The neutral hydrolysis of methyl acetate-Part 1. Kinetic experiments

Article abstract of DOI:10.1139/V09-003

The neutral hydrolysis of methyl acetate and catalysis of the reaction by the acetic acid product have been in the temperature range 90-110 °C. Extrapolated to 25 °C, the rate constants are 0.17×10^-8 s^-1 for the uncatalyzed and 1.4×10^-4 (mol/L) ^-1s^-1 for the catalyzed reaction. The acid catalysis is specific not general: at 90 °C rate constants for hydrochloric acid catalysis and catalysis by ionized acetic acid are the same as the rate constant, k_H = 1.4 × 10^-2 (mol/L)^-1s ^-1, determined in the neutral reaction.

Full text of DOI:10.1139/V09-003

539
The neutral hydrolysis of methyl acetate — Part 1.
Kinetic experiments
Yih-Huang Hsieh, Noham Weinberg, and Saul Wolfe
Abstract: The neutral hydrolysis of methyl acetate and catalysis of the reaction by the acetic acid product have been
studied in the temperature range 90–110 8C. Extrapolated to 25 8C, the rate constants are 0.17 ꢀ 10–8 s–1 for the uncata-
lyzed reaction and 1.4 ꢀ 10–4 (mol/L)–1s–1 for the catalyzed reaction. The acid catalysis is specific not general: at 90 8C
the rate constants for hydrochloric acid catalysis and catalysis by ionized acetic acid are the same as the rate constant,
kH = 1.4 ꢀ 10–2 (mol/L)–1s–1, determined in the neutral reaction.
Key words: specific acid catalysis, general acid catalysis, cooperative mechanism.
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Resume : Operant a des temperatures allant de 90 a 110 8C, on a etudie l’hydrolyse neutre de l’acetate de methyle et la ca-
talyse de la reaction par l’acide acetique produit. Extrapolees a 25 8C, les constantes de vitesse sont egales a 0,17 x 10–8 s–1
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pour la reaction non catalysee et a 1,4 x 10–4 (mol/L)–1 s–1 pour la reaction catalysee. La catalyse acide n’est pas generale; a
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90 8C, les constantes de vitesse pour la catalyse par l’acide chlorhydrique et pour la catalyse par l’acide acetique ionise
–1
sont les memes que la constante de vitesse, kH = 1,4 x 10–2 (mol/L)–1 s , determinee pour la reaction en milieu neutre.
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Mots-cles : catalyse acide specifique, catalyse acide generale, mecanisme cooperatif.
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[Traduit par la Redaction]
lysis by the product, and kH is the rate constant resulting
from specific acid catalysis by the product. Our interest is
in ko, but kHA, if important, may also involve cooperativity.
The specific objective of the present work is to obtain the
experimental values of ko and kHA or kH for the hydrolysis
of methyl acetate in water solvent, for comparison with the
values of ko and kHA calculated7 assuming cooperative me-
chanisms. Prior to this work, it had been reported that ethyl
Introduction
In recent publications,1–4 we have reported that the neutral,
