Effect of kosmotrope and chaotrope anions on rate and equilibria processes for the α-cyclodextrin catalysed reaction of 3-chloroperbenzoic acid with iodide

Article abstract of DOI:10.1007/s10847-012-0279-5

Solutes, ranging from small hydrophobic molecules to macromolecular structures such as proteins, are heavily influenced by the presence of salts, particularly the anionic component. It is well known that anions possessing a high charge density, such as sulfate, are able to reduce the solubility of these solutes, with this effect likely to be entropically driven and based on the influence the anions have on the structure of water. Other anions with a lower charge density, such as perchlorate, can have the opposite effect, improving solubility. The current consensus for the latter effect is that rather than being as a result of changes to water structure, it is due to specific binding of the anions to solute molecules, acting to reduce unfavourable interactions with the solvent. In this paper we report on how both sulfate and perchlorate influence binding and catalysis for the α-cyclodextrin mediated reaction of iodide with 3-chloroperbenzoic acid (MCPBA) for which the cyclodextrin-MCPBA complex is the catalytic species. Kinetic runs have been carried out over the temperature range 15-35 C in dilute nitric acid and with added sulfate and perchlorate, allowing the calculation of enthalpies and entropies for the rate and equilibria processes involved in this system. We show that both sulfate and perchlorate demonstrate the expected Hofmeister behaviours, with sulfate acting to exclude the MCPBA from solution into the cyclodextrin cavity, thus increasing the amount of the catalytic species, whereas perchlorate acts to solubilise the MCPBA. Perchlorate has a complex range of effects, since as well as forming an association complex with cyclodextrin, the thermodynamic parameters indicate that it may also associate with MCPBA and the MCPBA-cyclodextrin complex. These associations may explain the catalysis that is observed with increasing perchlorate concentration. The effects observed in this study may be relevant to other binding processes in chemistry and biology.

Full text of DOI:10.1007/s10847-012-0279-5

J Incl Phenom Macrocycl Chem
DOI 10.1007/s10847-012-0279-5
ORIGINAL ARTICLE
Effect of kosmotrope and chaotrope anions on rate and equilibria
processes for the a-cyclodextrin catalysed reaction
of 3-chloroperbenzoic acid with iodide
•
Michael E. Deary Salem M. Mousa
•
D. Martin Davies
Received: 18 July 2012 / Accepted: 20 December 2012
Ó Springer Science+Business Media Dordrecht 2013
Abstract Solutes, ranging from small hydrophobic mol-
ecules to macromolecular structures such as proteins, are
heavily influenced by the presence of salts, particularly the
anionic component. It is well known that anions possessing
a high charge density, such as sulfate, are able to reduce the
solubility of these solutes, with this effect likely to be
entropically driven and based on the influence the anions
have on the structure of water. Other anions with a lower
charge density, such as perchlorate, can have the opposite
effect, improving solubility. The current consensus for the
latter effect is that rather than being as a result of changes
to water structure, it is due to specific binding of the
anions to solute molecules, acting to reduce unfavourable
interactions with the solvent. In this paper we report on
how both sulfate and perchlorate influence binding and
catalysis for the a-cyclodextrin mediated reaction of
iodide with 3-chloroperbenzoic acid (MCPBA) for which
the cyclodextrin–MCPBA complex is the catalytic species.
Kinetic runs have been carried out over the temperature
range 15–35 °C in dilute nitric acid and with added sulfate
and perchlorate, allowing the calculation of enthalpies and
entropies for the rate and equilibria processes involved in
this system. We show that both sulfate and perchlorate
demonstrate the expected Hofmeister behaviours, with
sulfate acting to exclude the MCPBA from solution into
the cyclodextrin cavity, thus increasing the amount of the
catalytic species, whereas perchlorate acts to solubilise the
MCPBA. Perchlorate has a complex range of effects, since
as well as forming an association complex with cyclo-
dextrin, the thermodynamic parameters indicate that it
may also associate with MCPBA and the MCPBA–
cyclodextrin complex. These associations may explain the
catalysis that is observed with increasing perchlorate
concentration. The effects observed in this study may
be relevant to other binding processes in chemistry and
biology.
Keywords Chaotrope ꢀ Kosmotrope ꢀ Perbenzoic acid ꢀ
Cyclodextrin ꢀ Specific ion effects ꢀ Water structure ꢀ
Catalysis ꢀ Enthalpy–entropy compensation ꢀ Perchlorate ꢀ
Sulfate ꢀ Iodide ꢀ Hofmeister
Introduction
An understanding of how salts interact with water mole-
cules and solutes is fundamental to both chemistry and
biology [1, 2]. In this paper we report on how both sulfate
and perchlorate influence binding and catalysis in the
a-cyclodextrin mediated reaction of iodide with 3-chloro-
perbenzoic acid (MCPBA), and discuss the wider impli-
cations of our findings.
M. E. Deary (&)
Faculty of Engineering and Environment, Northumbria
University, Ellison Building, Newcastle upon Tyne,
NE1 8ST, UK
e-mail: michael.deary@northumbria.ac.uk
Cations and anions have been shown to produce effects
that can crudely be separated into ‘‘salting-in’’ and ‘‘salt-
ing-out’’ i.e. making the solute more or less soluble [1–3].
