Reactions within association complexes: Acid catalysed hydrolysis of substituted benzaldehyde acetals in the presence of detergents

Article abstract of DOI:10.1039/b203909p

First order rate constants for decomposition of substituted benzaldehyde dimethyl acetals at constant pH in solutions of sodium dodecyl sulfate (SDS) and Aerosol AOT (AOT) maintained at constant [Na⁺] are linear in surfactant concentration. When the only Na⁺ is that derived from the added SDS the kinetics exhibit a saturating rate law in SDS concentration. The present study shows that the curvature is not due to complexation of substrate but to ion exchange whereby increasing [Na⁺] expels H⁺ from the Stern region where reaction of the substrate occurs. Increasing [Na⁺] at constant [SDS] inhibits the SDS-catalysed acid hydrolysis of 4-isopropylbenzaldehyde dimethyl acetal and the dissociation constant of the Na⁺-SDS micelle complex is 0.0281 M which agrees with values calculated from previous data using different reactions. The complex to SDS of a neutral analogue of the acetals exhibits a dissociation constant (K_i) which is not significantly dependent on the overall concentration of Na⁺ ions in the range 0.07 to 0.42 M. Increasing concentrations of cetyltrimethylammonium bromide (CTAB; cetyl = hexadecyl) inhibit the acid catalysed hydrolysis; equilibrium constants (K_i) for the dissociation of the CTAB-complexed acetals fit a Hansch plot and possess no contribution from Hammett's sigma.

Full text of DOI:10.1039/b203909p

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Reactions within association complexes: acid catalysed hydrolysis
of substituted benzaldehyde acetals in the presence of detergents
Phillip D. R. Rose and Andrew Williams*
2
University Chemical Laboratory, Canterbury, Kent, UK CT2 7NH. E-mail: aw@ukc.ac.uk;
Fax: (44) (0)1227-82-7724; Tel: (44) (0)1227-82-3693
Received (in Cambridge, UK) 22nd April 2002, Accepted 4th July 2002
First published as an Advance Article on the web 30th July 2002
First order rate constants for decomposition of substituted benzaldehyde dimethyl acetals at constant pH in
solutions of sodium dodecyl sulfate (SDS) and Aerosol AOT (AOT) maintained at constant [Naϩ] are linear in
surfactant concentration. When the only Naϩ is that derived from the added SDS the kinetics exhibit a saturating rate
law in SDS concentration. The present study shows that the curvature is not due to complexation of substrate but to
ion exchange whereby increasing [Naϩ] expels Hϩ from the Stern region where reaction of the substrate occurs.
Increasing [Naϩ] at constant [SDS] inhibits the SDS-catalysed acid hydrolysis of 4-isopropylbenzaldehyde dimethyl
acetal and the dissociation constant of the Naϩ–SDS micelle complex is 0.0281 M which agrees with values
calculated from previous data using different reactions. The complex to SDS of a neutral analogue of the acetals
exhibits a dissociation constant (Ki) which is not significantly dependent on the overall concentration of Naϩ ions in
the range 0.07 to 0.42 M. Increasing concentrations of cetyltrimethylammonium bromide (CTAB; cetyl = hexadecyl)
inhibit the acid catalysed hydrolysis; equilibrium constants (Ki) for the dissociation of the CTAB-complexed acetals
fit a Hansch plot and possess no contribution from Hammett’s sigma.
and the explicit determination of the catalytic component
(kcat).