acetic acid-catalyzed and 2-hydroxypyridine(2-pyridone) cat-
alyzed aqueous hydrations of carbonyl compounds are para-
digms of cooperativity,5,6 by which we mean that C–O bond
formation and proton transfer to oxygen take place in each
case through an uncharged bridge of solvent and (or) catalyst
molecules. This mechanism provides a preferred neutral al-
ternative to multistep ionic processes in which charged inter-
mediates would be undergoing continuous solvation and
desolvation as they appear and disappear.5
The series of studies of which the present article is Part 1
was undertaken to determine if a neutral ester hydrolysis,
whether via a one-step (Scheme 1) or two-step (Scheme 2)
pathway, can also proceed via a cooperative mechanism.
The issue is complicated experimentally because neutral
ester hydrolysis produces a carboxylic acid, which can affect
kinetics by a combination of general and specific acid catal-
ysis, i.e.,
8
acetate exhibits ko = 2.47 ꢀ 10–10 s–1 and ethyl formate
exhibits ko = 5.0 ꢀ 10–6 s–1 at 298 K.9
Experimental
Materials
Methyl acetate was dried over anhyd. copper sulfate, dis-
˚
tilled under nitrogen, and stored over 4 A molecular sieves.
H216O was obtained from an Easypure UV/UF ultrapure
water system. Acetic acid (1.000 mol/L, Aldrich) was stand-
ardized by titration with 0.1 mol/L sodium hydroxide.
Neutral hydrolyses
½1ꢁ
kobs ¼ ko þ kHA½HAꢁ þ kH½Hþꢁ
Experiments were performed in sealed tubes, which had
been washed sequentially with chromic acid and water, and
dried at 110 8C. During reactions, all tubes were completely
immersed in a stirred, temperature-controlled oil bath for the
required time. Temperature control was achieved with a
Model 51 Thermotronic temperature controller. To stop re-
actions, tubes were removed, cooled in ice, and maintained
at –20 8C. Kinetics were monitored by measurement of the
integrals of the methoxy peaks of methyl acetate and metha-
nol. For reactions at 90 0.5 8C, methyl acetate (4.80 mL,
4.473 g, 60.3 mmol) was added to water (60.0 mL) to give a
1.00 mol/L solution, and this solution was transferred to
60 ꢀ 1 cm tubes, which were sealed and frozen until used.
For reactions at 100 0.8 8C and 110 0.8 8C, methyl ace-
tate (3.60 mL, 3.355 g, 45.3 mmol) was added to water
where ko is the rate constant of the neutral reaction of inter-
est, kHA is the rate constant resulting from general acid cata-
Received 27 August 2008. Accepted 23 October 2008. Published
on the NRC Research Press Web site at canjchem.nrc.ca on
12 March 2009.
Y. Hsieh,1 N. Weinberg,2 and S. Wolfe.2,3 Department of
Chemistry, Simon Fraser University, Burnaby, BC V5A 1S6,
Canada.
1Present address: Ningbo Smart Pharmaceutical Co. Inc. 315803
Ningbo, China.
2Present address: Department of Chemistry, University of the
Fraser Valley, Abbotsford BC V2S 7MA, Canada.
3Corresponding author (e-mail: swolfe@sfu.ca).
Can. J. Chem. 87: 539–543 (2009)
doi:10.1139/V09-003
Published by NRC Research Press

