3
190
J . Org. Chem. 1997, 62, 3190-3193
Effects of [Na OH] a n d [KBr ] on In tr a m olecu la r Gen er a l
Ba se-Ca ta lyzed Meth a n olysis of Ion ized P h en yl Sa licyla te in th e
P r esen ce of Ca tion ic Micelles
M. Niyaz Khan
Department of Chemistry, Universiti Malaya, 50603 Kuala Lumpur, Malaysia
Received October 14, 1996X
-
Pseudo-first-order rate constants obtained for methanolysis of ionized phenyl salicylate (PS ) at
constant [MeOH], [MeCN], [NaOH] or [KOH], and [KBr] and at 35 °C show a decrease with the
increase in [CTABr] (where CTABr represents cetyltrimethylammonium bromide) from 0.0-0.01
M. These observed data obey a pseudophase model of the micelle. The micellar binding constants
-
-
(K
S
) of PS , pseudo-first-order rate constants (k
M
) for methanolysis of PS in the micellar
pseudophase and cmc are almost unchanged with the change in [NaOH] from 0.005-0.050 M. The
-
1
S
increase in [KBr] from 0.0 to 0.3 M at 0.01 M KOH decreases K from 5140 to 653 M and cmc
-
4
-4
from 1.9 × 10 to 0.2 × 10 M. Pseudo-first-order rate constants, k
M
, are almost independent of
[
KBr] at 0.01 M KOH.
0
In tr od u ction
K ) KS - L[KBr]
(1)
S
Almost all kinetic data on the effects of micelles on
rates of chemical reactions have been interpreted in
The present study aimed to test eq 1 kinetically by
determining KS as a function of [KBr]. A solvolytic
reaction (methanolysis of ionized phenyl salicylate in
1
terms of a pseudophase model of the micelle. Among
2
the various assumptions involved in this model, perhaps,
the most crucial one is that the binding constants of two
or more than two chemical species with a micelle are
independent of each other. This model also assumes that
the presence of one chemical species does not affect the
presence of other chemical species in the micellar
pseudophase. This assumption, however, fails to explain
the observed data on the effects of cationic micelles on
mixed H O-MeOH-MeCN solvents) was chosen to avoid
2
the use of PIE model of micelles. The effects of CTABr
micelles on pseudo-first-order rate constants for metha-
nolysis of ionized phenyl salicylate have been determined
at different [NaOH] and [KBr]. These results and their
explanation(s) are described in this paper.
-
rates of reactions between OH (or any other anion) with
neutral substrates.3 In order to explain these observa-
Exp er im en ta l Section
tions, a pseudophase ion exchange (PIE) model has been
Ma ter ia ls. Phenyl salicylate and cetyltrimethylammonium
bromide (CTABr) were commercial products of reagent grade
from Fluka and Aldrich. All other chemicals used were also
of reagent grade and were from Fluka, BDH, or Aldrich. The
stock solutions of phenyl salicylate were prepared in acetoni-
trile.
proposed.3 Although this model works well for many
,4
5
reactions, its weaknesses have started appearing in the
6
literature. Probably, for the first time, Vera and Ro-
denas7 have observed the expulsion of anionic esters
(
(
substrates) from cetyltrimethylammonium bromide
CTABr) micelles by the added bromide ions in the kinetic
Kin etic Mea su r em en ts. In a typical kinetic run, the
reaction mixture (4.9 mL) containing all reaction ingredients
except phenyl salicylate was temperature equilibrated at 35
°C for about 10 min. The reaction was then initiated by adding
study on the effects of [KBr] on rates of reactions between
-
OH and anionic esters. In these reactions, the exchange
-
of both the reactants (anionic ester and OH) by bromide
0
.1 mL of 0.01 M phenyl salicylate (in MeCN) to the temper-
ions has occurred. But the PIE model takes care of only
ature-equilibrated reaction mixture (4.9 mL). Nearly 2.5 mL
of the reaction mixture was quickly transferred to 3 mL quartz
cuvette which was kept in the thermostated cell holder of the
spectrophotometer. The progress of the reaction was moni-
tored by recording the increase in absorbance (Aobs) as a func-
tion of time (t) at 290 nm using a double beam Shimadzu Model
UV-2102/310/PC UV-VIS-NIR spectrophotometer equipped
with thermostated cell holder. The constant temperature (35
°C) of the cell holder was maintained by using thermostated
circulating water bath.
-
-
7
the exchange between OH and Br . Vera and Rodenas
explained their observations in terms of PIE model
coupled with the assumption that the binding constants
(K
S
) of anionic esters with CTABr micelles varied with
[
1
KBr] according to the following empirical equation (eq
) where K and L are adjustable empirical parameters.
S
0
X
Abstract published in Advance ACS Abstracts, April 15, 1997.
1) Menger, F. M.; Portnoy, C. A. J . Am. Chem. Soc. 1967, 89, 4968.
2) Bunton, C. A. Catal. Rev. Sci. Eng. 1979, 20, 1.
(
(
(
All the kinetic runs were carried out until 3-6 halflives.
The observed data (Aobs versus t) were found to fit well to eq
2.
3) Romsted, L. S. In Surfactants in Solution; Mittal, K. L., Lindman,
B., Ed.; Plenum Press: New York, 1982; Vol. 2, p 1015.
4) (a) Romsted, L. S. In Micellization, Solubilization and Micro-
(
emulsions; Mittal, K. L., Ed.; Plenum Press: New York, 1977; Vol. 2,
p 509. (b) Bunton, C. A.; Gan, L.-H.; Hamed, F. H.; Moffatt, J . R. J .
Phys. Chem. 1983, 87, 336.
Aobs ) δapp[X0][1 - exp(-kobst)] + A0
(2)
(5) (a) Bunton, C. A.; Nome, F.; Quina, F. H.; Romsted, L. S. Acc.
Chem. Res. 1991, 24, 357. (b) Bunton, C. A.; Savelli, G. Adv. Phys.
Org. Chem. 1986, 22, 213.
The pseudo-first-order rate constant (kobs), apparent molar
extinction coefficient (δapp), and absorbance at t ) 0 (A ) were
calculated from eq 2 using the nonlinear least squares
technique. The standard deviations associated with kobs, δapp
and A were found to be <2% in all kinetic runs.
(6) (a) Bunton, C. A. In Surfactants in Solution; Mittal, K. L., Shah,
0
D. O., Eds.; Plenum Press: New York, 1991; Vol. 11, p 17. (b) Germani,
R.; Savelli, G.; Romeo, T.; Spreti, N.; Cerichelli, G.; Bunton, C. A.
Langmuir 1993, 9, 55.
,
(7) Vera, S.; Rodenas, E. J . Phys. Chem. 1986, 90, 3414.
0
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