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Part II Chapter 1: Introduction to crown ethersPart II Chapter 1: Introduction to crown ethersPart II Chapter 1: Introduction to crown ethersPart II Chapter 1: Introduction to crown ethers
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1.1 Introduction
In analytical chemistry, solvent extraction has come to the forefront in
recent years as a popular separation technique because of its elegance,
simplicity, speed and applicability to both tracer and macro amounts of metal
ions. The aspects of solvent extraction with its applicability are very well
explained [1, 2].
In the past century the technique of solvent extraction has grown into many
folds and has becomes most powerful unit of operation. Solvent extraction
enjoys a favored position because of its ease, simplicity, speed of operation and
wide scope. Solvent extraction is economically cheap as it does not require any
sophisticated apparatus or instrumentation; a separating funnel does the entire
job of separation. Solvent extraction separation technique is a convenient and
useful method. Its applications are innumerable and extend to a wide range of
industries such as chemical, metallurgical, nuclear, petrochemical, food,
pharmaceutical as well as in waste management. It can be employed to
concentrate and separate metal ions and to determine stoichiometries and
stability’s of complexes extracted into an immiscible liquid phases. The
importance of solvent extraction leads back to the rapid growth of Science and
Technology. Solvent extraction permits very simple and clean separation of
materials at both micro as well as macro concentrations, hence it is employed
very widely in both fundamental research and technology.
Solvent extraction operation consists of following steps:
1. Intimate contacting of solvent with the aqueous phase containing solute
so that the solute is transferred from aqueous phase to the organic phase.
2. Equilibration of two phases.
3. Separation of two immiscible phases.
4. Back extraction of solute from the organic phase to aqueous phase by
the use of suitable strippants.
The proceedings of International Conference on Solvent Extraction
[ISEC] [3-20] are very important sources of information on the various
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aspects of solvent extraction and provide valuable records of the latest
developments and trends in diverse areas of the field. The various aspects of
solvent extraction are very well covered in several monographs by Treybal
(21), Morrison and Freiser [22], Alders [23], De [24], Starry [25], Marcus
and Kertes [26], De, Khopkar and Chalmers [27], Hanson [28], Sekine and
Hasegawa [29], Zolotov [30].
1.2 Histology of Crown Ethers
Crown ether is a generic name given to macrocyclic polyethers
containing ethylene bridges separating electronegative oxygen atoms. The
discovery of macrocyclic polyethers is interesting in recent developments of
separation chemistry. They typically contain central electron rich hydrophilic
cavity with diameter varying from 1.2-6.0 Ao. The hydrophilic cavity is ringed
with electronegartive binding hetero atoms such as oxygen, nitrogen, sulphur
etc., which in turn are surrounded by a collar of -CH2 groups forming a frame
work which is flexible and exhibits hydrophobic behavior. The hydrophobic
exteriors allow them to solubilize ionic substances into non-aqueous solutions
and in membrane media. Such properties facilitate for their use as extractants
and memebrane carriers. Macrocyclicpolyethers form much more stable
complexes than open chain analogues. Apparantly, this “macrocyclic effect” is
due to the fact that cation is being completely surrounded by a cyclic
macrocycle. Thus, when the inorganic cation fits into the cavity of crown ether
or sandwiched between two crown ether molecules it becomes a lipophilic
species. This property of crown ethers, converting inorganic cation into
lipophilic species can be utilized in extractive separation analysis.
Pure chance discovered the first crown ether [31]. C. J. Pedersen was
working as an industrial chemist for Du Pont. A project was initiated in the fall
of 1961 by him, to find new vanadium containing catalysts for
the polymerization of olefins. Pedersen decided to study the effects of uni and
multidentate phenolic ligands on the catalytic properties of vanadyl group,
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quinque-dentate ligand selected was bis-{2-(o-hydroxy-phenoxy)ethyl} ether
‘C’ and the synthesis was began according to the route outlined in Fig-1.5.1
During the synthesis of ligand from catechol a small quantity of byproduct in
the form of silky crystals was obtained, which showed strange solubility
behavior, which led to further spectroscopic studies and resulted in the
discovery of the first crown ether which was named as Dibenzo-18-Crown-6.
Since then large number of crown ethers were synthesized. The discovery of
crown ethers led to an enormous advances in chemistry.