Probably the most well-known example of this is the
Hofmeister effect and its resulting series of anions, origi-
nally ranked in their ability to precipitate egg white protein
S. M. Mousa
Department of Chemistry, Sebha University, Sebha, Libya
D. Martin Davies
Faculty of Health and Life Sciences, Northumbria University,
Ellison Building, Newcastle upon Tyne, NE1 8ST, UK
123

J Incl Phenom Macrocycl Chem
from solution, (F- & SO42-[HPO42-[MeCO2-[Cl-
[NO3- [Br- [ClO3- [I- [ClO4- [SCN- [3]), but
in numerous subsequent studies shown to hold true for a
range of other proteins and indeed for solutes more gen-
erally, including small molecules and dissolved gases [3].
Whilst anions have been found to be most effective, cations
can also exert an influence (Na?[K?[Li?[K?[Ba2?
consequently reducing the configurational entropy of each
of the species in solution. Thus in addition to the entropic
gains that occur in pure water, aggregation of non-polar
molecules in the presence of kosmotrope anions gives rise
to a favourable configurational entropy change. An alter-
native explanation is provided in Moelbert’s adapted MLG
model, in which the solvent domain, comprising bulk sol-
vent, and solute hydration shells, is divided into ordered
and disordered states with associated enthalpy and entropy
values. The model, which is valid for small solute species,
draws upon the classical view that solute molecules have a
surrounding hydration shell in which the water molecules
are highly ordered because of the strong hydrogen bonding
interactions and reduced degrees of freedom due to the
adjacent solute molecule. In the presence of a kosmotropic
anion the hydration shell is even more ordered than in pure
water and, as a consequence, there is a significant entropic
gain from aggregation or complexation. At the same time
the enthalpy becomes less negative as a result of reduced
hydrogen bonding interactions [9].
[Rb?[ Ca2? [Ni2? [Co2? [Mg2? [Fe2? [Zn2?
[
Cs? [ Mn2? [ Al3? [ Fe3? [ Cr3? [ NH4? [ H? [1]),
though they are less consistent in their effects, partly due to
the fact that they cannot form hydrogen bonds with the sol-
vating waters and also because of a reduced ability for charge
transfer [2]. The terms kosmotrope (order-making) and
chaotrope(order-breaking)areoftenappliedtosalting-outand
salting-in anions respectively, though some authors have
argued that such terms are not helpful and should be avoided
because of the underlying assumptions therein [4]. Recent
evidence from a variety of spectroscopic and calorimetric
methods has shown that the order making influence of these
anions is restricted to only one or two solvation shells and sois
limited with respect to the bulk solution [3]. As Collins con-
cludes:‘‘thedominantforcesofionsinsolutionare short range
and chemical in nature’’ [2]. Moreover, Zangi et al. [5] have
suggested from the results of molecular dynamics simulations
of solution interactions involving ions of different charge
densities, that in thepresenceofso-calledkosmotropicanions,
the bulk water beyond the ordered hydration clusters is actu-
ally less ordered than in pure water (in terms of the average
number of hydrogen bonds formed with neighbours). Con-
versely, the same study shows that chaotropic anions, whilst
disrupting water structure intheir immediatevicinity, increase
the structure of bulk water overall.
Turning now to low charge density anions such as per-
chlorate, the explanation for their ability to increase solu-
bility of solutes is not simply a matter of reversing the
arguments for kosmotropes. Rather, there appears to be a
consensus that the phenomenon is due to specific binding
of anions to solute, [3, 9–13] and the associated enthalpic,
but particularly entropic, energy changes that this entails.
Anions with charge densities such that the anion-water
interactions are weaker than the water–water interactions
are poorly hydrated and tend to be excluded from solution,
preferentially associating at the solute–solvent interface [3,
9–14]. Collins [15] has coined the term ‘sticky ions’ for
cations and anions that behave in this way. On a general
level this process can be viewed as acting to reduce the
unfavourable solute-solvent interaction, thus making the
presence of the solute in solution more likely. In the case of
cyclodextrins this results in a reduction in the stability of
inclusion compounds [16]. Zangi et al.’s [5] molecular
dynamics simulation model proposes that this ‘‘salting-in’’
phenomenon is determined by the balance of specific, but
opposite, entropic effects. Firstly, the binding of chaotropic
anions at the solute–solvent interface is entropically unfa-
vourable because of the loss of translational entropy of
these anions. At the same time, and contrary to the com-
monly held view, because the presence of chaotropic
anions in solution has been shown in molecular dynamics
simulations to increase the order of bulk water molecules
[11], the loss of anions to the solute–solvent interface
results in a less ordered bulk solvent and thus a favourable
entropy change. It is this latter process that wins out and so
overall the solubilisation of non-polar solutes by chaotropic
anions is entropy driven [5]. Moelbert’s model also involves
the exclusion of chaotropic anions to the solute–solvent
In considering the effect of salts on aggregation and
inclusion processes, it is useful to first look at the energy
contributions within a solution containing non-polar solute
in pure water. The solute will be surrounded by a hydration
shell in which the water molecules have approximately
similar numbers of hydrogen bonds as in the bulk solution,
but have a reduced entropy due to the presence of the
adjacent solute molecule (though the molecules approach
the bulk entropy at high temperatures [6]). In the classical
hydrophobic effect there is a significant entropic gain when
the extent of the solvent–solute interface is reduced i.e.
