- Water
- Intermolecular Force
- Polymer
- Atomic And Molecular Structure
- Phase Rule
- Aromatic System And Molecular Structure
Unit: WATER (H2O)
SR
TOPICS
NO.
1.
INTRODUCTION
2.
HARDNESS OF WATER-UNITS,NUMERICAL PROBLEMS
3.
HARDNESS OF WATER-(EDTA METHOD)
NUMERICAL PROBLEMS
4.
SOFTENING OF WATER-(ION EXCHANGE METHOD)
5.
BOD & COD (WITH NUMERICALS)
6.
PURIFICATION OF WATER(REVERSE OSMOSIS)
Introduction:
The pure water is composed of two parts of hydrogen and one part of oxygen by volume and dissolves
many substances. These dissolved salts are the impurities in water. Water is a very good solvent. So it is
called as universal solvent.
Hardness of water: - The water which does not give lather with soap is called Hard water. The Hard
water contains dissolved calcium & magnesium salts.
Soft water: - The water which can give lather with soap easily is called as soft water.
Na-stearate + H 2 O
Soap
(soft water)
Types of Hardness:- Hardness in water is of two types.
● Temporary hardness:-
The hardness that can be removed simply by boiling is called the temporary hardness. It is due to the
presence of boiling. On boiling Ca(Hco3)2, Mg(Hco3)2 are precipitated as insoluble salts. This type of
hardness can be identified as the carbonates and bicarbonates.
● Permanent Hardness:-
Permanent hardness cannot be removed by boiling . It is due to CaCl2, CaSO4, MgCl2, MgSO4 and
nitrates in H2O. These salts cannot remove this hardness. Fe3+, Al3+ & Mn2+ also cause hardness in
water. . This type of hardness can be identified as the non-carbonates and
non-bicarbonates.
Units of Hardness:-
(1) Parts per million (ppm):- It is the number of parts of equivalents of CaCO3 hardness causing salt
present in one million parts(10 6 parts ) of water.
(2).Milligram per litre(mg/l):-It is the number of milligrams of equivalentof CaCO3 per litre of hard water.
E.g.:- 1mg/li means 1 mg of equivalent caco3 present in litre of hardwater.
(3)Degree Clarke (o cl)
(4)Degree French (o Fr)
Inter conversion: - 1ppm=1mg/l == 0.07 o cl = 0.10 Fr
1 o cl =1.43o Fr = 14.3 ppm=14.3 mg/l
The most commonly used units are ppm or m=Mg/L
NOTE:
UNIT CONVERSION IMPORTANT FOR NUMERICAL PROBLEMS
Determination of Hardness of Water: -
(1)EDTA method:- In EDTA methods, the known water sample is titrated against standard EDTA solution
using EBT as indicator in the presence of basic buffer solution(PH=10). At the end point the wine red
color changes to blue.
Solved examples:
Determination of Hardness of Water by Winkler’s method:
FORMULAE:
D.O = Normality of Hypo×V×8×1000
Volume of the Sample
Numerical problems on Hardness of Water :
Equivalent weight =
molecular weight
n
Where n is valence electrons or the electrons shared.
Example: Al 2 (SO 4 ) 3
HERE n=6 because Aluminium sharese electrons
3. One litre of water from an underground reservoir in tirupati town in Andhra Pradesh showed
the following analysis for its contents. Mg (HCO 3 ) 2 = 42 Mg, Ca(HCO 3 ) 2 = 146 Mg, CaCl 2 = 71
Mg, NaOH= 40 Mg, MgSO 4 =48 Mg,organic impurities=100 Mg, Calculate temporary,
permanent and total hardness?
SOLUTION:
Hardness causing
Quantity (H.C.S)
Mol.Wt.of (H.C.S
Equivalent CaCO 3
salt (H.C.S)
of
CaCl2
71
111
71*100
= 64
111
MgSO4
48
120
48*100
= 40
120
Ca(HCO 3 ) 2
146
162
146*100
= 90.1
162
Mg(HCO 3 ) 2
42
146
42*100
= 28.1
146
NaOH
40
-
-
Temporary Hardness= Mg(HCO 3 ) 2 +Ca(HCO 3 ) 2
= 28.7+90.1=118.8ppm
Permanent Hardness=CaCl 2 +MgSO 4
= 64+40 = 104ppm
Total Hardness=Temporary Hardness + Permanent Hardness
= 118.8+104=222.8ppm
(4) A sample of hard water contains the following dissolved salts per liter CO 2 =44Mg, Ca (HCO 3 ) 2
=16.4Mg, Mg (HCO 3 ) 2 =14.6 Mg CaCl 2 =111 Mg, MgSO4=12 Mg, &CaSO 4 =13.6 Mg. Calculate the
temporary & Permanent hardness of water in °Fr &° Cl. (2013)
SOLUTION:
S.No
Constituent
Amount Mg/l
Mol.wt. of salt
Equivalent of
CaCO3(Mg/l)
1.
CO2
44
44
44*100 = 100
44
2.
