+
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Hydrolysis of Aspirin Studied by Spectrophotometric and Fluorometric
Variable-Temperature Kinetics
X
†
‡
G
IUSEPPE
A
LIBRANDI* , NORBERTO
M
ICALI†, SEBASTIANO
T
RUSSO
,
AND
A
NTONINO
V
ILLARI
Received December 11, 1995, from the *Dipartimento di Chimica Inorganica, Analitica e Struttura Molecolare, Universita` di Messina and
Istituto di Chimica e Tecnologia dei Prodotti Naturali (ICTPN-CNR), Sezione di Messina, Salita Sperone 31, Villaggio S. Agata, 98166
Messina, Italy, †Istituto di Tecniche Spettroscopiche, CNR, Salita Sperone 31, Villaggio S. Agata, 98166 Messina, Italy, and ‡Dipartimento
Farmaco-Chimico, Facolta` di Farmacia, Universita` di Messina, Villaggio Annunziata, Messina, Italy.
Final revised manuscript
received March 11, 1996.
Accepted for publication June 7, 1996X.
d[A]
dt
Abstract
acetylsalicylic acid as a function of temperature have been obtained by
variable-temperature kinetic experiments. method, based on
0 Pseudo-first-order rate constants for the hydrolysis of
-
) {kobs[Pari(t)]}Par*Par [A]
(3)
i
A
a
of two varying terms [A] ) [A](t) and kobs ) kobs[Pari(t)], where
Pari(t) is the parameter i varying with the time. The integral
form, shown in eq 4, describes the concentration of the
reacting species A during the process. The differential form
generalization of non-isothermal analysis, has been used that takes
advantage of the capabilities of modern data collection and processing
systems. Both spectrophotometric and, for the first time under non-
isothermal conditions, fluorometric measurements have been carried out.
The results obtained are identical to those obtained under the same
conditions but using traditional constant-temperature kinetic runs. This
provides the possibility of reducing the amounts of time and chemicals
usually spent in collecting kinetic data in mechanistic studies in solution
by an order of magnitude.
[A] ) [A] exp{- tkobs[Pari(t)] dt}
(4)
∫
0
0
(eq 3) can be used to obtain the kobs(Pari) profile, simply from
the ratio -(d[A]/dt)/[A]. When the mathematical form of the
function kobs[Pari(t)] is known, both differential and integral
forms can be used for a direct best fitting of the experimental
data to obtain the optimized values of the terms regulating
the dependence of kobs on that parameter.
The reaction studied here is the hydrolysis of acetylsalicylic
acid (aspirin) to salicylic acid, where the importance of this
method can be fully appreciated because the formulation of a
mechanistic scheme for this, as for many other similar
(prostacyclin,6 penicillins,7 securinine,8 etc.) systems, requires
tens of kinetic runs to delineate the dependence of kobs on pH,
on temperature, on buffer concentration, and on ionic strength.2
The reaction has been extensively studied in the past9 so
that the mechanism is well established and a lot of compara-
tive data are available in the literature. Furthermore, this
system continues to be used as a model.10
Introduction
The mechanistic investigation of a chemical reaction is a
very important step in the effort to understand the submi-
croscopic world and is often very useful for practical applica-
tions.1 Unfortunately a great deal of kinetic work is usually
necessary to clarify in detail the behavior of the systems
studied,2 in this way discouraging an extensive involvement
and limiting the results obtained in such a key sector of
scientific production.
The aim of this paper is to illustrate a new method recently
proposed,3,4 based on a generalization of non-isothermal
analysis,5 that makes it possible to obtain kinetic data much
more easily than before. It is applied to a well-known reaction
of pharmaceutical interest in a way that allows the saving of
time and chemicals.
Here the dependence of the pseudo-first-order rate constant
on temperature at different pH values has been obtained by
single variable-temperature kinetic (VTK) runs using about
one-tenth of the time usually necessary and saving substrate
and chemicals.
The method consists in carrying out kinetic experiments
while varying, in a known way, the value of a parameter (T,
P, [H+], etc.) and in obtaining, with a single kinetic run,
instead of a single rate constant, the entire dependence of the
rate constant on that parameter. Consider a generic reaction,
Variable-temperature kinetic experiments are not new and
are all derived from “non-isothermal reaction analysis”,
extensively used in thermochemistry.5 (Here the definition
“variable-temperature” instead of “non-isothermal” is pre-
ferred because of the more general context of variable-
parameter kinetics.) Some interesting examples of applica-
tions in the pharmaceutical field were reported in this
J ournal.11 Nevertheless, only the availability of fast comput-
ers for acquisition and processing of experimental data, as
well as of computer-aided devices for accurately varying the
parameter inside the reaction vessel, makes possible an easy
application of this method and can explain why so far almost
all the data given in the literature have been obtained by
traditional isothermal kinetics.
The experiments have been carried out spectrophotometri-
cally and fluorometrically. Spectrophotometry represents by
far the most powerful and utilized method of monitoring the
progress of a chemical reaction,1 for several reasons, particu-
larly, good sensitivity, simple apparatus, and good control of
the temperature. This, together with the easy connection to
a computer for real time collection of the experimental data,
makes it the best choice for VTK experiments. Care must be
A f B
(1)
The general rate law, for first-order or pseudo-first-order
conditions, is given by eq 2,
d[A]
dt
-
) k(T,P,[Yi],µ,...)[A]
(2)
where k is the rate constant, which depends on many
parameters such as temperature, pressure, concentration of
other reagents, and ionic strength. Usually, all the values of
these parameters are maintained constant during the kinetic
run. If a parameter changes during the course of the reaction,
eq 2 maintains its validity and can be written in the form of
eq 3, to emphasize that the rate depends now on the product
X Abstract published in Advance ACS Abstracts, August 1, 1996.
© 1996, American Chemical Society and
American Pharmaceutical Association
S0022-3549(95)00506-5 CCC: $12.00
Journal of Pharmaceutical Sciences / 1105
Vol. 85, No. 10, October 1996