aggregation, thus releasing solvent molecules into the bulk
solution [3, 7]; in addition there may be accompanying
enthalpic contributions. The question then is how the
addition of kosmotropic ions changes this situation so that
increased aggregation or, in the case of cyclodextrins [8],
increased complexation, is favoured? One proposal, based
on solution molecular dynamics simulations [5], posits that
the presence of high charge-density anions leads to the
formation of increased numbers of hydration clusters in
solution, reducing the volume of bulk water molecules and
123

J Incl Phenom Macrocycl Chem
interface. This results in an overall entropic gain, thus
favouring the presence of solutes in solution rather than as
aggregates or complexes [9]. Whilst these models have both
had some success in explaining observed properties of
chaotropes and kosmotropes, it is clear that, despite signif-
icant recent progress in the field, there is still a great deal
that is yet to be understood [1].
Duran Buchner funnel (75 ml) with a sintered disc (45 mm)
of 16–40 lm pore diameter. The concentration of MCPBA
was determined iodometrically. The required concentration
for the working solution was obtained by further dilution in
distilled water. The stock solution of the MCPBA required
regular standardisation due to decomposition. Potassium
iodide, sodium perchlorate, nitric acid and sodium sulfate
were all obtained from Sigma-Aldrich.
The reaction used to probe the effects of chaotropic and
kosmotropic salts in this study is the a-cyclodextrin med-
iated reaction between MCPBA and iodide. The uncatal-
ysed reaction has been well characterised and involves
nucleophilic attack of the iodide on the outer peroxidic
oxygen of the peroxide, yielding IOH in the rate deter-
mining step as shown in Eq. 1 [17]. The IOH reacts with a
further iodide to form iodine (Eq. 2), which equilibrates
with iodide to form triodide (Eq. 3) [18].
Kinetics
Kinetic runs were made on an Applied Photophysics SX 17
stopped-flow spectrophotometer at temperatures of 15, 20,
25, 30 and 35 °C. The temperature of the spectrophotometer
cell was maintained by circulating water from a thermostated
bath around the injection syringes. The runs were conducted
by monitoring the increase in absorbance due to formation of
the triiodide anion at 351 nm, and absorbance versus time
plots were fitted to an equation for a single exponential to
obtain the observed pseudo first order rate constants. The
quotient of this value and the iodide concentration yields
the observed second order rate constant. The concentration
RPhCO3H + Iꢁ + Hþ ꢁ! RPhCO2H + IOH
IOH + Iꢁ + Hþ ꢁ! I2 + H2O
Iꢁ þ I2 ꢀ Iꢁ3
ð1Þ
ð2Þ
ð3Þ
We have shown from linear free energy relationships
derived from rate data for the analogous reaction between
phenyl methyl sulfides and MCPBA in the presence of a-
cyclodextrin that the reactive species is the MCPBA–
cyclodextrin complex in which the peroxide group is
located at the primary end of the cyclodextrin cavity; such
an orientation activates the peroxide [19]. The uncatalysed
electrophilic reactions of peroxy acids with a range of
substrates, including sulfides and halides, have been shown
to be heavily influenced by the nature of the solvent, with
protic solvents able to donate hydrogen bonds that stabilise
the transition state by circumventing charge separation
[20]. In aprotic solvents it has been proposed that an
internally hydrogen bonded configuration is likely for the
peroxyacid, which again will circumvent charge separation
in the transition state [20]. It is likely that the cyclodextrin
effects catalysis of perbenzoic acids either via stabilisation
in a reactive internally hydrogen bonded configuration, or
that the primary hydroxyls participate in catalysis in place
of the solvent [19, 21].
of the iodide and MCPBA were 0.0015 M and 5 9 10-6
M
respectively. The measurements were performed in 0.003 M
nitric acid at pH 2.5, which is below the pKa of the MCPBA,
ensuring the peracid was completely protonated. Sodium
sulfate and sodium perchlorate were added to give concen-
trations shown in the captions to Figs. 1, 2, 3 and 4.
Experimental section
Materials
Fig. 1 Plots showing the effect of a-cyclodextrin on the observed
second order rate, kobs, for the reaction of 0.0015 M iodide and
5 9 10-6 M MCPBA acid at a range of temperatures in 0.003 M
nitric acid. The symbols are: open circles 15 °C, filled circles 20 °C,
open squares 25 °C, filled squares 30 °C, and open triangles 35 °C.