540
Can. J. Chem. Vol. 87, 2009
Scheme 1.
Scheme 2.
dCE
dt
(45.0 mL) to give a 1.00 mol/L solution, and this solution
was transferred to two sets of 45 ꢀ 1 cm tubes, which were
sealed and frozen until used.
½2ꢁ
n ¼ ꢂ
¼ k1CE ꢂ kꢂ1CACB
where CE, CA, and CB are the concentrations of ester, acetic
acid, and methanol. Since the process is catalyzed by the
product,
Catalysis by hydrochloric acid at 90 8C
0.93 mmol/L HCl
½3ꢁ
k1 ¼ k10 þ k1H½Hþꢁ
Methyl acetate (2.40 mL, 2.239 g, 30.2 mmol) and hydro-
chloric acid (0.30 mL, 0.100 mol/L) were added to water
(30.0 mL) to give a solution (1.00 mol/L in methyl acetate
and 0.93 mmol/L in hydrochloric acid). This solution was
transferred to 33 ꢀ 1 cm tubes, which were sealed and fro-
zen until needed.
and
½4ꢁ
kꢂ1 ¼ kꢂ0 1 þ kꢂH1½Hþꢁ
where k10 and kꢂ0 1 are the neutral rate constants and k1H and
kꢂH1 are the specific acid-catalyzed rate constants of the for-
ward and reverse reactions. The contribution of kꢂ0 1 can be
neglected because in the initial stages of the reaction there
is no reverse reaction, and as hydrolysis proceeds, the acid-
catalyzed reverse reaction becomes dominant. Since CA and
CB are stoichiometrically related to CE and its initial value
CE0, eq. [2] becomes
4.8 mmol/L HCl
Methyl acetate (1.60 mL, 1.491 g, 20.1 mmol) and hydro-
chloric acid (1.0 mL, 0.100 mol/L) were added to water
(20.0 mL) to give a solution (1.00 mol/L in methyl acetate
and 4.8 mmol/L in hydrochloric acid). This solution was
transferred to 22 ꢀ 1 cm tubes, which were sealed and fro-
zen until needed.
dCE
2
½5ꢁ
v ¼ ꢂ
¼ ðk10 þ k1HÞCE ꢂ kꢂH1ðCE ꢂ CEÞ
6.5 mmol/L HCl
0
dt
Methyl acetate (2.40 mL, 2.239 g, 30.2 mmol) and hydro-
chloric acid (2.1 mL, 0.100 mol/L) were added to water
(30.0 mL) to give a solution (1.00 mol/L in methyl acetate
and 6.5 mmol/L in hydrochloric acid), and this solution was
transferred to 33 ꢀ 1 cm tubes, which were sealed and fro-
zen until needed.
The required specific acid concentration [H+] can be
found from eq. [6],
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
ꢂðHo þ KaÞ þ ðHo þ KaÞ þ 4ðCE ꢂ CEÞKa
0
½6ꢁ
½Hþꢁ ¼
2
where Ho is 10–7 mol/L and Ka is the dissociation constant
9.1 mmol/L HCl
of acetic acid, 1.7 0.2 ꢀ 10–5.10
Methyl acetate (2.40 mL, 2.239 g, 30.2 mmol) and hydro-
chloric acid (3.0 mL, 0.100 mol/L) were added to water
(30.0 mL) to give a solution (1.00 mol/L in methyl acetate
and 9.1 mmol/L in hydrochloric acid). This solution was
transferred to 33 ꢀ 1 cm tubes, which were sealed and fro-
zen until needed.
The Verlet algorithm11 was employed to integrate eq. [5]
with a time step dt for a given set of the parameters k10, kꢂ0 1
,
and kꢂH1
1
½7ꢁ
CEðt þ dtÞ ¼ CEðtÞ þ vðtÞdt þ aðtÞdt2
2
Catalysis by acetic acid at 90 8C
The required acceleration of the reaction, a(t), was found
by differentiation of eq. [5] with respect to time
Methyl acetate (0.168 mL, 0.1562 g, 2.11 mmol) was
added to acetic acid (21.0 mL, 1.00 mol/L) to give a solu-
tion (0.10 mol/L in methyl acetate and 1.00 mol/L in acetic
acid). This solution was transferred to 21 ꢀ 1 cm tubes,
which were sealed and frozen until needed.
dv
½8ꢁ
aðtÞ ¼
¼ f2kꢂH1½ðCE ꢂ CEÞ ꢂ ðk10 þ k1HÞꢁ½HþꢁvðtÞ
0
dt
þ kꢂH1½ðCE ꢂ CEÞ ꢂ ðk10 þ k1HÞCEꢁg
2
0
"
#
ꢂKa
Analysis of kinetic data
Kinetic data for the neutral hydrolyses were treated using
Scheme 3 and eq. [2],
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ꢀ
vðtÞ
2
ðHo þ KaÞ þ 4KaðCE ꢂ CEÞ
0
Published by NRC Research Press

