several analysis to be performed
on each tube.
A sample (0.2 ml) of very dilute solution (in ppm)
of this standard solution in acetonitrile was pipetted
into the reaction tube which was then sealed under
vacuum and kept inside the pyrolyzer for 600 seconds
at a temperature where 10–20% pyrolysis is deemed
to occur. The volume of the reacation tube was about
BIBLIOGRAPHY
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and M. Abdelkhalik, Int. J. Chem. Kinet., in press.
4. N. A. Al-Awadi, M. H. Elnagdi, and T. Mathew, Int. J.
Chem. Kinet., 27, 517 (1995).
5. N. A. Al-Awadi, R. F. Al-Bashir, and O. M. E. El-
Dusouqui, J. Chem. Soc., Perkin Trans. 2, 579 (1989).