Introduction
Reactions which are catalysed by micelles are of interest in the
study of artificial enzyme systems because the micelle has a size
The specific acid catalysed hydrolysis of benzaldehyde
dimethyl acetals at constant [Naϩ] (by adding compensating
commensurate with that of many enzymes and complexation
quantities of NaCl) was measured in varying SDS concen-
with the substrate is necessary for the catalytic function. The
trations to attempt to measure the dissociation constant of
analogy with enzymes is only loose as the micelle has a fluxional
SDS–acetal complex (KS) but the kinetics exhibited only linear
structure and can complex more than one substrate molecule at
plots up to 0.2 M in [SDS]. The linear data indicate that sub-
large substrate concentrations altering the micellar structure
substantially. Even though a catalytic micelle may possess the
groups necessary for catalysis observed in an enzyme system it
has no analogous structural integrity to enable the groups to
stantial complexation of acetal to SDS is not occurring at the
concentrations employed. The Hammett ρ values of the previ-
ous work are called into question because of the use of arbi-
trary concentrations of SDS to determine kcat. The saturation
phenomenon needs reinvestigation under conditions of constant
ionic atmosphere in the Stern layer so that the substituent effects
could be appraised satisfactorily. Knowledge of the substituent
perform at their optimum efficiency. Although micellar and
enzymic catalysis have similar characteristics no micellar sys-
tem has the specificity and catalytic advantage of an enzyme.1
Artificial enzyme systems are composed of relatively large
dependenceofKS andkcat wouldenableustodeterminethesource
complex molecules, usually based on macrocycles, possessing
of the kinetic advantage of the micellar catalysis.
their own characteristic kinetic properties which are different
from those of enzymes.2 Polar groups need to be attached to the
Experimental
artificial enzyme to solubilise it and these are often charged
species. Aggregation phenomena and ion exchange are thus
very likely complications in the development of an artificial
enzyme; experience gained from the study of micellar catalysis
is therefore apposite if an appraisal of the catalytic advantage
of an artificial enzyme is to be made.
Materials
Sodium dodecyl sulfate (SDS) was obtained from BDH, cetyl-
trimethylammonium bromide (CTAB) from Sigma and Aerosol
AOT (AOT) from Lancaster Synthesis.
The catalysis of the hydrolysis of acetals under conditions
of mild acid pH in the presence of sodium dodecyl sulfate
(SDS) has been studied under conditions of varying Naϩ con-
centration;3,4,6 the influence of increasing SDS concentrations
on the hydrolysis of acetals involves an initial increase up to
about 3.6 mM [SDS] followed by a decrease at higher SDS
concentrations. This was attributed to association of the
acetal with the micellar particle and expulsion of Hϩ at higher
Naϩ concentrations. Substituent effect studies assumed the
catalytic rate constant to be that of the optimal rate constant
and that the data represented the hydrolysis of the fully com-
plexed acetal.3–6 We reinvestigated this system because of the
possibility it appeared to offer for experimentally determining
the dissociation constant of the acetal–micelle complex (KS)
The substituted benzaldehyde dimethyl acetals were prepared
by mixing trimethyl orthoformate (10 mL), methanol and sub-
stituted benzaldehyde in equimolar proportions. A single drop
of concentrated HCl was added whereupon the mixture became
warm. The product solution was allowed to stand overnight at
room temperature and neutralised with solid K2CO3. The mix-
ture was then diluted with petroleum ether (bp 40–60Њ), filtered
and evaporated in vacuo. The resultant oil was taken up in more
petroleum ether (bp 40–60Њ), filtered to remove any remaining
solid and evaporated in vacuo. The oil was distilled in vacuo and
then stored over solid K2CO3. Boiling point ranges are recorded
in Table 1 and those for the known acetals agree with the litera-
ture values. 2-Chorophenyl acetate was donated by Dr A. B.
Maude.
DOI: 10.1039/b203909p
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This journal is © The Royal Society of Chemistry 2002
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All solutions for kinetics were prepared with water double
distilled from glass and degassed. Buffer materials and salts
to maintain cation or anion concentrations were either
of A.R. grade or purified before use.
Methods
A series of buffered solutions for studies of rates at different
SDS concentrations (0–0.2 M) were prepared by dilution of a
stock solution containing the SDS (0.2 M) and buffer (0.01 M)
with a diluent solution of NaCl (0.2 M, identical in respect of
pH, ionic strength and buffer identity and concentration) but
without SDS. Dilution of the stock SDS with the diluent
altered the pH slightly due to differential absorption of buffer
species into the micelles. Experiments with CTAB at varying
concentrations were carried out using stock CTAB and diluent
so that the total BrϪ (and buffer anion) concentrations were
invariant.
