CHEMISTRY (2CY101)

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SELECTED TOPICS IN CHEMISTRY PHASE TRANSFER CATALYSIS In nucleophilic substitutions the substrate is usually insoluble in water and other polar solvents, while

the nucleophile is anions, which is soluble in water but insoluble in the substrate medium or other organic solvents.

When the two reactants are brought together, their concentrations in the same phase are too low for convenient reaction rates.

To overcome this difficulty either use a solvent that will dissolve both the reactant species or another way called phase transfer catalysis.

A phase transfer catalyst or PTC in chemistry is a type of chemical compound, which facilitates the migration of a particular chemical component in one phase into another phase in a heterogeneous system. The chemical component is soluble in one phase but insoluble in the other unless the phase transfer catalyst is present.

In this method, a catalyst is used to carry the nucleophile from aqueous phase into the organic phase.

For example, simply heating and stirring a two-phase mixture of 1-chlorooctane for several days with aq.NaCN gives no yield of 1-cyanooctane. But if a small amount of an appropriate quaternary ammonium salt is added, the product is quantitatively formed in about 2 hrs.

CH3-(CH2)6-CH2-Cl NaCN no reaction

CH3-(CH2)6-CH2-Cl NaCN PTC

CH3-(CH2)6-CH2-CN + NaCl

+

+

There are two principal types of phase transfer catalyst. But both get the anions into the organic phase and allow it to react with the substrate.

1) QUATERNARY AMMONIUM or PHOSPHONIUM SALTS

In the case of NaCN, the uncatalysed reaction does not take place because the CN¯ ions cannot

cross the interface between the two phases. The reason is that Na+ ions are solvated by the water, and this salvation energy would not present in the organic phase. The CN¯ ions cannot cross

without the Na+ ions. In contrast to Na+ ions, quaternary ammonium (R4N

+) and phosphonium (R4P+) ion with

sufficiently large �R� (alkyl) groups are poorly solvated in water and prefer organic solvents. If a small amount of such a salt is added, three equilibria are set up:

Q + C N - + R C l R C N + Q + C l -

N a +C N - + Q + C l -Q + C N - + N a + C l -

4

2

3

1

organ ic phase

aqueous phase

Q + = R 4N + / R 4P + where

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The Na+ ions remain in the aqueous phase. The Q+ ions cross the interface and carry an anion with them. At the beginning of the reaction, the anion CN¯ gets carried into the organic phase

(equilibrium 1) where it is react with RCl to produce RCN and Cl¯. The Cl¯ then gets carried into the aqueous phase (equilibrium 2). Equilibrium 3, taking place entirely in the aqueous phase, allows Q+CN¯ to be regenerated. All the equilibria are normally reached much faster than the actual reaction (4), so it is the rate-

determining step.

2) CROWN ETHERS

Crown ethers are cyclic polyethers. These compounds are called Crown Ethers because of their crown-like shape.

The first number in their common names indicates the ring size, and the second number indicates the number of oxygen atoms in the ring.

O

OO

OOO

O

O

O

O

OOO

O O

O

O

O

O

15-Crown-5 18-Crown-612-Crown-4 14-Crown-4

O

OO

OO

O

O

O

O

O

O

Dibenzo-[18]-Crown-6Benzo-[15]-crown-5 In place of O atom other heteroatoms like S and N may also be present. If S is the heteroatoms,

these are called thiacrown ether and if N is the heteroatom, called azacrown ether.

N

NN

NN

S

S

S

S

Crown ethers have the unique property of forming complexes with positive ions like Na+, K+, Li+

and transition metal ions. These ions are held tightly in the centre of the cavity.

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For example [12]-crown-4 bind Li+ but not K+ while Dicyclohexyl-[18]-crown-6 binds K+ but not Li+. Similarly [15]-crown-5 bind Hg2+ but not Cd2+ or Zn2+ or Sr2+.

This complexing ability is so strong that ionic compounds can be dissolved in organic solvents that contain some crown ether. For example Potassium permanganate (KMnO4) is soluble in water but insoluble in benzene. However, if [18]-crown-6 are dissolved in benzene, it is possible to extract the potassium permanganate from water into benzene. The resulting �Purple Benzene� contains free permanganate ions and is a powerful oxidizing agent.

SYNTHESIS of Dibenzo-[18]-crown-6

This was the first compound synthesized by Pedersen. It is obtained by the treatment of catechol

with a base to form a dianions; this nucleophile is made to react with 2,2�-dichlorodiethyl ether by

refluxing in n-butanol.

OH

O

O

O

OH

OH

OH

+Cl

O

Cl

NaOH, n-BuOH

reflux

O

O

O

O OO

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3) CRYPTANDS or CRYPTATES

Cryptands are three- dimensional analogues of the crown ethers but are more selective and

complex the guest ions more strongly. The resulting complexes are lipophilic.