For many years the co-ordination chemistry of alkali metal ions was
completely ignored by chemists. The synthesis, properties and various
applications of crown ethers have appeared in several monographs and review
articles [32-97] giving new ideas in the use of crown ethers in separation
science. Indeed, there was strong doubt that such coordination chemistry could
even exists since it was universally accepted that the alkali ions in solution
were inert, i.e. they were highly resistant to solvolytic, redox, or complexation
reactions. A new era in the coordination chemistry of alkali elements was
inaugurated by the discovery of crown ethers by Pedersen in 1967. During the
past few decades inventive scientists have found many applications of crown
ethers which includes synthetic organic chemistry, membrane transport,
chromoionophores, fluoroionophores, ion chromatography, isotope separation,
ion selective electrodes, molecular recognition, phase transfer catalysis and
extractive separation analysis.
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Fig-1.2 Discovery of First Crown Ether
1.3 Classification of Crown Ethers
The organic neutral ligands are classified into three groups:
a) Podands b) Coronands c) Cryptands.
a) Podands: These are the open chain compounds and are characterized by
lacking ring and bridge structures.
b) Coronands: These are cyclic compounds. Coronands containing oxygen as
donor atoms are called crown ethers, those containing oxygen and nitrogen as
donor atoms are called as aza-crown ethers and others containing oxygen and
OR
O
O
O
ROt-Butanol
OH
O
O
O
HO O
O
O
O
O
O
H+ MeOH 2H2O
+
( BY-PRODUCT )
2 A + O ( CH2-CH2-Cl )2
+ 2 NaOH
CBis-[ 2-(o-hydroxy phenoxy) ethyl ] ether
D
OH
OH
OH+
Ether
O
OH
O O
OH
R
A
+
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sulphur as donor atoms are called as thio-crown ethers. Various crown ethers
are shown in Fig-1.3.1.
c) Cryptands: These are macropolycyclic polyethers and are classified into
bicyclic, tricyclic and tetracyclic.
1.4 Nomenclature of Crown Ethers
The IUPAC names for the crown ethers are very long and it was not
easy to use such names in routine days. It was very difficult to use such lengthy
names for repeated use. Therefore a system of adhoc name, based upon the
number and kinds of hydrocarbon rings, the total number of atoms in the ring,
the class name “crown” and the number of oxygen atoms in the polyether ring,
was developed for the purpose of naming. e.g., 1,4,7,10,13,16-hexaoxa cyclo
octadecane is designated as 18-Crown-6. Here number 18 indicates the total
number of atoms in the polyether ring while the number 6 denotes the number
of donor oxygen atoms in polyether ring. Additional substituents or sites of
condensation like dibenzo or dicyclohexano are written first, e.g., Dibenzo-18-
Crown-6, Dicyclohexano-18-crown-6. The various crown ethers with their
structures are shown in figure 1.4
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Figure 1.4 structures various crown ethers
C. J. Pedersen [1967] synthesized
crown ethers and awarded the Nobel
Prize in Chemistry in 1987 without
having a Ph. D.
12-crown-4 15-crown-5 18-crown-6
Dibenzo-18-crown-6 Diaza-18-crown-6
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1.5 Synthesis of various Crown ethers
Synthesis of crown ethers are done by straight forward condensation
method using vicinal diols such as catechol and divalent organic group
containing (-CH2-CH2-O)n -CH2-CH2- moiety as shown in Figure1.4.1. The
saturated crown ethers are prepared from the corresponding aromatic ones by
catalytic hydrogenation in 2-butanol at higher temperatures and pressures over
ruthenium catalyst. Recovery of the product is done by column
chromatography on alumina.
Figure 1.5 Synthesis of Crown Ethers
.
+ 2 NaOH + Cl-R-Cl
OH
OH
+ 2 NaOH + Cl-R'-Cl
OH
O
OH
O
R
O O
R
O OR'
OH
OH
2 + 4 NaOH + 2 Cl-R"-Cl
O O
O O
R"
R"
O
O
R
+ 2 NaCl + 2H2O
+ 4NaCl + 4H2O
+ 2 NaCl + 2H2O
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1.6 Characteristics of Various Crown Ethers
Various crown ethers are characterized by their different properties
such as colour, solubility, melting point and characteristic absorption peaks.
Crown ethers with aromatic side rings are colorless crystalline compounds.
The saturated crown ethers are colorless viscous liquids or solids of low
melting point. They are very much more soluble in all solvents than their
aromatic precursors.
