Effect of the reactant ratio on the kinetics of methanolysis of acyl chlorides

Article abstract of DOI:10.1023/B:RUJO.0000010204.53254.11

The apparent second-order rate constants for the reaction of methacryloyl chloride with methanol regularly increase on raising the acyl chloride-to-alcohol ratio. The variation of the rate constants is described by the Michaelis-Menten equation, indicating that the process involves initial formation of an associate with a probable structure of charge-transfer complex (CTC). Thermodynamic parameters of the CTC formation and rate constants and activation parameters for the transformation of CTC into the products have been determined.

Full text of DOI:10.1023/B:RUJO.0000010204.53254.11

Russian Journal of Organic Chemistry, Vol. 39, No. 9, 2003, pp. 1221 1226. Translated from Zhurnal Organicheskoi Khimii, Vol. 39, No. 9, 2003,
pp. 1298 1303.
Original Russian Text Copyright
2003 by Marshalok, Oglashennyi, Makitra, Yatchishin.
Effect of the Reactant Ratio on the Kinetics of Methanolysis
of Acyl Chlorides
G. A. Marshalok, Yu. I. Oglashennyi, R. G. Makitra, and I. I. Yatchishin
L’vivs’ka politekhnika State University, ul. S. Bandery 12, L’viv, 79646 Ukraine
Received February 12, 2003
Abstract The apparent second-order rate constants for the reaction of methacryloyl chloride with methanol
regularly increase on raising the acyl chloride-to-alcohol ratio. The variation of the rate constants is described
by the Michaelis Menten equation, indicating that the process involves initial formation of an associate with
a probable structure of charge-transfer complex (CTC). Thermodynamic parameters of the CTC formation
and rate constants and activation parameters for the transformation of CTC into the products have been
determined.
Kinetic parameters of the reaction of carboxylic
acid chlorides with alcohols are quite sensitive to
both solvent nature [1 3] and alcohol structure [4, 5].
The rates and activation parameters of the examined
processes cannot be determined unambiguously by
some single parameter but are a complex function
of several factors which may be summarized via
multiparameter equations based on the linear free-
energy relation. Here, the main factor affecting the
process is the basicity of the medium, which slows
down the reaction [3]. It should also be noted that the
electron-donor species. This is confirmed by both
extremely low donor numbers of acyl chlorides (e.g.,
1.0 for CH3COCl and 2.3 for C6H5COCl [7]) and
identification of individual CTC derived from acyl
chlorides with such compounds as dioxane [8, 9],
acetone (by refractometry) [10] and DMF (by con-
ductometry [11] and spectrophotometry [12]) but
primarily with pyridine [13, 14] and other tertiary
amines [15, 16]. In the case of amines, the structure
of CTCs was also proved, specifically the presence
of a bond at the carbon atom with possible charge
transfer from the amino nitrogen atom to the oxygen
[15, 17]. In solvents with high dielectric constants,
charge-transfer complexes can dissociate into radical
ions [15].
On the other hand, alcohols exist mainly as cyclic
dimers (especially in weakly polar media); therefore,
they possess an appreciable basicity. Their basicities
B according to Pal’m (i.e., shift of the OH band of
phenol in the IR spectrum in the presence of a given
compound in carbon tetrachloride) range from 208
(benzyl alcohol) to 260 (tert-pentyl alcohol) and are
comparable with the basicities of other electron-donor
compounds, such as diethyl ether (280), dioxane
(237), DMF (291), acetone (224), pyridine (472), and
DMSO (362) [18].
effect of alcohol structure is characterized not only by
*
Hammett Taft constants
and Es but also by its
solvation properties according to Koppel’ Pal’m
since alcohol acts as both reagent and solvent in the
alcoholysis of acyl chlorides. The dependence on
solvation parameters is observed when the reaction is
carried out with excess alcohol, as well as with
an equimolar amount of alcohol in an inert solvent
[1, 5], regardless of the structure assigned to primary
reaction complex: whether it is a charge-transfer
complex (CTC) B [6] or an H-complex A which then
rearranges into CTC:
Unfortunately, the available scales of electron-
acceptor power [19] (Gutmann acceptor numbers AN,
Reichardt parameter ET, and Kamlet Taft parameter
) lack data for acyl chlorides, so that we cannot com-
pare them with alcohols. However, it is known that
the enthalpies of mixing of strong electron-donors,
The formation of CTC between RCOCl and ROH
is quite feasible, for carboxylic acid chlorides are
electron acceptors capable of forming CTCs with
1070-4280/03/3909-1221$25.00 2003 MAIK Nauka/Interperiodica