Cryptands are a family of synthetic bi- and polycyclic multidentate ligands for a variety of

cations. The Nobel Prize for Chemistry in 1987 was given to Donald J. Cram, Jean-Marie Lehn,

and Charles J. Pedersen for their efforts in discovering and determining uses of cryptands and

crown ethers, thus launching the now flourishing field of supramolecular chemistry.

The three-dimensional interior cavity of a cryptand provides a binding site for "guest" ions. The

complex between the cationic guest and the cryptand is called a cryptate. Cryptands form

complexes with many "hard cations" including NH4+, lanthanides, alkali metals, and alkaline

earth metals. In contrast to typical crown ethers, cryptands bind the guest ions using both nitrogen

and oxygen donors. Their three-dimensional encapsulation mode confers some size-selectivity,

enabling discrimination among alkali metal cations.

Cryptands although they are more expensive and more difficult to prepare offer much better

selectivity and strength of binding than other complexants for alkali metals, such as crown ethers.

N

O O

NOO

OO

Crptand [2.2.2]

NO

NOO

OO

Crptand [2.2.1] ADVANTAGE OF PHASE TRANSFER CATALYSIS

Do not require vigorous conditions and the reactions are fast.

Do not require high temperature.

There is no need for anhydrous conditions since water is used as one of the phase.

Phase transfer catalysts are especially useful in green chemistry -by allowing the use of water, the

need for organic solvents is reduced.

With the helps of PTC, the anion is available in organic solvents and so its nucleophilicty

increases.

PTC increases the yields and purity of the product and also suppresses the side products.

PTC provides a much simpler procedures for the reactions and for the isolation of the products.

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CHEMISTRY (2CY101)

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FULLERENES Fullerene is an allotrope of carbon. molecules composed entirely of carbon, in the form of a hollow sphere, ellipsoid, or tube. Spherical

fullerenes are also called buckyballs, and cylindrical ones are called carbon nanotubes or buckytubes.

Fullerenes are similar in structure to graphite, which is composed of a sheet of linked hexagonal rings, but they also contain pentagonal (or sometimes heptagonal) rings that prevent the sheet from being planar.

Fullerenes were discovered in 1985 by Robert Curl, Harold Kroto and Richard Smalley and are named after Richard Buckminster Fuller, an American architect who designed geodesic dome structures based on hexagons and pentagons. The shape of C60 resembles that of such domes designed by Fuller.

PREPARATION Fullerenes are prepared by vapourizing a graphite rod in a helium atmosphere. Mixture of Fullerenes like C60, C70, etc, are formed which are separated by solvent extraction. Pure C60 is isolated from this mixture by column chromatography. STRUCTURE The C60 molecule has a truncated icosahedron structure. An icosahedron is a polygon with 60

vertices and 32 faces, 12 of which are pentagonal and 20 hexagonal. A carbon atom is present at each vertex of this structure. The molecule is aromatic and has several resonance structures. The valencies of each C atom are satisfied by two single and one double bond. C60 is also known as bulky ball as it is a spherical cluster of C atoms arranged in series of 5- and 6-

membered rings to form a soccer ball shape. PROPERTIES

It is black powdery materials. It forms deep magenta colour solution, when dissolve in benzene. It is rigid and thermally stable. It can be compressed to lose 30% of its volume without destroying its carbon cage structure.

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APPLICATIONS It is suitable for use as a lubricant due to its spherical structure. It can be used as superconductor when mixed with alkali metals. It can be used as soft ferromagnets. Other possible areas of uses are:- (1) electronic and microelectronic devices.

(2) non- linear optical devices. (3) composites. (4) drug delivery to their respective sites

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CHEMISTRY (2CY101)

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NANOMATERIALS

Nanoscience

Is the study of phenomena and manipulation of materials at atomic, molecular and

macromolecular scales, where properties differ significantly from those at larger scale.

Nanotechnologies

Are the design, characterization, production and application of structures, devices and systems by

controlling shape and size at nanometer scale.

Nanomaterials

Nanomaterials are those, which have structured components with at least one dimension less than

100nm. Materials that have one dimension in the nanoscale (and are extended in the other two

dimensions) are layers, such as a thin films or surface coatings. Some of the features on computer chips

come in this category. Materials that are nanoscale in two dimensions (and extended in one dimension)

include nanowires and nanotubes. Materials that are nanoscale in three dimensions are particles, for

example precipitates, colloids, and quantum dots (tiny particles of semiconductor materials).