The curves are the best fits to Eq. 10 in which k1obs, Kp and KI were
fitted parameters with the values shown in Table 1; ko was input as a
directly measured rate constant and k2obs was set at zero
MCPBA was purchased from Sigma-Aldrich. It had a purity
of 80 %, with the main impurity being the parent acid,
3-chlorobenzoic acid. As the MCPBA solid is sparingly
soluble in water, solutions were normally prepared by adding
the required amount to distilled water and stirring for about
an hour using a magnetic stirrer. This was filtered with a
123

J Incl Phenom Macrocycl Chem
Fig. 2 Plots showing the effect of a-cyclodextrin on the observed
second order rate, kobs, for the reaction of 0.0015 M iodide and
5 9 10-6 M MCPBA acid at a range of temperatures in 0.003 M nitric
acid and 0.35 M sodium sulfate. The symbols are: open circles 15 °C,
filled circles 20 °C, open squares 25 °C, filled squares 30 °C, and open
triangles 35 °C. The curves are the best fits to Eq. 10 in which k1obs, Kp
and KI were fitted parameters with the values shown in Table 1; ko was
input as a directly measured rate constant and k2obs was set at zero
Fig. 4 Plots showing the effect of a-cyclodextrin on the observed
second order rate, kobs, for the reaction of 0.0015 M iodide and
5 9 10-6 M MCPBA acid at a range of temperatures in 0.003 M nitric
acid and 1.0 M sodium perchlorate. The symbols are: open circles 15 °C,
filled circles 20 °C, open squares 25 °C, filled squares 30 °C, and open
triangles 35 °C. The curves are the best fits to Eq. 10 in which k1obs and
Kp were fitted parameters with the values shown in Table 1; ko was input
as a directly measured rate constant, KI was a constant obtained from the
runs conducted in nitric acid alone and k2obs was set at zero
reaction of MCPBA with iodide. It is known that a-
cyclodextrin forms 1:1 complexes with both MCPBA [21],
and iodide [22], the equilibria for which are shown in
Eqs. 4 and 5 respectively.
Equilibria for inclusion compound formation
KP
CD + MCPBA ꢀ CD,MCPBA
KI
ð4Þ
ð5Þ
CD + Iꢁ ꢀ CD,Iꢁ
From a consideration of these equilibria we can see that the
system of reactions will involve both bound and free reactants
in various combinations. Thus, in addition to the uncatalysed
reaction, ko, (Eq. 6), three possible catalysed reaction path-
ways may exist, as follows: (a) the unbound iodide reacting
with bound MCPBA, k1a (Eq. 7), (b) the unbound MCPBA
reacting with bound iodide, k1b, (Eq. 8) and (c) the reaction of
both reactants in a bound state, k2 (Eq. 9).
Fig. 3 Plots showing the effect of a-cyclodextrin on the observed
second order rate, kobs, for the reaction of 0.0015 M iodide and
5 9 10-6 M MCPBA acid at a range of temperatures in 0.003 M
nitric acid and 0.2 M sodium perchlorate. The symbols are: open
circles 15 °C, filled circles 20 °C, open squares 25 °C, filled squares
30 °C, and open triangles 35 °C. The curves are the best fits to Eq. 10
in which k1obs and Kp were fitted parameters with the values shown in
Table 1; ko was input as a directly measured rate constant, KI was a
constant obtained from the runs conducted in nitric acid alone and
k2obs was set at zero
Reactions
ko
MCPBA þ Iꢁ ꢁ! Products
ð6Þ
ð7Þ
ð8Þ
ð9Þ
k1a
CD,MCPBA þ Iꢁ ꢁ! Products
MCPBA þ CD,Iꢁ ꢁ! Products
CD,MCPBA þ CD,Iꢁ ꢁ! Products
k1b
k2
Results
2
ko þ k1obs½CDꢂ þ k2obs½CDꢂ
À
Á
kobs
¼
ð10Þ
We first need to consider the equilibria and reactions that
1 þ Kp þ KI ½CD] þ KpKI½CD]2
are likely to be relevant to a-cyclodextrin catalysed
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J Incl Phenom Macrocycl Chem
Equation 10, derived from Eqs. 4 to 9, together with a
consideration of the mass balances on cyclodextrin, MCPBA
and iodide (not shown), describes the relationship between
the observed second order rate constant, kobs, and the con-
centration of cyclodextrin. Here, ko, k1obs and k2obs are,
respectively, the observed zero, first and second order
dependencies in cyclodextrin (second, third and fourth order
overall), with the latter two being composite rate constants,
as defined in Eqs. 11 and 12.
very small contribution to the overall mass balance on
cyclodextrin.
½CDꢂt
½CD]f ¼
ð13Þ
KClO4 ꢀ ½ClOꢁ4 ꢂ þ 1
.
Figures 3 and 4 show that the decline in rate observed
at high cyclodextrin concentrations for reactions carried
out in sulfate and nitric acid alone is not evident in the
presence of perchlorate because of competitive binding
between this species and cyclodextrin; this essentially
equates to a situation where the reaction is carried out in a
much lower cyclodextrin concentration, one at which the
influence of iodide is not observed. Consequently,
attempting to include KI as a parameter is not valid and so
the KI values obtained at each temperature in nitric acid
were input as constants. This is reasonable because, as
shown in Table 1, KI values are not strongly influenced by
the addition of salt and in any case the influence of this
variable on the overall rate is small. Thus only two
parameters were fitted for this data, which is acceptable
given the limited curvature in the plots at the higher per-
chlorate concentration. Unfortunately, this meant that we
were unable to determine KI in the presence of the two
perchlorate concentrations. All curve fitting analysis was
carried out using Grafit version 7 [25].