Hsieh et al.
541
Fig. 3. Hydrolysis of methyl acetate at 100 8C.
Scheme 3.
Fig. 1. Hydrolysis of methyl acetate at different temperatures.
Fig. 4. Hydrolysis of methyl acetate at 110 8C.
Fig. 2. Hydrolysis of methyl acetate at 90 8C.
Fig. 5. Arrhenius plot of k10.
Results
Tables 1–3 of the Supplementary data and Fig. 1 provide
the primary data for the hydrolysis of methyl acetate in water
at 908, 1008, and 110 8C.
The rate constants k10, k1H, and kꢂH1 were obtained by the
best root-mean-square fit of the integrated C(t) to these data
(Figs. 2–4).
Figures 5 and 6 are the Arrhenius plots for k10 and k1H, and
Table 4 summarizes the results. At 25 8C, the extrapolated
neutral hydrolysis rate constant k10 is 1.7 1.1 ꢀ 10–9 s–1.
Catalysis by HCl
Catalysis of the hydrolysis by HCl was examined to ob-
tain an independent determination of k1H and kꢂH1. Reactions
Published by NRC Research Press

542
Can. J. Chem. Vol. 87, 2009
Fig. 6. Arrhenius plot of k1H.
Fig. 8. Analysis of hydrochloric acid catalysis of the hydrolysis of
1.0 mol/L methyl acetate at 90 8C.
Table 4. Hydrolysis of methyl acetate at different temperatures.
Temperature (8C)
110 0.8
100 0.8
90 0.5
k10 (s–1)
4
2.4 0.8 ꢀ 10–7
1.4 0.8 ꢀ 10–7
0.17 0.11 ꢀ 10–8a
k1H (s–1)
1 ꢀ 10–7
4.2 0.3 ꢀ 10–2
2.2 0.2 ꢀ 10–2
1.4 0.2 ꢀ 10–2
1.4 0.3 ꢀ 10–4a
Fig. 9. Pseudo-first-order plot of the hydrolysis of 0.10 mol/L
methyl acetate in the presence of 1.0 mol/L acetic acid.
25
aExtrapolated value.
Fig. 7. Hydrochloric acid catalysis of the hydrolysis of 1.0 mol/L
ethyl acetate at 90 8C.
1.00 mol/L acetic acid at 90 8C, and Fig. 9 is a pseudo-first-
order plot of the data, giving kobs = 5.2 0.5 ꢀ 10–5 s–1.
From eq. [6] and the dissociation constant of acetic acid,
the specific hydrogen-ion concentration of 1.0 mol/L acetic
acid is 4.16 ꢀ 10–3 mol/L. If acetic acid functions by spe-
cific acid catalysis, i.e., kobs = kH[H+], kH is calculated to be
1.3 0.1 ꢀ 10–2 s–1, identical to the value found in catalysis
by HCl and in the analysis of neutral hydrolysis. It can be
concluded that the hydrolysis of methyl acetate does not
proceed with general acid catalysis by the product.
were performed at 90 8C using 1.0 mol/L methyl acetate and
1, 5, 7, and 9 mmol/L HCl. Tables 5–8 of the Supplemen-
tary data and Fig. 7 summarize the results.
The data of Fig. 7 were fitted to theoretical curves ob-
tained by numerical integration of the rate equation, using
the Verlet algorithm as before, with the results shown in
Fig. 8. The values of k1H = 1.4 0.2 ꢀ 10–2 s–1(mol/L)–1 and
kꢂH1 = 1.4 0.2 ꢀ 10–3 s–1(mol/L)–2 are the same as in Fig. 2.
Supplementary data
Supplementary data for this article are available on the
journal Web site (canjchem.nrc.ca) or may be purchased
from the Depository of Unpublished Data, Document Deliv-
ery, CISTI, National Research Council Canada, Ottawa, ON
Catalysis by acetic acid
Table 9 of the Supplementary data summarizes the hydrol-
ysis of 0.10 mol/L methyl acetate in the presence of
Published by NRC Research Press

Hsieh et al.
543
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Acknowledgement
This research was supported financially by a Discovery
Grant from the Natural Sciences and Engineering Research
Council of Canada (NSERC).
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