Properties of Nanomaterials

Two principal factors cause the properties of nanomaterials to differ significantly from other

materials: increased relative surface area, and quantum effects. These factors can change or enhance

properties such as reactivity, strength, and electrical characteristics.

The properties of materials can be different at the nanoscale for two main reasons. First,

nanomaterials have a relatively larger surface area when compared to the same mass of material produced

in a larger form. This can make materials more chemically reactive, and affect their strength or electrical

properties. Second, quantum effects can begin to dominate the behaviour of matter at the nanoscale -

particularly as the structure or particle size approaches the smaller end - affecting the optical, electrical

and magnetic behaviour of materials. Materials can be produced that are nanoscale in one dimension (for

example, very thin surface coatings), in two dimensions (for example, nanowires and nanotubes) or in all

three dimensions (for example, nanoparticles).

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Applications of Nanomaterials

a) Sunscreens and Cosmetics

Nanosized titanium dioxide and zinc oxide are currently used in some sunscreens, as they absorb

and reflect ultraviolet (UV) rays and yet are transparent to visible light and so are more appealing to the

consumer.

b) Composites

An important use of nanoparticles and nanotubes is in composites, materials that combine one or

more separate components and which are designed to exhibit overall the best properties of each

component. This multi-functionality applies not only to mechanical properties, but extends to optical,

electrical, and magnetic ones. Currently, carbon fibres and bundles of multi-walled CNTs are used in

polymers to control or enhance conductivity, with applications such as antistatic packaging. The use of

individual CNTs (Carbon Nano Tubes) in composites is a potential long-term application. A particular

type of nanocomposite is where nanoparticles act as fillers in a matrix; for example, carbon black used as

a filler to reinforce car tyres.

c) Coatings and Surfaces

Wear and scratch-resistant hard coatings are significantly improved by nanoscale intermediate

layers (or multilayers) between the hard outer layer and the substrate material. The intermediate layers

give good bonding and graded matching of elastic and thermal properties, thus improving adhesion. A

range of enhanced textiles, such as breathable, waterproof and stainresistant fabrics have been enabled by

the improved control of porosity at the nanoscale and surface roughness in a variety of polymers and

inorganics.

d) Tougher and Harder Cutting Tools

Cutting tools made of nanocrystalline materials, such as tungsten carbide, tantalum carbide and

titanium carbide, are more wear and erosion-resistant, and last longer than their conventional (large-

grained) counterparts. They are finding applications in the drills used to bore holes in circuit boards.

e) Fuel Cells

Engineered surfaces are essential in fuel cells, where the external surface properties and the pore

structure affect performance. The potential use of nano-engineered membranes to intensify catalytic

processes could enable higher-efficiency, small-scale fuel cells. These could act as distributed sources of

electrical power.

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f) Displays

The huge market for large area, high brightness, flat-panel displays, as used in television screens

and computer monitors, is driving the development of some nanomaterials. Nanocrystalline zinc selenide,

zinc sulphide, cadmium sulphide and lead telluride are candidates for the next generation of light-

emitting phosphors. CNTs are being investigated for low voltage field-emission displays; their strength,

sharpness, conductivity, and inertness make them potentially very efficient and long-lasting emitters.

g) Medical Implants

Current medical implants, such as orthopedic implants and heart valves, are made of titanium and

stainless steel alloys, primarily because they are biocompatible. Unfortunately, in some cases these metal

alloys may wear out within the lifetime of the patient. Nanocrystalline zirconium oxide (zirconia) is hard,

wearresistant, bio-corrosion resistant and bio-compatible. It therefore presents an attractive alternative

material for implants. It and other nanoceramics can also be made as strong, light aerogels.

Nanocrystalline silicon carbide is a candidate material for artificial heart valves primarily because of its

low weight, high strength, and inertness.

h) Paints

Incorporating nanoparticles in paints could improve their performance, for example by making

them lighter and giving them different properties. Thinner paint coatings (�lightweighting�), used for

example on aircraft, would reduce their weight, which could be beneficial to the environment. However,

the whole life cycle of the aircraft needs to be considered before overall benefits can be claimed. It may

also be possible to substantially reduce solvent content of paints, with resulting environmental benefits.

New types of fouling-resistant marine paint could be developed and are urgently needed as alternatives to

tributyl tin (TBT), now that the ecological impacts of TBT have been recognized. Anti-fouling surface

treatment is also valuable in process applications such as heat exchange, where it could lead to energy

savings. If they can be produced at sufficiently low cost, fouling-resistant coatings could be used in

routine duties such as piping for domestic and industrial water systems. Other novel, and more long-term,

applications for nanoparticles might lie in paints that change colour in response to change in temperature

or chemical environment, or paints that have reduced infra-red absorptivity and so reduce heat loss.