Saturated crown ethers do not show any absorption above 220 nm, the
aromatic crown ethers show absorption band near 270 nm (in methanol) which
are characteristics for catechol and its ethers. Complexing with cation brings
about distinctive changes in this band generally by appearance of a second peak
at about 280 nm, with changed absorbance of the main band. The molar
absorptivity of these compounds varies from 1.2 - 8.4 x 103 cm
-1mole
-1. The
infrared spectra of aromatic as well as aliphatic crown ether shows the presence
of ether linkages by a strong broad band around 1230 cm-1
for aromatic-O-
aliphatic and a band at 1100 cm-1
for aliphatic-O-aliphatic group.
For a given polyether ring the melting point rises with a number of
benzo- groups. Crown ethers containing more than one benzo- groups are
nearly insoluble in water and sparsely soluble in alcohols and many other
common solvents at room temperature. They are readily soluble in methylene
chloride, ethylene chloride, chloroform, nitrobenzene. The saturated crown
ethers are colorless viscous liquids or solids of low melting point. They are
very much more soluble in all solvents than their aromatic precursors.
1.7 Extraction equilibria involved in solvent extraction with crown ethers
It is worthwhile to consider the extraction equilibria with reference to
crown ether as an extractant. The extraction equilibrium between an aqueous
phase of metal ion Mn+
, counter anion A- and an organic phase of the crown
ether L can be represented by
Mn+
+ Lo + nA- (MLAn)o
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[ MLAn ]o
Kex =
[ Mn+
] [ L ]o [ A-]
n
Where Kex is the extraction equilibrium constant, the subscript ‘O’ refers to
organic phase. The extraction equilibrium is considered to be consists of the
following constituent equilibria.
The distribution of free crown ether between the organic and aqueous
phase:
L Lo
[ L ]o
DL = ---------------
[ L ]
Where DL is the distribution constant of crown ether. The complexation
reaction of crown ether with metal ion in the aqueous phase is represented by
Mn+
+ L MLn+
[ MLn+
]
KML = -------------------
[ Mn+
] [ L ]
Where KML is the complex formation constant.
The ion pair extraction of crown ether-metal ion complex with the
counter anion in the aqueous phase is given by the equation
MLn+
+ A- ( MLAn )O
[ MLAn ]O
Kex’ = ----------------------------
[ MLn+
] [ A- ]
n
If a non polar solvent is used as an organic phase, the dissociation of an ion
pair MLAn in the organic phase will be negligible.
KML . Kex’
Kex = ---------------------
[ MLn+
] [ A- ]
The overall distribution ratio of metal ion can be represented by:
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[ M ]O
D = ------------
[ M ]
Where [M]O and [M] represent total metal concentration in organic phase and
aqueous phase respectively. Since the ion pair formation in the aqueous phase
is minimum due to high dielectric constant of water, we have
[ MLAn ]o
D = -----------------------------
[ Mn+
] + [ MLn+
]
The [ MLAn ]o can be obtained experimentally. If [ Mn+
] >> [ MLn+
] then
D = Kex [ A- ]
n [ L ]O
From the above equations it is evident that in solvent extraction of metals
(Mn+
), with crown ether (L) in presence of a counter anion (A-), the value of D
will be maximum if the [A-] in organic phase is large or [L] in aqueous phase is
large. Alternatively, the magnitude of D will be large if the value of Kex is
largest, which is possible by having large value of (MLAn)O with minimum
value for the concentration of uncomplexed metal ion or maximum value for
the concentration of crown ether in the aqueous phase. Since Kex is directly
proportional to KML and Kex’ and inversely proportional to DL , it is imperative
to have maximum value for KML. This is possible by having maximum
interaction of metal ion with crown ether in the aqueous phase. Further Kex’ can
be largest provided if [MLAn]O is highest. In order to get maximum value for D
and consequently Kex, the magnitude of DL should be smallest, which is
possible by having concentration of crown ether largest in the aqueous phase.
1.8 Factors influencing extraction by crown ethers
Complexes are usually made up of two or more species which are held
together by a force, the binding forces involved are either a pole-pole, pole-
dipole or dipole-dipole in nature. The interaction of cation and crown ether is
primarily of ion-dipole Interaction type. The various factors which affect
extraction with crown ethers include, Distribution of ligand in the aqueous
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phase, Size of the cation, Charge on the cation, Nature of donar atoms, nature
of the diluent and Nature of counter anion (A-). These factors are discussed
below.
a) Distribution of ligand in the aqueous phase
The distribution of crown ether between the aqueous and organic phase
influences the extraction of metal ions. The small DL value gives large value
for Kex If few cryptands are soluble in water then its salt complex is also
soluble in water which inturn lowers the extractability of the complex.
b) Size of the cation
The cavity diameter of the crown ethers and the diameters of the various
cations are shown in Table 1.8.The size of the cation and the cavity diameter of
the crown ether are of great importance. If the size of the cation matches with
the cavity diameter of the crown ether then the metal ion gets extracted [98].
