Journal of the Chinese Chemical Society, 2007, 54, 957-962
957
Spectrophotometric Determination of the Stability Constant of the
Inclusion Complexes of Some Catechol Derivatives with b-Cyclodextrin
Based on their Reaction with Iodate
Abbas Afkhami* and Lida Khalafi
Faculty of Chemistry, Bu-Ali-Sina University, Hamadan 65174, Iran
The effects of b-cyclodextrin (b-CD) inclusion on the kinetics of the oxidation of several cathechol
derivatives, including 4-tert-butylcatechol, 3-methylcatechol and 3-methoxycatechol, with iodate was
studied spectrophotometrically. The rate of the oxidation reactions decreased by increasing b-CD concen-
tration as a result of inclusion. The stability constants for the inclusion complexes of the investigated com-
pounds were determined based on the changes in the rate constants as a function of b-CD concentration at
pH 3.0. The rate constants for the free and complexed forms and also the stability constants for the inclu-
sion complexes were calculated. The role of the hydrophobic effect was evaluated by studying the influ-
ence of the presence of different amounts of ethanol on the b-CD: guest interaction. In a given H
mixture the stability of b-CD complexes shows the order of 4-tert-butylcatechol » 3-methylcatechol »
-methoxycatechol. Increasing ethanol content caused a decrease in the stability constant of the inclusion
2
O-EtOH
3
complexes and an increase in observed rate constants.
Keywords: Catechol derivatives; Inclusion complexes; Spectrophotometry; Stability constant.
INTRODUCTION
The unusual properties of cyclodextrins (CDs) origi-
uses in pharmaceuticals, foods, cosmetics, or chemicals
5
have been summarized in some recent monographs.
-14
Inclusion of organic molecules in the CD cavity is
confirmed by changes in spectroscopic parameters in aque-
ous solution subsequent to complexation. Parameters which
have been monitored include UV-vis absorption spectrom-
nate in their unique structure. They are a group of cyclic
oligosaccharides with six (a) seven (b) or eight (g) glucose
residues linked by a-[1-4] glycosidic bonds in a truncated
cone shape structure. In general, cyclodextrins are fairly
soluble in water. Despite a hydrophilic surface, cyclodex-
trins contain a hydrophobic cavity. It is the presence of this
cavity that enables cyclodextrins to entrap hydrophobic
1
5
16
etry, intensity of fluorescence, and nuclear magnetic
1
7
resonance. In conjunction with spectrophotometric tech-
niques, CDs have been widely used in fluorimetric analysis
mainly to increase analyte sensitivity. Moreover, reports of
the usage of CDs as analytical reagents in UV-vis spectro-
1
molecules. b-CD with an internal diameter of 0.78 nm can
1
8
accommodate only relatively small and medium-sized or-
photometry are scarce. Chemical reactions pertaining to
the included guest may take place, and the effects of inclu-
sion on the reactivity vary widely depending on the guest,
ganic molecules in which entrapment/inclusion occurs
2
,3
without the formation of formal chemical bonds. The ex-
tent of complex formation, however, depends upon the po-
larity of the guest molecule. The stability of the inclusion
complex may be increased by the presence of hydrophobic
substitutents, such as the tert-butyl group, on the aromatic
1
9
the CD, and the reaction examined.
Among the first cyclodextrin inclusion complexes to
be investigated were those containing substituted phenol
3
and benzenediol molecules as guests. The chemical or
4
ring.
electrochemical oxidation of catechol derivatives and cate-
2
0
Because of this inclusion, a change of the chemical or
physical properties of the guest molecule is generally ob-
served, opening a wide field of applications in areas such as
organic and analytical chemistry. Their actual or potential
cholamines has long been studied. Catechol derivatives
can be oxidized chemically to their corresponding o-qui-
nones by proper oxidants such as iodate.
Imonigie and Macartney reported the effects of a and
*
Corresponding author. E-mail: afkhami@basu.ac.ir