Buffered solutions of AOT required the addition of
acetonitrile in order to prepare solutions with a reasonably high
AOT concentration. The protocol for mixing was as for the
SDS solutions except that 30% acetonitrile–water (v/v) was
required as solvent and a maximum [AOT] of only 0.1 M was
attainable before the onset of cloudiness.
Reactions were initiated by the addition of an aliquot
(0.01 to 0.05 mL) of a solution of the benzaldehyde acetal
(in acetonitrile) to 2.5 mL of buffered solution, contained in
a 1 cm path-length silica cuvette in the thermostatted cell
compartment of a Pye Unicam SP 8-200 Uv/visible spectro-
photometer. The pH of the solution was measured before and
after each kinetic run using a Radiometer PHM62 pH-meter
with a Russell CMAWL combined electrode, calibrated with
BDH buffers. Wavelengths for studying the kinetics of the
reactions were determined initially by repetitively scanning
the spectrum during a reaction and are recorded in Table 1.
The reactions were followed kinetically by measuring the
absorbance (At) at the optimal wavelength as a function of time.
Kinetics of the reaction between 2-chlorophenyl acetate and
imidazole were determined as previously described.7
Pseudo-first order rate constants were obtained by fitting
the value of At (which increases with time in these cases) to
the equation At = A∞ Ϫ (A∞ Ϫ Ao)exp(Ϫkt). Data were fit to
theoretical equations by use of standard software. In the case of
the more weakly reactive 4-nitrobenzaldehyde dimethyl acetal
the rate constants were obtained from initial rates (ϪdAo/dt) by
division by the final absorption change measured at lower pH
1
where the reaction is fast. H-NMR spectra were determined
using CDCl3 solvent and a 270 MHz spectrometer.
Results
Repetitive scanning of the ultra-violet absorption spectrum of
the reacting solutions and comparison of the final spectra with
those of the corresponding aldehydes show that all the acetals
hydrolysed in aqueous solutions of micelles to yield aldehyde as
product in stoichiometric yield. The scanning spectra exhibit
sharp isosbestic wavelengths consistent with a 1 : 1 reaction
with any intermediate present in very low concentration
compared with that of reactants and products. The reactions,
followed at constant wavelength (Table 1), exhibit excellent
pseudo first order kinetics up to at least 90% of the total
progress of the reaction. The pseudo first order rate constants
were measured as a function of pH at constant [SDS] and con-
stant [Naϩ] and shown to be proportional to the hydrogen ion
concentration (Fig. 1) in the case of the parent acetal. It is
assumed that this proportionality holds for all the substrates.
The rate constants increase with increasing [SDS]. Depend-
ing on the buffer in use the pH varied showing a slight increase
from [SDS] = 0 M to [SDS] = 0.2 M. The effect of the increase is
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advantage. The hydrolysis of 4-isopropylbenzaldehyde dimethyl
acetal was carried out in buffer at constant [SDS] and increas-
ing total [Naϩ]. The hydrolysis was inhibited by the addition
of Naϩ and a dissociation constant of 0.0281 0.003 M was
estimated for the complex of Naϩ with SDS by fitting the data
to eqn. (3).
kobs/[Hϩ] = koKexch./(Kexch. ϩ [Naϩ])
(3)
Similar values may be calculated from previous data for the
effect of added Naϩ on the SDS catalysed hydrolysis of methyl
orthobenzoate.6 The rate constants for acid catalysed hydrolysis
of acetals follow a rate law derived from Romsted’s pseudo
phase theory when there is no cation compensation.4,8
Acid-catalysed acetal hydrolysis is inhibited in the presence
of increasing concentrations of CTAB because the micelle does
not exchange Hϩ into its Stern layer. The second order rate
constants obtained from kobs/[Hϩ] decrease with increasing
[CTAB] according to eqn. (4).