1222
MARSHALOK et al.
Table 1. Enthalpies of mixing of electron-donor solvents
with PhCOCl, MeOH, and EtOH at 25 C according to
[21] (the amount of the second component is given in
parentheses)
desired 1:1, their comparison with the repsective
values for alcohols provides at least qualitative
estimation of their acceptor power. It is seen that
in all cases the enthalpies are positive; however,
processes of mixing of the above substances with
alcohols are considerably more endothermic than with
PhCOCl. Obviously, the energy of hydrogen bond
formation with alcohols does not compensate the
energy consumed for decomposition of the dimer. On
the other hand, in most cases the enthalpies of mixing
of alcohols with PhCOCl are close to zero; therefore,
in some media solvation of PhCOCl with the solvent
should predominate over formation of CTC PhCOCl
ROH or hydrogen bond ROH B. This explains both
appreciable deceleration of the process in basic
solvents and fairly large positive values of H and
G ; the latter were estimated by us at 80 90 kJ/mol.
The enthalpies of mixing with PhCOCl are also
positive but are small, which may be due to some
dimerization of acyl chloride molecules as shown
below. However, there are no published data on the
existence of such dimers.
H, kcal/mol
Solvent
PhCOCl
MeOH
EtOH
Acetone
Acetonitrile 108 (19.0)
31.2 (19.6)
129 (24.3) 168 (17.1)
160 (77.9) 208 (18.2)
112 (65.4) 136 (65.2)
172 (35.0) 216 (34.0)
Benzene
Dioxane
Anisole
39.6 (64.9)
30.4 (17.5)
6.3 (16.0)
148 (18.0)
[22]
87.9 (11.0)
8.2 (33.0)
e.g., ethylenediamine, with organic substances are
roughly proportional to the electron-acceptor param-
eters (AN or ET) of the latter [20]. Therefore, com-
parison of the enthalpies of mixing of some strong
electron donor with alcohols and with acyl chlorides
should make it possible to estimate, at least roughly,
the relative electron-acceptor power of these sub-
stances and hence to determine the preferential way
of solvation, RCOCl:B or ROH:B. Table 1 gives
the corresponding enthalpies of mixing taken from
[21, 22]. Although the available data on the enthalpies
of mixing with PhCOCl are few in number and were
obtained at a different component ratio than the
We previously found [23] that the melting diagrams
for the systems PhCOCl alcohol considerably deviate
from the Schroeder equation and that even a peritectic
point is observed in the system PhCOCl cyclo-
hexanol. This indicates formation of compounds with
an incongruent melting point in such systems. More-
over, in the presence of alcohols the UV absorption
maximum of PhCOCl ( 241 nm) shifts to the short-
wave region with simultaneous increase in intensity,
which is typical of charge-transfer complexes.
Study of the kinetics of PhCOCl reaction with
methanol in toluene at 45 C showed that the apparent
second-order rate constants change on variation of
the reactant ratio and that their behavior is described
by the Michaelis Menten equation for reactions
involving association equilibria [24, 25]:
Here, kap = Keq ktr, where kap is the experimental
second-order rate constant, Keq is the equilibrium con-
stant for formation of CTC by initial reactants, and ktr
is the rate constant for the transformation of CTC into
the products. Then we have
Fig. 1. Temperature dependences of the apparent rate con-
stants for the methanolysis of methacryloyl chloride in
benzene at different molar ratios RCOCl:MeOH: (1) 1:1,
(2) 2:1, (3) 3:1, (4) 4:1, (5) 5:1, (6) 6:1, (7) 7:1.
1
1
CA + CB
=
+
.
kap
ktr Keq
ktr
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EFFECT OF THE REACTANT RATIO ON THE KINETICS OF METHANOLYSIS
1223
