Part II Chapter 1 - INFLIBNETshodhganga.inflibnet.ac.in/bitstream/10603/10067/6/06_chapter 1 part ii.pdfPart II Chapter 1: Introduction to crown ethers Part II Chapter 1: Introduction

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


91

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,


92

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.


93

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

+


94

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


95

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


96

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


97

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


98

[ 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:


99

[ 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


100

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].


101

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


102

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


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


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


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.


106

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