Fig. 1 Plot of kobs versus [Hϩ] at constant [SDS] (0.04 M) for
benzaldehyde dimethyl acetal at 25 ЊC (total [Naϩ] = 0.06 M); line is
theoretical from eqn. (1) and parameters in Table 1.
kobs/[Hϩ] = kHKi/(Ki ϩ [CTAB])
(4)
The kinetic parameters are recorded in Table 2.
We attempted to find a catalytic system where the complex-
ation constant (KS) as well as the catalytic constant (kcat) could
be measured independently unlike that of the SDS system. This
proved not to be possible with the materials available.9 The low
solubility of Aerosol AOT in aqueous solution required the use
of 30% acetonitrile–water compositions to obtain kinetically
useful concentrations. Plots of kobs/[Hϩ] versus [AOT] exhibit a
catalytic advantage and obey eqn. (1) in the concentration
range between 0 M and 0.1 M (Table 3). Other micelle-forming
anionic surfactants such as those with phosphate head groups
could exhibit complexation within the concentration range
available before aggregation but these systems were not
investigated. The main problem is that aggregation of the
micelles is likely to occur unless the head group is very polar;
the sulfate group studied here is probably the best candidate to
keep the micelle in solution up to tenth molar concentrations
of the surfactant molecules. We chose to employ CTAB as an
analogue of SDS and investigate its dissociation constant (Ki)
from inhibition studies.
Studies of acid-catalysed aromatic ketal hydrolysis and
addition reactions at benzaldehyde derivatives in the absence of
detergents have indicated that a Yukawa–Tsuno-type equation
correlates the data better than the Hammett equation; this is
due to the development of resonance between aromatic sub-
stituent and the reaction centre.10,11 The parameters of Table 1
fit Young’s modified Yukawa–Tsuno equation11 (log k = ρσ ϩ
ρr(σϩ Ϫ σ) ϩ log kЊ, eqns. (5) and (6)) and the Ki parameters of
Table 2 fit the Hansch equation as follows (eqn. (7)); values of σ
and σϩ employed in the correlations are from Hansch and Leo12
and Leffler and Grunwald.13 (kSDSЊ and kHЊ are rate constants
for the unsubstituted benzaldehyde acetal.) The deviations in
the Hansch plot are due to the relatively large discrepancy
between the micelle-binding equilibrium and the reference
system for π,14 the transport of a substituted phenyl species
between water and n-octanol. The fit to the Hansch equation
is not particularly good in common with other Hansch
relationships.15
Fig. 2 Plot of kobs/[Hϩ] versus [SDS] for 4-isopropylbenzaldehyde
dimethyl acetal at 25 ЊC; line is theoretical from eqn. (1) and parameters
in Table 1.
corrected by division of kobs by [Hϩ] and the resulting second
order rate constants are linearly dependent on [SDS] in the
concentration range employed (Fig. 2 and eqn. (1)).
kobs/[Hϩ] = kH ϩ ksurfactant[surfactant]
(1)
The intercept at [SDS] = 0 M is the second order rate constant
for acid-catalysed hydrolysis in the absence of micelle (kH). The
parameter ksurfactant for SDS (kSDS) is a third order rate constant
which measures the micelle-catalysed acid reaction. The
parameters are recorded in Table 1.
It is necessary to determine the effect of change in Naϩ ion
concentration on the dissociation constant of the complex
between acetal and SDS micelle. Because KS cannot be deter-
mined explicitly for the acetals (the rate law is linear in [SDS]
over the range studied) it was decided to employ 2-chlorophenyl
acetate as an analogue and the dissociation constant for this
ester is available from studies on its reaction with imidazole in
the presence of increasing concentrations of SDS.7 The binding
of a neutral substrate, 2-chlorophenyl acetate, to SDS micelles
was studied by measuring the rate constants for reaction of the
ester with imidazole in the presence of increasing amounts of
SDS for a range of total [Naϩ] values as described previously.7
The data are analysed according to eqn. (2) where kimidazole is the
second order rate constant for reaction of neutral imidazole
with ester in the absence of surfactant; the values of Ki are 8.79
1.50 mM (0.07 M [Naϩ]), 6.90 0.29 mM (0.22 M [Naϩ]) and
6.67 0.34 mM (0.42 M [Naϩ]).