Table 2. Apparent rate constants of methanolysis of methacryloyl chloride in benzene at different reactant ratios and
activation parameters of the process at 303 K
1
kap 103, l mol 1 s
1/kap
Eap,
Hap,
Sap,
Gap,
Ratio
XA: MeOH
1
kJ/mol kJ/mol J mol 1 K
kJ/mol
283 K 293 K 303 K 313 K
283 K 293 K 303 K 313 K
1: 1
2: 1
3: 1
4: 1
5: 1
6: 1
7: 1
0.25 0.42 0.60 0.94
0.27 0.50 0.69 1.14
0.31 0.55 0.85 1.67
0.37 0.60 1.03 1.82
0.41 0.74 1.28 3.33
0.47 1.18 2.36 5.00
0.74 1.61 4.37 15.39
32.3
34.0
40.5
40.9
50.1
57.2
74.2
29.7
31.5
38.0
38.4
47.6
54.7
71.6
208.6
201.7
178.4
175.6
143.3
114.8
53.8
92.9
92.6
92.1
91.6
91.0
89.5
87.9
4050
3700
3270
2730
2450
2120
1360
2381
1990
1810
1664
1353
849
1658
1460
1174
976
783
424
1060
880
600
549
300
200
65
621
229
The data of [23] refer to an aromatic carboxylic
acid chloride; moreover, the reaction was studied at
a single temperature and with a relatively small excess
of benzoyl chloride over methanol (from 1:1 to 3:1).
According to [24, 25], in order to obtain reliable data
on the formation of intermediate compound, the reac-
tion should be carried out with a considerable excess
of reagent B with respect to substrate A. Therefore,
we performed a more detailed study of carboxylic acid
chloride alcoholysis using methacryloyl chloride and
methanol as reactants and benzene as solvent.
Table 2 contains the apparent second-order rate
constants obtained in the temperature range from 283
to 313 K; the acyl chloride-to-alcohol molar ratio
was varied from 1:1 to 7:1. As expected, the rate
constant regularly increases as the above ratio rises.
This tendency becomes stronger on raising the tem-
perature. On the other hand, at each reactant ratio,
second-order kinetic relations are fulfilled with a satis-
factory accuracy up to a conversion of 60 70%.
stant at all reactant ratios and are within the range
91.6 92.9 kJ/mol. Some reduction of Gap to 88 89
kJ/mol is observed only for large escess of RCOCl
(6:1, 7:1). Therefore, we can presume a common
reaction mechanism throughout the examined range of
reactant ratios. However, the facts that the reaction
is a two-step process and that kap increases with rise
in the RCOCl:MeOH ratio indicate formal character
of the activation parameters determined in such a way.
Furthermore, the value of Gap determined at a react-
ant ratio of 1:1 (92.9 kJ/mol) is close to those found
for the reaction of crotonoyl chloride with methanol
in toluene (91 kJ/mol) [26] or of , -dimethylacryloyl
chloride with methanol in benzene (83.2 kJ/mol) [3].
On the other hand, the energy of activation, Eap
=
The temperature dependence of the apparent rate
constants fits the Arrhenius equation (Fig. 1); there-
fore, the energies of activation and the other activation
parameters of the overall process may be calculated
with a high accuracy (r > 0.99; Table 2). These data
indicate regular increase of the apparent energy of
activation and hence of the apparent enthalpy of
activation with increase in excess of RCOCl and con-
comitant increase of kap. In the first case, the correla-
tion coefficient R is 0.95; exclusion of the most
deviating data for the acyl chloride-to-alcohol ratio
equal to 7:1 gives r = 0.97.
In the entire series of experiments, an excellent
linear correlation (r = 0.99) is observed between the
apparent entropy and enthalpy of activation, i.e.,
an isokinetic relation exists (Fig. 2). The Gibbs
energies of activation Gap remain essentially con-
Fig. 2. Correlation between the apparent entropy and
enthalpy of activation in the methanolysis of methacryloyl
chloride in benzene at 303 K; RCOCl MeOH molar ratio
1:1 to 7:1.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 39 No. 9 2003