Smaller cations are strongly solvated and more energy is required for
desolvation. On the contrary, larger cations are unable to attract the ligand; on
the whole, the selectivity of metal ions for crown ethers can not always be
ascertained on the basis of the size of the cation as well as on the cavity
diameter of crown ether. Thus, 15-crown-5 shows maximum affinity for
potassium in methanol even though the diameter of sodium ion is closer to the
cavity diameter of 15-crown-5 Some times, the complex formation with
relatively smaller ion in comparison with the cavity diameter of the crown
ether, offers a large electrostatic stabilization energy e.g., extraction of gallium
from hydrochloric acid medium with 18-crown-6 [99].
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Table-1.8 Cation, Ionic and Crown Ether Cavity Diameter
Cation Ionic Diameter
(Ao)
Crown Ether Cavity
Diameter (Ao)
Li+ 1.36 All 12-Crown-4 1.2-1.5
Na+ 1.94 All 15-Crown-5 1.7-2.2
K+ 2.66 All 18-Crown-6 2.6-3.2
Rb+ 2.94 All 21-Crown-7 3.4-4.3
Cs+ 3.34 All 24-Crown-8 4.5-5.0
Ca2+
1.98 All 30-Crown-10 < 6.0
Sr2+
2.24
Ba2+
2.68
Pb2+
1.34
Mo6+
1.22
U6+
1.66
c) Charge on the cation
Charge on the cation plays very important role on the extraction of metal
ions by crown ethers. The cation selectivity’s of various ions with different
charge, can be explained in terms of the difference in their Kex values. When
the charge on the ion is large, invariably the size of the ion will be small,
consequently complex formation will offer a large electrostatic stabilization
energy. e.g., lithium will not form complex with 18-crown-6 because of small
ionic size and low charge but gallium with small size and higher charge , forms
strong complex with 18-crown-6 [99]. Similarly potassium ion (2.66 Ao) and
barium ion (2.68 Ao) are almost identical in size but the selectivity with 18-
crown-6 in methanol is more for barium than potassium. For smaller cations
e.g., sodium (1.94 Ao) and calcium (1.98 A
o) which are similar in size, the
selectivity for sodium ion is more than calcium ion. Thus for 18-crown-6, the
cations which are larger in size and capable of fitting cavity of crown ether
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have the selectivity scale divalent cation > monovalent cation, but for cations
with smaller size the selectivity scale is reversed. This rule is not applicable
everywhere, e.g. Tl+(2.94 A
o ) and Rb
+ ( 2.94 A
o ), the KML values with 18-
crown-6 are 1.23 and 0.62 respectively [100].
d) Nature of donor atoms
The nature of the donor atom plays an important role in determining the
selectivity of the complex, e.g., if the ligand contains sulfur atoms, it enhances
the complexation, thus for silver, dithia-18-crown-6 is better extractant than 18-
crown-6. The stability of Ag+-dithia-18-crown-6 is greater than Ag
+-18-crown-
6 complex. The large size of sulphur atom (1.85 Ao) increases the value of
cation dipole distance and also the increased van der Waal’s repulsion between
the sulphur atom and adjacent oxygen atoms result in the loss of complex
stability [101-102]. If the nitrogen atom is substituted for oxygen atom in 18-
crown-6, the stability of the resulting complex decreases because the van der
Waal’s radius of nitrogen atom (1.5 Ao) is larger than that for oxygen atom (1.4
Ao) and the dipole moment of nitrogen donor atom group is smaller than that of
oxygen donor atom group [103-104], with pyridine nitrogen the stability drops
only slightly [105-106]. When ether oxygen in crown ether is replaced by ester
oxygen groups, the stability of complexes decreases significantly.
e) Nature of diluents
Nature of diluents largely affects the extraction by crown ethers. In
solvent extraction, the extractability and selectivity, with crown ethers are
greatly affected by the organic solvents. The dielectric constant of the diluent
as well as the solubility of crown ether in the organic phase is extremely
important. Danesi et al. [107] investigated the extraction of alkali metal
picrates by dibenzo-18-crown-6 into nitrobenzene- toluene mixtures, in which
the dielectric constant varies from 3.4-35.0. The extraction equilibrium
constant shows that the extractibility of alkali metal ions decreases, as the
diluent composition is varied from pure nitrobenzene to pure toluene. Similar
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103
results were obtained for the selective extraction of KCl with dibenzo-18-
crown-6 and dicyclohexano-18-crown-6 [108-110]. This was explained in
terms of extractibility from the view point of anion solvation [111]. Rais et al.