log kH = Ϫ3.30 0.03σ Ϫ 0.69
0.32(σϩ Ϫ σ) ϩ log kHЊ (r = 0.9971) (5)
kobs/[imidazole] = kimidazoleKi/(Ki ϩ [SDS])
(2)
log kSDS = Ϫ4.21 0.17σ Ϫ 1.62 0.41(σϩ Ϫ σ) ϩ
log kSDSЊ (r = 0.9972) (6)
The rate constants for acetal hydrolysis (kobs/[Hϩ]) are
lower than those previously determined for SDS4 because the
relatively large Naϩ counterion concentration exchanges for the
Hϩ in the micellar Stern layer thus reducing its overall catalytic
log Ki = Ϫ0.702 0.072π Ϫ 1.742 0.048 (r = 0.9553) (7)
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Table 2 Acid-catalysed hydrolysis of substituted benzaldehyde dimethyl acetals in the presence of increasing concentrations of CTAB and constant
[BrϪ]a
Substituent
Ki/mMc
∆kobs × 103/sϪ1 d
∆pHe
Nf
kSDSKi/MϪ1 sϪ1 b ≡ kcat
3-CH3O
4-CH3O
H
2-Cl
4-Cl
3-NO2
3-Cl
4-NO2
4-CN
4-Isopropyl
4-CH3
13.9 0.27
14.6 0.413
22.4 0.8
7.95 0.54
3.91 0.11
14.1 0.81
3.66 0.35
17 4.5
30.3–8.52
5.05–1.63
1.58–0.61
3.58–0.81
19.4–2.23
0.357–0.104
4.31–0.584
0.0598–0.0172
0.407–0.201
5.82–0.42
2.87–2.77
5.25–5.09
4.43–4.51
2.89–2.65
2.91–2.74
2.89–2.73
2.9–2.72
2.89–2.73
2.89–2.70
4.41–4.28
4.47–4.18
5
5
4
5
5
6
5
6
4
5
5
10.1
1270
17.9
0.0744
1.24
0.0197
0.0842
0.0135
0.0349
13.1
28.4
6
1.32 0.16
9.41 1.1
6.03–1.23
104
a [CTAB] = 0.01–0.05 M, total [BrϪ] = 0.05 M (made up with KBr), 25 ЊC. Buffers are chloroacetate up to pH 2.73 and acetate up to 5.09. Acetal
concentrations as in Table 1 footnote f. b This value is proportional to kcat (see eqn. 13). c See footnote j of Table 1. d Range of observed rate constants.
e Range of pH values for kinetics. f Number of data points not including duplicates.
Figs. 3 and 4 illustrate the free energy relationships of the
parameters. The corresponding Yukawa–Tsuno–Young co-
Fits of the parameters kH, kSDS and kAOT to regular Hammett
equations have poorer correlation coefficients than those from
the Yukawa–Tsuno–Young equation.
efficients for kH in the AOT system are: ρr = Ϫ1.18
0.54,
ρ = Ϫ2.71
0.54, log kHЊ = 0.926
0.097 (r = 0.9804) and
The values of kH and the ρ coefficient are similar to those
obtained by Fife,16 Cordes,5 Jensen17 and Young10 in aqueous
and water–organic solvent mixtures.
for kAOT: ρr = Ϫ0.86 0.46, ρ = Ϫ3.14 0.47, log kAOTЊ = 1.93
0.08 (r = 0.9866).