k1obs ¼ k1a ꢀ Kp þ k1b ꢀ KI
k2obs ¼ k2 ꢀ Kp ꢀ KI
ð11Þ
ð12Þ
Figures 1, 2, 3 and 4 show the relationship between
kobs and cyclodextrin concentration in the presence of
four different aqueous environments: respectively, no
added salt, 0.35 M sulfate, 0.2 M perchlorate and 1.0 M
perchlorate, each at five temperatures (15, 20, 25, 30 and
35 °C). All solutions contained 0.003 M nitric acid in order
to keep the MCPBA in its protonated form and provide
hydrogen ions for the reaction (Eq. 1). The curves are the
best fits to Eq. 10 with the parameters listed in Table 1. It
should be stressed that whilst this equation does include
five parameters, not all of these are used as fitted variables
in the analysis: ko is input as the directly measured value
for the reaction in the absence of cyclodextrin and k2obs
is set at zero because we have previously shown from
the reaction of MCPBA with substituted phenyl methyl
sulfides that the catalytic species is the bound peracid, and
that reactions involving bound versions of both reactants
do not make a significant contribution to the overall rate
[19, 21]. Of the remaining three parameters, k1obs and Kp
are responsible for determining the main curvature of
the plots, and in the case of the reactions carried out in
sulfate and nitric acid alone, non productive binding of KI
accounts for the small reduction in observed rate at higher
cyclodextrin concentrations. There is sufficient definition
in the plots shown in Figs. 1 and 2 to determine these three
parameters unambiguously.
Discussion
Figure 5, which is extracted from the runs at 25 °C in
Figs. 1, 2, 3 and 4, shows very nicely the effects of the
different salt solutions on the relationship between cyclo-
dextrin concentration and the observed second order rate
constant. The presence of sulfate appears to accelerate the
reaction compared to nitric acid, whereas perchlorate,
because of the competitive binding mentioned earlier,
results in a lower effective cyclodextrin concentration and
consequently a situation where there is still a significant
amount of unbound MCPBA at higher cyclodextrin
concentrations. In the following paragraphs we discuss
the effect of these different aqueous environments on the
binding of the reactants and the rates of reaction. The
results are also discussed within the context of water
structure and of enthalpy–entropy compensation.
For the reactions studied in the presence of perchlorate,
the fitting is complicated by the fact that this species also
binds to a-cyclodextrin, (literature value of KClO4 = 29.4
dm3 mol-1 at 25 °C [23]), which must be accounted for in
the fitting routine by calculating the free cyclodextrin
concentration after complexation with perchlorate (Eq. 13,
in which [CD]t is the total cyclodextrin concentration and
[CD]f the free component). Values of KClO4 were deter-
mined from independent kinetic runs and are given in
Table 2. It is also the case that the product of the reaction,
triiodide (I3-), binds very strongly with a-cyclodextrin
(ca. 20,000 M-1 [24]), however the maximum concentra-
tion of this species attainable in the reactions was 5 9
10-6 M, i.e. the concentration of MCPBA, representing a
Before discussing the rates of reaction it is important to
first consider the effects of different salts on binding pro-
cesses, since this will affect the concentration of the
MCPBA–cyclodextrin complex that is responsible for
catalysis, as well as that of the non-productive iodide–
cyclodextrin complex. Table 1 shows Kp values for each of
the aqueous environments over the temperature range
15–35 °C; the resultant DG°, DH° and DS° parameters are
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J Incl Phenom Macrocycl Chem
Table 1 Temperature dependence of rate constants and transition state pseudoequilibrium constants ( standard error) for the reaction of
3-chloroperbenzoic acid with iodide in the presence of a-cyclodextrin in different aqueous environments
Added salt
Temp/
°C
Binding constants
Rate constants and transition state pseudoequilibrium constants
Kp/
dm3 mol-1
KI/
dm3 mol-1
ko/103 dm3
k
1obs/103 dm6
k1a/103 dm3
KTS1/
dm3 mol-1
mol-1 s-1
mol-2 s-1
mol-1 s-1
0.003 M HNO3
15
20
25
30
35
15
20
25
30
35
15
20
25
30
35
15
20
25
30
35
647 40
491 25
412 19
321 13
247 10
804 47
669 30
540 21
428 14
331 10
550 33
359 18
277 12
271 10
8.9 0.7
3.44 0.01
4.40 0.03
5.47 0.03
6.93 0.04
8.60 0.05
3.60 0.01
4.40 0.02
5.53 0.04
6.93 0.03
8.64 0.06
3.35 0.02
4.53 0.02
5.19 0.03
6.71 0.05
8.46 0.04
3.12 0.02
4.02 0.03
5.11 0.01
6.69 0.07
8.12 0.11
9,010 410
8,820 330
9,010 290
8,950 240
8,360 210
11,550 500
11,550 400
12,080 340
11,970 280
11,720 240
10,160 380
7,940 250
7,060 200
8,240 190
6,690 130
13,900 1,150
10,800 740
9,130 620
10,670 70
8,500 390
13.9 1.1
18.0 1.2
21.9 1.2
27.8 1.4
33.8 1.6
14.4 1.1
17.3 1.0
22.4 1.1
28.0 1.1
35.4 1.3
18.5 1.3
22.1 1.3
25.4 1.3
30.5 1.3
39.1 1.4
36.4 13.9
40.7 11.8
53.3 17.1
66.5 15.3
72.6 12.0
2,620 119
2,010 76
1,649 54
1,290 35
972 25
9.3 0.7
5.9 0.6
5.7 0.5
3.9 0.5
0.35 M Na2SO4
0.2 M NaClO4
1.0 M NaClO4
11.0 0.9
3,209 139
2,624 92
2,183 63
1,727 41
1,356 29
3,030 115
1,752 56
1,360 38
1,228 30
790 16
7.5 0.5
6.5 0.5
4.8 0.4
5.1 0.4
–
–
–
–
–
–
–
–
–
–
171
5
382 143
265 74
171 54
160 37
117 22
4,450 370
2,689 185
1,785 121
1,595 20
1,046 50
Kp and KI are the binding constants for the 1:1 complexes of a-cyclodextrin with MCPBA and iodide respectively. ko and k1obs are the zero and
first order dependencies in cyclodextrin and k1a is the second order rate constant for the reaction of iodide with the MCPBA–cyclodextrin
complex. KTS is the transition state pseudo-equilibrium constant for the stabilisation of the activated complex by one molecule of cyclodextrin
Table 2 Temperature dependence of the 1:1 formation constant
-
between a-cyclodextrin and ClO4 in 0.003 M nitric acid
Temperature/°C
K
ClO4/dm3 mol-1
15
20
25
30
35
62 16
38
28
25
17
7
7
6
4
shown in Table 3. It is clear that sulfate causes an increase
in Kp (and KI) values relative to 0.003 M HNO3 alone,
whereas perchlorate causes a decrease. It is important to
note here that the competitive binding of the perchlorate
has already been accounted for, so these are the real rather
than apparent binding constants.