[112] extracted sodium and cesium dipicryl aminate by dibenzo-18-crown-6
into various solvents such as chloroform, nitrobenzene, methylene chloride,
chlorobenzene, propylene carbonate and nitromethane. The distribution ratio
for Na+ and Cs
+ are the largest for chloroform where as for polypropylene
carbonate and nitromethane it was smallest even though these two diluents are
polar solvents. Iwachido et al. determined distribution ratio for potassium
between aqueous potassium picrate and 57 organic solvents in presence of or
absence of 18-crown-6. The presence of 18-crown-6 enhances the extractibility
of potassium when the halogenated hydrocarbons were used but only slight
enhancement was observed for oxygen containing solvents. Amongst the
various diluents studied for the extraction of same element with crown ethers,
methylene chloride was found to be the efficient solvent for the extractive
separation analysis [113-115].
f) Nature of counter anion
In solvent extraction of metals with crown ethers, the nature of counter
anion is very important. For the same crown ether-metal ion complex, the
extractibility of ion pair into the same organic solvent is largely governed
by the chemical nature of counter anion. In general the counter anion with
large molar volume, such as picrate, dipicryl aminate, tetraphenyl borate
and dinitrophenolate, makes the ion pair extraction more efficient. Other
anions such as thiocyanate [116], iodide [117-119], bromide and
perchlorate [120] have a degree of organophilicity which allow the metal
ion extraction with crown ethers. Yakshin et al. [121] extracted dibenzo-18-
crown-6-alkali metal ion complexes with various inorganic anions in to
ethylene chloride. The extractibility sequence of anion was ClO4- > I
- >NO3
-
> Br- > OH
- > Cl
- > F
-. Crown ethers alone in low polarity solvents do not
extract s- block elements or first row transition elements from dilute
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104
mineral acid solutions, this is because of large amount of energy required to
remove the water of hydration from the anion or to transfer the hydrated
anion to the organic phase [122]. In order to increase the distribution of
metal complex with hard anion such as Cl-, NO3
- and SO4
-2, carboxylic acid
crown ethers were used [123-128], for which metal extraction does not
involve concomitant transfer of the aqueous phase anion into the organic
phase.
The extraction of 13 lanthanides with mixture of 8-hydro-quinoline
(HQ) and crown ethers, dibenzo-18-crown-6 and dibenzo-24-crown-8 in
dichloroethane from chloride medium were investigated [129]. A study on
the highly efficient extraction of cesium ion by using calyx crown ether,
bis(2-propyloxy)calyx[4] crown-6 was reported [130]. Novel pyrazolones,
HPMP-A15C5 and HPMP-A18C6 were used for the extraction of divalent
metal ions such as Mn2+
, Co2+
, Ni2+
, Cu2+
, Zn2+
, Cd2+
and Pb2+
[131]. By
using 18C6, DB18C6, DCH18C6 and DBP18C6 extraction and separation
of La3+,
Ce3+
, Pr3+
, Eu3+
and Er3+
cations in DMSO/water binary mixed
solvent was carried out to inspect the influence of the trichloroacetic acid
as counter ion on the stability and selectivity of the complexes formed
between these cations and macrocyclic ligands [132]. Extraction ability and
selectivity of tetra-aza-crown ethers for transition metal cations were
studied [133]. The derivative of crown such as N-phenylbenzo-18-crown-6-
hydroxamic acid was reported for the extraction and separation of thorium
from monazite sand [134].
1.9 Scope and Methodology
Crown ethers have been used for the various studies pertaining to
extraction equilibrium constant, stability constant and for spectrophotometric
determination of some of the alkali and alkaline earth elements and other
elements from p, d and f-block elements. No systematic efforts were made for
the application of crown ethers in separation studies of various elements in
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105
multicomponent mixtures. Crown ethers have been used extensively in the
field of organic synthesis, for membrane transport studies, phase transfer
catalysis, ion selective electrodes, molecular recognition, chromoionophores,
fluoroionophores and for extractive separation analysis using chromatography
and solvent extraction techniques. Therefore it was thought worthwhile to
undertake systematic extraction and binary mixture separation studies of
thorium(IV), uranium(VI), barium(II), strontium(II), beryllium(II), Lithium(I),
sodium(I), potassium(I), Cesium(I), rubidium(I), cobalt(II), nickel(II),
cadmium(II) and zinc(II) using crown ethers.
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107
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