1224
MARSHALOK et al.
313 K: 1/kap
=
165.9CA + 1180.
From the slopes of these relations we can determine
the rate constants ktr for the transformation of inter-
mediate H-complex or CTC into the products (ester
and HCl), and from the intercepts on the y axis, which
1
are equal to Keq ktr 1, the equilibrium constants for
formation of CTC (Keq) can be found (Table 3). It
should be noted that, unlike the data of [23], kap
values increase rather than decrease with rise in
RCOCl excess.
The equilibrium constant for formation of charge-
1
transfer complex RCOCl MeOH are of a 10 order
of magnitude, and (what is surprising) they increase
as the temperature rises (Fig. 3). The most probable
reason is that the process of decomposition of
methanol dimer into monomeric molecules is energe-
tically more favorable than decomposition of the
RCOCl MeOH charge-transfer complex, and the
equilibrium is displaced to the right. On the other
hand, the dependence of lnKap on 1/T is strictly linear
(r = 0.998, Fig. 4), and the positive values of Geq,
Heq, and Seq found for the complex formation and
Fig. 3. Plots of the apparent rate constants for the
methanolysis of methacryloyl chloride in benzene versus
reactant molar ratio in the temperature range from 283
to 313 K.
the low values of Keq suggest that the formation of
CTC is not favorable from the viewpoint of energy.
In addition, the constant Keq for formation of charge-
transfer complex between methanol and benzoyl
chloride [23] is higher by an order of magnitude,
presumably due to greater electron-acceptor power of
PhCOCl as compared to methacryloyl chloride.
The rate constants for the subsequent transforma-
tion of intermediate complex into the products are
greater, on the average, by an order of magnitude than
the apparent rate constants of the overall process at
the same temperature (Tables 2, 3). This means that
intermediate charge-transfer complex is unstable,
which is consistent with the low energy of activation
for its transformation into the products ( 0.02 kJ/mol)
and with the negative value of the enthalpy of activa-
tion ( 2.5 kJ/mol). The negative Htr value originates
from formation of stable HCl molecule. We shall not
presume whether the transformation of CTC into the
Fig. 4. Temperature dependence of the equilibrium con-
stant for complex formation in the reaction of methacryloyl
chloride with methanol in benzene.
32.3 kJ/mol, is considerably lower than the values
reported in [3] and [26], 51.8 and 64.5 kJ/mol,
respectively.
products is a one- or multistep process. The negative
1
entropy of activation, Str
=
0.3 kJ mol 1 K ,
suggests that the rate-determining stage of the process
is decomposition of CTC. It should also be noted that
the overall Gibbs energy variation at both stages,
formation of CTC ( Geq = 5.3 kJ/mol) and its decom-
position ( Gtr = 88.1 kJ/mol), is close to the apparent
Gibbs energy of activation of the overall process,
Gap = 92.9 kJ/mol.
The variation of the apparent rate constants versus
reactant ratio is excellently described in terms of the
Michaelis Menten equation (Fig. 3). The following
linear relations (r > 0.99) were obtained:
283 K: 1/kap
293 K: 1/kap
303 K: 1/kap
=
=
=
430.4CA + 4533;
286.4CA + 2670;
241.1CA + 1922;
Thus the results of our study are consistent with
the existing views on alcoholysis of acyl chlorides,
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 39 No. 9 2003

EFFECT OF THE REACTANT RATIO ON THE KINETICS OF METHANOLYSIS
1225
Table 3. Equilibrium constants Keq and energy parameters for the formation of charge-transfer complex and rate
constants ktr and activation parameters for the transformation of charge-transfer complex into the products at 303 Ka
Etr,
kJ/mol
H ( H ),
kJ/mol
S ( S ),
G ( G ),
kJ/mol
Parameter
283 K
293 K
303 K
313 K
1
J mol 1 K
Keq, l/mol
lnKeq
0.0949
2.355
2.324
2.634
0.1073
2.232
3.492
2.457
0.1254
2.076
4.148
2.382
0.1399
1.967
6.028
2.220
9.709
2.497
14.7
5.255
88.1
1
ktr, 103 s
0.022
299
logktr
a
1
For PhCOCl and MeOH at 318 K: ktr 103 = 0.073 l mol 1 s , Keq = 1.09 l/mol [23].
which is assumed to proceed in two steps with initial
formation of a charge-transfer complex between the
reactants:
were taken). The accuracy of determination of the
kinetic and activation parameters was estimated by
the least-squares procedure with a confidence prob-
ability
of 0.95.
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