The observed trends are as expected from the literature.
Sulfate, commonly referred to as a kosmotrope, or water
structure maker, acts to reduce the solubility of solutes, i.e. the
so-called ‘salting-out’ effect, as discussed in the introduction.
In the presence of cyclodextrin, the relatively hydropho-
bic cyclodextrin cavity provides a suitable environment into
Fig. 5 Plots showing the effect of a-cyclodextrin on the observed
second order reaction rate, kobs, for the reaction of 0.0015 M iodide
and 5 9 10-6 M MCPBA acid at 25 °C in 0.003 M nitric acid with
added salts. The symbols are: open circles no added salt, filled circles
0.35 M sodium sulfate, open squares 0.2 M sodium perchlorate, filled
squares 1.0 M sodium perchlorate. The curves are the best fits to
Eq. 10 with the parameters listed in Table 1
123

J Incl Phenom Macrocycl Chem
Table 3 Calculated thermodynamic parameters for the reaction of 5 9 10-6 M MCPBA and 0.0015 M iodide in 3.0 9 10-3 M nitric acid and
in the presence of added salts
Parameters
0.003 M HNO3
0.35 M Na2SO4
0.20 M NaClO4
1.00 M NaClO4
DH0(Kp), kJ mol-1
TDS0(Kp), kJ mol-1
DG0(Kp), kJ mol-1
DH0(KI), kJ mol-1
TDS0(KI), kJ mol-1
DG0(KI), kJ mol-1
DH0(KTS), kJ mol-1
TDS0(KTS), kJ mol-1
DG0(KTS), kJ mol-1
-34.6 1.4
-19.8 1.3
-14.8 1.2
-31.6 6.6
-27.0 6.1
-4.6 1.5
-35.8 1.4
-17.5 1.3
-18.3 1.7
-32.7 1.6
-17.2 1.4
-15.5 1.6
-29.4 5.9
-24.7 5.4
-4.7 1.5
-31.6 1.4
-12.6 1.3
-19.0 2.2
-38.7 5.7
-24.6 5.2
-14.1 3.9
–
-42.5 4.8
-26.4 4.4
-13.1 2.6
–
–
–
–
–
-45.9 5.0
-27.8 4.6
-18.1 3.8
-50.6 5.1
-31.7 4.7
-18.9 3.6
which these solutes may be excluded, thus accounting for the
increase in binding constants of both MCPBA and iodide
compared to nitric acid alone. Perchlorate, often classed as a
chaotrope or water structure breaker demonstrates the
expected effect of ‘salting in’, increasing the proportion of
the unbound MCPBA in solution. The effect of perchlorate is
more complex than sulfate because of its ability to form a
complex with cyclodextrin [16, 23, 26], thus acting to physi-
cally exclude the MCPBA from the cyclodextrin cavity.1
There is evidence from partial molal volume changes to show
that anions such as perchlorate do not fully include within the
a-cyclodextrin cavity [27]. This is supported by crystallo-
graphic data [16, 26], together with a more recent molecular
dynamics study [28] that shows perchlorate mainly associat-
ing with the primary hydroxyls of the cyclodextrin, though
there is some evidence for additional conformations, includ-
ing association with the secondary hydroxyls. If the per-
chlorate was located the primary hydroxyl end this would
effectively cap the cavity, preventing MCPBA from taking up
its preferred orientation where the peroxo group is located at
the primary end of the cavity. In addition to this specific and
measurable association, our results show that perchlorate acts
to stabilise the MCPBA in solution, as evidenced by the much
reduced Kp values shown in Table 1. A developing consensus
is that chaotrope anions or cations exert their effect by pref-
erential association with the solute, reducing the extent of
the solute-water interface, thereby increasing its solubility
[3, 9–13]. There is strong supporting evidence that these ions
are attracted to interfaces [2]. The thermodynamic data for
MCPBA complexation in the presence of sulfate and per-
chlorate (Table 3), are consistent with the current ideas on
how anions may affect water structure [5, 9, 11], as discussed
in the introduction.