Discussion
It is commonly accepted that the specific acid-catalysed
hydrolysis of benzaldehyde acetals in aqueous solution involves
rate limiting formation of the oxocarbenium ion (eqn. (8)) and
subsequent fast degradation to hemi-acetal, benzaldehyde
hydrate and finally benzaldehyde (eqn. (9)). The lifetime of the
oxocarbenium ion (PhCHOCH3ϩ) has been estimated to have a
half life of 3.5 × 10Ϫ9 s in water.18 In the presence of nucleo-
philes more reactive than water the oxocarbenium ion is
diverted away from the hemi-acetal intermediate. Existence of
a concerted displacement by the nucleophile depends on the
lifetime of the oxocarbenium ion in the presence of the nucleo-
phile and inverting glycosidases involve acylal intermediates
formed by nucleophilic attack from a neighbouring carboxylate
anion.19
Hydrolysis may be accelerated by complexing the acetal in
the Stern region of a micelle bearing anionic groups possibly by
stabilisation of the protonated acetal (Scheme 1 and eqn. (10)).
The substrate will be located so that only the acetal function
Fig. 3 Yukawa–Tsuno–Young dependence of kSDS, kH and kcat. Data
are from Table 1 and lines are theoretical from eqns. (5), (6) and (14).
Fig. 4 Hansch dependence of Ki. Line is theoretical from eqn. (7);
data are from Table 2.
Scheme 1
(8)
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Table 3 Acid-catalysed hydrolysis of substituted benzaldehyde dimethyl acetals in the presence of increasing concentrations of AOT and constant
[Naϩ]a
Substituent
kAOT/MϪ2 sϪ1 b
kH/MϪ1 sϪ1
103ؒ∆kobs/sϪ1 c
λ/nmd
∆pHe
3-CH3O
4-CH3O
Parent
4-Cl
4-CH3
52.8
1510
61.5
21.8
595
3.29
184
13.5
3.02
29.2
33.1
2.9–5.7
73–140
2.3–6.6
0.67–1.7
10.3–15
6.1–20
254
283
256
246
256
270
2.76–3.43
3.12–3.65
3.31–3.90
3.25–3.82
3.29–3.87
3.21–3.93
4-(CH3)2CH
271
a [AOT] 0–0.1 M, total [Naϩ] = 0.11 M (made up with NaCl) (lowest concentration of AOT was 0.02 M), 25 ЊC. Buffer is chloroacetate. Acetal
concentrations as in footnote f of Table 1. b See footnote j of Table 1. c Range of observed rate constants. d Wavelengths for kinetic studies. e Range of
pH values for kinetics.
(9a)
(9b)
(10)
resides in the Stern region; the fact that catalysis occurs rather
than inhibition indicates that the substrate is not wholly
immersed in the hydrophobic core of the micelle (although its
aromatic component resides in this region).
KS = aKi
(12)
(13)
log kcat = log kSDS ϩ log Ki ϩ log a
The linearity of the dependence of kobs/[Hϩ] on SDS concen-
tration precludes the explicit kinetic determination of KS values
for the acetal/SDS and acetal/AOT systems and experiments
with other anionic micelles (SHDS and STDS)9 as models
have encountered solubility problems when concentration
dependencies have been attempted. Values of Ki are easily
obtained for the CTAB system because they fall within the
solubility range of this surfactant. CTAB partitions the acetal
from aqueous solution (eqn. (11)) in this system but does not
exchange protons thus inhibiting hydrolytic attack by the bulk
solution of protons.
Table 2 collects the data for kcat (≡ kSDSؒKi) determined
when a is defined arbitrarily set to unity and the resultant
k
cat fits a Yukawa–Tsuno–Young equation (14) (log kcatЊ = 0.861
0.21 ϩ log a). The absolute magnitude of kcat is not known
but its variation as a function of σ is independent of the value
of a.
log kcat = Ϫ3.68 0.42σ Ϫ 2.28
1.0(σϩ Ϫ σ) ϩ log kcatЊ (r = 0.9807) (14)
The corresponding parameters for the AOT-catalysed
hydrolysis for kcat are: ρr = Ϫ1.34 1.50, ρ = Ϫ3.12 1.50, log
(11)
k
catЊ = 0.0051 0.27 ϩ log a (r = 0.8986).