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i) Catalysts

In general, nanoparticles have a high surface area, and hence provide higher catalytic activity.

Nanotechnologies are enabling changes in the degree of control in the production of nanoparticles, and

the support structure on which they reside. It is possible to synthesise metal nanoparticles in solution in

the presence of a surfactant to form highly ordered monodisperse films of the catalyst nanoparticles on a

surface. This allows more uniformity in the size and chemical structure of the catalyst, which in turn

leads to greater catalytic activity and the production of fewer byproducts. It may also be possible to

engineer specific or selective activity. These more active and durable catalysts could find early

application in cleaning up waste streams. This will be particularly beneficial if it reduces the demand for

platinum-group metals, whose use in standard catalytic units is starting to emerge as a problem, given the

limited availability of these metals.

j) Magnetic Materials

It has been shown that magnets made of nanocrystalline yttrium�samarium�cobalt grains possess

unusual magnetic properties due to their extremely large grain interface area (high coercivity can be

obtained because magnetization flips cannot easily propagate past the grain boundaries). This could lead

to applications in motors, analytical instruments like magnetic resonance imaging (MRI), used widely in

hospitals, and microsensors. Overall magnetisation, however, is currently limited by the ability to align

the grains� direction of magnetisation.

Nanoscale-fabricated magnetic materials also have applications in data storage. Devices such as computer

hard disks depend on the ability to magnetize small areas of a spinning disk to record information. If the

area required to record one piece of information can be shrunk in the nanoscale (and can be written and

read reliably), the storage capacity of the disk can be improved dramatically. In the future, the devices on

computer chips which currently operate using flows of electrons could use the magnetic properties of

these electrons, called spin, with numerous advantages. Recent advances in novel magnetic materials and

their nanofabrication are encouraging in this respect.

k) Military Battle Suits

Enhanced nanomaterials form the basis of a state-of- the-art �battle suit� that is being developed

by the Institute of Soldier Nanotechnologies at MIT. A short-term development is likely to be energy-

absorbing materials that will withstand blast waves; longer-term are those that incorporate sensors to

detect or respond to chemical and biological weapons (for example, responsive nanopores that �close�

upon detection of a biological agent). There is speculation that developments could include materials

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which monitor physiology while a soldier is still on the battlefield, and uniforms with potential medical

applications, such as splints for broken bones.

l) Machinable Ceramics

Ceramics are hard, brittle and difficult to machine. However, with a reduction in grain size to the

nanoscale, ceramic ductility can be increased. Zirconia, normally a hard, brittle ceramic, has even been

rendered superplastic (for example, able to be deformed up to 300% of its original length).

Nanocrystalline ceramics, such as silicon nitride and silicon carbide, have been used in such automotive

applications as high-strength springs, ball bearings and valve lifters, because they can be easily formed

and machined, as well as exhibiting excellent chemical and high-temperature properties. They are also

used as components in high-temperature furnaces. Nanocrystalline ceramics can be pressed into complex

net shapes and sintered at significantly lower temperatures than conventional ceramics.

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ZIEGLER-NATTA SYNTHESIS

Zeigler-Natta catalyst is very important for polymerizing ethene into polyethene.(coordination Polymerization)

Zeigler-Natta catalyst is the solutions of triethyl aluminium [Al(C2H5)3]and titanium tetrachloride (TiCl4) in a hydrocarbon solvent.

The triethyl aluminium [Al(C2H5)3]and titanium tetrachloride (TiCl4) catalyst is of great commercial importance. It produces stereo-regular polymers (i.e polymers where the molecules have the same orientation). These polymers are stronger and have higher melting points than random polymers.

Practically any alkene can be polymerized.

n CH2=CH2 CH2-CH2[ ]n

Z.N catalyst

How The Catalyst Work? The active species is Ti3+ and the Al(C2H5)3 can reduce TiCl4 to TiCl3 in situ. The one of the Cl atoms is replaced by an ethyl group. A possible mechanism is that the double bond in ethene attaches itself to a vacant site on a Ti

atom on the surface of the catalyst. A carbon shift reaction occurs, and the ethene migrates and is inserted between Ti and C in the Ti-

C2H5 bond. This extends the C chain from two to four atoms, leaving a vacant site on Ti. The process is repeated, and the C chain grows in length.

Ti

ClCl

ClCl

Al(C2H5)3 TiCl

ClCl

CH2

CH3

TiCl

ClCl

CH2

CH3

CH2=CH2

TiCl

ClCl

CH2

CH3

CH2

CH2

TiCl

ClCl

CH2

CH3

CH

2

CH2

TiCl

ClCl

CH2

CH2

CH2

CH3

CH2=CH2Ti

Cl

ClCl

CH2

CH2

CH2

CH3

CH2

CH2

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