Turning now to the kinetic data, literature studies on the
reaction between peroxyacids and iodide (Eq. 1) have
shown the reaction to be unaffected by pH and ionic
strength [17, 29–31]. Our results for the uncatalysed
reaction in the presence of 0.35 M sulfate, compared to
weak nitric acid solution alone, agree with this observation
at all temperatures. However, the presence of perchlorate
depresses the rate of the uncatalysed reaction at all tem-
peratures, with larger effects observed at higher perchlorate
concentration. This observation is at odds with the work of
Secco and Venturin [17] who altered the ionic strength
using sodium perchlorate (up to 1.0 M), though perbenzoic
acid was the peroxyacid used.
For the catalysed reaction, k1obs, the presence of 0.35 M
sulfate and 1.0 M perchlorate accelerates the rate at all
temperatures, whereas for 0.2 M perchlorate the rates are
increased at lower temperatures and decreased at higher
temperatures. This complex behaviour for perchlorate is
due to the fact that k1obs is a composite term that includes
equilibrium constants for cyclodextrin complexes with
iodide and MCPBA, as defined in Eq. 11: an increase in
temperature decreases the extent of complexation, and thus
of the concentration of the catalytic species, but increases
the rate of reaction.
k1obs
k1a
¼
ð14Þ
Kp
Nevertheless, it is possible to make a more insightful
analysis of this data if we make the reasonable assumption
that the k1bꢀKI term in Eq. 11 is negligible compared to
k1aꢀKp, i.e. the catalytic species is the bound peracid, which
we have previously shown to be the case [19]. Equation 11
can be then simplified to Eq. 14 that allows us to calculate
k1a, a second order rate constant for the reaction of the
MCPBA–cyclodextrin complex with iodide; these values
are shown in Table 1. It is clear that in the presence of
sulfate and nitric acid alone, the k1a values are very similar
at all temperatures, i.e. the observed increase in k1obs in
1
It should be noted that we are probing this system using kinetic
techniques and so there is a possibility that the configuration of the
perchlorate–cyclodextrin complex will allow the MCPBA to com-
plex, but inhibit its reactivity.
123

J Incl Phenom Macrocycl Chem
sulfate is entirely due to the exclusion or ‘salting out’ effect
and the resulting increase in concentration of the catalytic
MCPBA–cyclodextrin species.
whereas at 1.0 M the transition state is stabilised at all
temperatures.
A further advantage of the transition state pseudoequi-
librium approach is that the corresponding DH° and DS°
values can be plotted on enthalpy–entropy plots alongside
the thermodynamic data for the binding constants, as
shown in Fig. 6. These plots provide an intuitive way of
visualising thermodynamic data and also allow compari-
sons to be made between different series. For example, we
have also included KTS data from our recent study of
reaction of MCPBA with phenyl methyl sulfides, which
was the first reported enthalpy–entropy plot for these
parameters [19]. It should be noted that TDS° is used as the
entropy measure, where T = 298.15 K, because this gives
a slope of unity when there is complete compensation
between enthalpy and entropy.
The situation is very different for perchlorate, where k1a
is increased compared to the reaction in nitric acid alone.
Admittedly, this increase is relatively small (ca. twofold
in the case of 1.0 M perchlorate), but even accounting for
the large errors at the highest perchlorate concentration, at
which the greatest effects are seen, the increase is signifi-
cant. The possibility that the accelerative effects of per-
chlorate on k1a are as a result of the cyclodextrin catalysed
oxidation by this species can be discounted on the basis of
work carried out by Hamai [32].
Table 4 shows the DGà, DHà and DSà values for the un-
catalysed reaction, ko, and also for the second order reaction
between the bound MCPBA and free iodide, k1a. The overall
free energy of activation for the uncatalysed reaction is
unaffected by the presence of added salts in contrast to the
catalysed reaction where the presence of perchlorate, but not
sulfate, results in a more favourable DGà. Looking at the
enthalpic and entropic contributions to the free energy of
activation, we see that in the presence of perchlorate, DHà is
more favourable for the catalysed reaction and is likely to be
the driving force for the observed rate enhancements. The
DSà values are less favourable, indicating a more ordered
transition state.
The linear relationships formed by all of the series is
referred to as enthalpy–entropy compensation, though such
correlations are not universal, with non-linear relationships
being observed in some cases [12]. Compensation effects
can be observed for binding processes involving series of
structurally related molecules [21, 36], but also for the
binding of the same substance under different conditions.
As an example of the latter, a-cyclodextrin binding con-
stants for p-nitrophenol and p-nitrophenolate, determined
under a variety of different conditions, form linear rela-
tionships when plotted in this way [37]. The origin of this
compensation between enthalpy and entropy is disputed,
with some authors suggesting that it is merely a statistical
artefact [38, 39], whilst others infer information about
conformational and desolvation requirements of the mole-
cules involved [40–43]. We have recently used these plots
to infer information about the nature of the transition state
for the a-cyclodextrin mediated reaction between phenyl
methyl sulfides and MCPBA [19].