Perusal of the data for the acid hydrolysis of benzaldehyde
CTAB has a longer aliphatic chain than SDS and its critical
micelle concentration occurs at a lower value (0.92 mM)1
compared with that of SDS (8.1 mM)1 indicating that CTAB
should have a greater propensity to complex hydrophobic
species than SDS. The effect of substituents on the value of
Ki for binding of benzaldehyde dimethyl acetals to CTAB is
therefore a reasonable measure of the effect of their com-
plexation with SDS. Previous work indicates that the dissoci-
ation constants for complexes of aryl acetates to SDS and to
CTAB both fit Hansch equations and Ki for SDS and CTAB
are proportional to each other for these substrates. If we
assume for the acetal hydrolysis system that KS for SDS is
proportional to Ki for CTAB (eqn. (12)) then we can determine
kcat from kSDS (eqn. (13)).
diethyl acetals in micellar solutions where the counter anion is
not maintained at a constant concentration or ionic strength
indicates that there is no simple relationship between reactivity
(kobs/[Hϩ]) and surfactant concentration.5 A normalised plot of
kobs versus [SDS] (normalised on the maximal rate in each case)
has approximately the same shape for each substrate and the
maximal rate constant occurs at approximately the same value
of [SDS] (Fig. 5). The complexation to SDS micelles of small
neutral substrates analogous to the acetals (such as phenyl
acetates) is markedly affected by the substituent and the dis-
sociation constant (KS) for a range of substituents obeys a
Hansch relationship.7 Values of KS for phenyl acetates with
SDS and CTAB7 vary over approximately 2 powers of 10. The
rate–concentration curves for acid-catalysed hydrolysis of
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catalytic advantage of the SDS micelle in acetal hydrolysis.
The KS value is unlikely to be greater than 1 M (although this
concentration would be unobtainable for SDS) so that an
estimate of the upper limit of the catalytic advantage of the
micelle is only (797 × 1)/61 ∼ 13-fold.
Comparison of the ρ parameter for kcat with that of kH
(Ϫ3.68 and Ϫ3.30 respectively) indicates that the reaction of
complexed acetal with Hϩ is slightly more sensitive to sub-
stituent than is that of the free acetal; this indicates that positive
charge development in the complexed acetal is slightly greater
than in the free acetal in the transition state for reaction with
the proton.
The kinetic parameter, kcat, registers the free energy difference
between complexed acetal and the transition state including
the proton (Scheme 1). The equilibrium constant for the
protonation of complexed acetal is included in kcat but its
measurement cannot be effected for the present results.
Nevertheless, the data indicate that there is no significant
electronic difference between complexed acetal and free acetal
for the binding to CTAB because there is no Hammett depend-
ence of Ki (eqn. (7)).
Fig. 5 Plots of kobs/[Hϩ] (normalised on the maximal rate constant)
versus [SDS] for non-compensated counterions. Data from Dunlap,
Ghanim and Cordes.5 ᭿, 4-NO2 (2.25); ᭹, 4-Cl (5.36); ꢀ, 4-F (4.76); ᭢,
H (5.36); ଙ, 4-CH3 (6.4); ᭜, 4-OCH3 (6.4). The cmc is not quoted in
Cordes’ paper for the experiments on SDS catalysed hydrolysis of
acetals.
If the oxocarbenium ion were formed as an intermediate at
the surface of the micelle it is possible that even the weakly
nucleophilic –SO3 head group would react to form the
acetals in SDS should exhibit maxima at SDS concentrations
with a wide range of values if the curvature were due to sub-
strate complexation. It is therefore likely that the curvature
previously observed in the SDS5 case is due to an ion exchange
effect of increasing [Naϩ] and expelling proton from the Stern
region rather than due to complexation although complex
formation will certainly be occurring (but not to saturation).
A change in cmc caused by change in ionic strength will also
cause non-linearity.