A more intuitive approach to looking at the effect of
cyclodextrin on reactions is to use the transition state
pseudoequilibrium approach of that was originally devel-
oped for enzymology by Kurz [33] and then adapted for
cyclodextrin systems by Tee [34, 35]. The transition state
pseudoequilibrium constant, KTS is calculated from the
quotient of k1obs and ko and can be taken to indicate the
stabilisation imparted to the transition state as a result of its
association with one molecule of cyclodextrin. These val-
ues are listed in Table 1 and the thermodynamic parame-
ters derived from them listed in Table 3. The KTS values
show that in the presence of sulfate, compared to nitric acid
alone, the transition state is stabilised at all temperatures as
a result of the association with a cyclodextrin molecule.
For perchlorate at 0.2 M the trend is similar to nitric acid,
For complexation processes, the standard explanation
for compensation effects is that structural changes that
result in strengthened bonding interactions will always
have an entropic cost in terms of reduced degrees of
freedom of the molecules involved; this results in a smaller
than expected or negligible change to the overall free
Table 4 Enthalpies and entropies of activation for the reaction of 5 9 10-6 M MCPBA and 0.0015 M iodide in 3.0 9 10-3 M nitric acid and in
the presence of added salts
Parameters
0.003 M HNO3
0.35 M Na2SO4
0.20 M NaClO4
1.00 M NaClO4
DHà(ko), kJ mol-1
DSà(ko), J mol-1 K-1
DGà(ko), kJ mol-1
DHà(k1a), kJ mol-1
DSà(k1a), J mol-1 K-1
DGà(k1a), kJ mol-1
31.4 0.3
-68.1 0.6
51.7 0.7
30.2 0.7
-60.5 1.0
48.2 1.3
30.1 0.7
-72.3 1.3
51.6 1.5
31.2 1.1
-56.8 1.5
48.1 2.2
31.6 0.9
-67.6 1.6
51.7 2.0
24.4 1.9
-78.6 4.1
47.8 4.4
33.3 0.7
-62.2 1.1
51.8 1.5
25.2 2.5
-70.1 4.7
46.1 5.6
123

J Incl Phenom Macrocycl Chem
complex. Nevertheless, given the magnitude of the error
bars of the underlying data (Table 3), it is not possible to
state that this deviation from unity is significant.
Conclusions
This study has shown that both sulfate and perchlorate have
significant effects on the cyclodextrin catalysed reaction of
MCPBA with iodide. The effects can in part be explained
by the influence these anions exert upon the binding of the
reactants to cyclodextrin. In the case of sulfate the increase
in observed rate is explained by the expected kosmotropic
effect of ‘salting-out’, whereby the MCPBA is excluded
from solution into cyclodextrin, thus increasing the con-
centration of the catalytic species. Perchlorate has a more
complex range of effects: it forms a specific, measurable,
association complex with cyclodextrin, thus acting to pre-
vent the binding of MCPBA, but it also shows the expected
chaotropic property on ‘salting-in’, increasing the solubil-
ity of MCPBA, thus reducing the stability constant of the
cyclodextrin–MCPBA complex.
Fig. 6 Enthalpy–entropy compensation plot for the 1:1 complexa-
tion, of a-cyclodextrin with MCPBA, (Kp) and iodide, (KI) in
different aqueous environments, and with the transition state, (KTS
)
for the reaction of iodide with MCPBA in the same environments.
The KTS data for the reaction of MCPBA with a range of substitutes
phenyl methyl sulfides are shown for comparison
energy [42, 44, 45]. For the current data, we are looking at
the same processes but under different conditions, i.e. with
kosmotropic or chaotropic ions present: clearly the effects
exerted by the salts are compensatable. There is consis-
Probably the most interesting result to emerge from this
work is the accelerating effect that perchlorate has on the
reaction between the cyclodextrin–MCPBA complex and
iodide (k1a). The origin of this catalysis may be a bulk
solvent effect due to the presence of perchlorate: the un-
catalysed reaction, ko, is slightly inhibited with increasing
perchlorate concentrations and so it is not unreasonable to
expect the catalysed reaction to be also affected in some
way. Nevertheless, the presence of sulfate affects neither
the catalysed nor uncatalysed reaction (k1a) and this may
hint that perchlorate is acting in a different way, possibly
by a specific association with the cyclodextrin–MCPBA
complex. A ternary complex involving perchlorate and
MCPBA may result in a conformational change that makes
the complex more reactive. It is interesting to note that
there is a literature precedent for the involvement of per-
chlorate in ternary complexes with cyclodextrin [16].
-
tency between the KTS and Kp series, with 1 M ClO4
showing the most negative entropy and enthalpy, as already
discussed, and sulfate the least.
In Fig. 6 the position of the individual linear relation-
ships with respect to the DH° axis is determined by the
relative magnitude of the average DG° for the series. Thus
we can see that the KI series for different aqueous envi-
ronments is located at a much less negative DH° than the
more strongly binding MCPBA (Kp). Similarly the KTS
series for the reaction of MCPBA with iodide is located at a
less negative DH° than that for sulfides, reflecting the
greater stabilisation imparted to the transition state for the
latter. This is an interesting point because, as we have
discussed, the bound peracid is the reactive species for the
reaction with both substrates, and so similar considerations
may have been expected with regard to stabilisation. The
decrease in stability of the iodide series may reflect the
desolvation requirements of iodide in the transition state
compared to the neutral phenyl methyl sulfides.
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