Ϫ
sulfonylal (Scheme 2). There are no data for the reactivity of a
sulfonylal (RSO2–O–CH(ORЈ)Ar)20 but if it were formed in
this system it would have to decay rapidly to aldehyde because
the product UV spectrum indicates that only aldehyde is
formed and gives no evidence for a stoichiometric amount of
intermediate. It is not possible to distinguish the stepwise pro-
Ϫ
cess from the one where the –SO3 group displaces methanol
from the protonated acetal in a concerted pathway (similar to
that of eqn. (9a)).
The small spread of Ki values determined for an analogous
neutral substrate (2-chlorophenyl acetate) with SDS over the
[Naϩ] range 0.07 to 0.42 M indicates that the structure of
the SDS micellar interface would not have much effect on
the binding of the neutral acetals. The values of Kexch. for the
exchange of Naϩ with the SDS micelle calculated from
the literature and that determined from the hydrolysis of the
4-isopropylbenzaldehyde dimethyl acetal (0.0281 M) are con-
sistent with the curvature of Fig. 5 being due to ion exchange
rather than substrate complexation.
The present data indicate that when the ionic atmosphere is
maintained constant the fit is to a linear equation up to 0.2 M
SDS in all the cases shown. This fact does not mean that com-
plexation is absent or is not important in the catalytic sequence
of reactions. It merely indicates that the complex between sub-
strate and micelle possesses a much larger dissociation constant
than can be determined at the concentrations of SDS available
in the experiments (without detergent precipitation). The lower
limit of the value of the dissociation constant of the complexed
acetal is 0.2 M, the maximum concentration of SDS employed
in the experiments.
The relative reactivity of complexed and free benzaldehyde
dimethyl acetals cannot be measured absolutely because the
value of a in eqn. (12) is unknown. However, a lower limit can
be determined for kcat from the maximum concentration of SDS
employed because KS > [SDS]max. Let us consider the parent
acetal where kSDS = 797 MϪ2 sϪ1. The lower limit on kcat is 0.2 ×
797 MϪ1 sϪ1 = 159 MϪ1 sϪ1 which compares with the value
61.1 MϪ1 sϪ1 for kH. The reaction of Hϩ with complexed acetal
is therefore >159/61 more efficient than with uncomplexed
substrate. It is possible to speculate on an upper limit of the
The method involved in dissection of kSDS into its component
kcat and KS parameters introduces uncertainty additional to that
of kSDS because the relationship between Ki and KS involves
non-systematic deviations caused by the different surfactant
systems. These uncertainties are reflected in the magnitudes of
the standard deviations for ρ and ρϩ in both SDS and AOT
cases. The value of ρ for kcat is more negative than that of kH
but the difference is within the confidence limits of both
measurements and little emphasis can be placed on the differ-
ences regarding the relative charges in the transition states.
The existence of only a small catalytic advantage in the
surfactant-catalysed reaction is consistent with this conclusion.
The relative values of ρr appear to indicate that there is a larger
resonance contribution in the transition state for the surfactant
catalysed reactions but the confidence limits on the measure-
ments indicate that it would be unsafe to conjecture on their
significance at this stage.
Conclusions
The saturation curve obtained in studies of the acid catalysed
hydrolyses of acetals in the presence of increasing concentra-
tions of SDS without Naϩ counterion compensation is due to
ion exchange by Naϩ depleting the proton concentration
within the micelle and is not due to a complexation effect with
the substrate. Values of the dissociation constants, KS, cannot
therefore be determined explicitly for this system. The differ-
ence in ρ and ρr between acid catalysed hydrolysis of free acetal
and complexed acetal indicates that there is a marginally greater
Scheme 2
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J. Chem. Soc., Perkin Trans. 2, 2002, 1589–1595

View Article Online
9 Both sodium tetradecyl sulfate (STDS) and sodium hexadecyl
positive effective charge in the transition state in the micellar
catalysed reaction.
sulfate (SHDS) clouded in water (at 0.2 M [Naϩ]) at too low a
concentration for them to exhibit saturation effects in the acid
catalysed hydrolysis of acetals.
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