Ca(HCO 3) 2
16.4
162
16.4*100 = 10
162
3.
Mg(HCO 3 ) 2
14.6
146
14.6*100 = 10
146
4.
CaCl2
111
111
111*100 = 100
111
5.
MgSO4
12
120
12*100 = 10
120
6.
CaSO4
13.6
136
13.6*100 = 10
136
Temporary hardness of water= CO2+Ca(HCO 3 ) 2 +Mg(HCO 3 ) 2
=100+10+10=120 mg/l
Permanent hardness of water=CaCl 2 +MgSO 4 +CaSO 4
=100+10+10=120 mg/l
Conversion of hardness:-
1ppm = 1 mg/l = 0.07 °cl = 0.1 °fr
Temporary hardness = 120 mg/l, 120 ppm, 120*0.07 = 8.4 °cl
= 120*0.1 = 12° French
Permanent hardness = 120 mg/l, 120 ppm, 120*0.07 = 8.4°cl
= 120*0.1= 12°french.
Practice Problems :
1) One liter of water from khammam Dist in A.P showed the following analysis. Mg(HCO3)2=0.0256
gms,Ca(HCO3)2=0.0156 gms, NaCl=0.0167gms, CaSO4=0.0065gms, and MgSO4=0.0054gms. Calculate
temporary, Permanent & total hardness.
2) Calculate the temporary & permanent hardness of 100 litre of water containing the following
impurities per litre MgCl2=19 mg, MgSO4=60 mg, NaCl=36.5 mg, CaCl2=11.1 mg, Ca(HCO3)2=32.4 mg &
Mg(HCO3)2=7.3 mg
3) Calculate the lime and soda needed for softening 50,000 litres of water containing the following salts:CaSO4
= 136 mg/l, MgCl2=95mg/l, Mg(HCO3)2 = 73 mg/l, Ca(HCO3)2= 162 mg/l. given that molar mass of
Ca(HCO3)2 is 162 and that of MgCl2 is 95.
Ion exchange process (or) deionization or demineralization:-
Ion exchanges are of two types:
•Anionic
•Cationic.
These are co-polymers of styrene & divinyl benzene.i.e. Long chain organic polymers with a micro porous
structure.
• Cation exchange resins:- The resins containing acidic functional groups such as -COOH,-SO3H etc. are
capable of exchanging their H+ ions with other cations are cation exchange resins , represented as RH +
• Anion exchange resins:- The resins containing amino or quaternary ammonium or quaternary
phosphonium(or) Tertiary sulfonium groups, treated with “NaoH solution becomes capable of
exchanging their oHioans with other anions. These are called as Anion exchange resins represented as ROH-
Numerical problem:
Disadvantages of hard water in domestic and Industrially In Domestic use:-
Washing:- Hard water, when used for washing purposes, does not producing lather freely with soap. As
a result cleaning quality of soap is decreased and a lot of it is wasted. Hard water reacts with soap it
produces sticky precitates of calcium & Mg soaps. These are insoluble formations.
(a) Bathing:- Hard water does not produce lather freely with soap solution, but produces sticky scum on
the bath-tub and body. Thus, the cleaning quality of soap is depressed and a lot of it is wasted.
(b) Cooking:- The boiling point of water is increased because of presence of salts. Hence more fuel and
time are required for cooking.
(c) Drinking:- Hard water causes bad effects on our digestive system. Moreover, the possibility
of formg calcium oxalate crystals in urinary tracks is increased.
Industrial use:-
(a) Textile industry:- Hard water causes wastage of soap. Precipitates of calcium and magnesium
soaps adhere to the fabrics and cause problem.
(b) Sugar Industry:- Water containing sulphates, nitrates, alkali carbonates etc. if used in sugar
refining, causes difficulties in the crystallization of sugar. Moreover, the sugar so produced may be de-
liquiscent.
(c) Dyeing industry:- The dissolved salts in hard water may reacts with costly dyes forming precipitates.
(d) Paper Industry:- Calcium, magnesium, iron salts in water may affect the quality of paper.
(e) Pharmaceutical Industry:-Hard water may cause some undesirable products while preparation
of pharmaceutical products.
(f) Concrete making:- water containing chlorides and sulphates, if used for concrete making, affects
the hydration of cement and the final strength of the hardened concrete.
(g) Laundry:- Hard water, if used in laundry, causes much of the soap used in washing to go as
waste iron salts may even causes coloration of the cloths.
Disinfection:-
The process of destroying/killing the disease producing bacteria, micro organisms, etc, from the water
and making it safe are, is called Disinfection.
Disinfectants:- The chemicals or substances which are added to eater for killing the bacteria. The
disinfection of water can be carried out by following methods.
(a) Boiling:- Water for 10 -15 min.boiled,all the disease producing bacteria are killed and water
become safe for use.
(b) Bleaching powder:-
It is used to purity the drinking water from micro organisms. The purification process is achieved by dissolving
1 kg of bleaching powder in 1000 kilo litres of water. This dissolved water solution is left undisturbed for many
hours when bleaching powder is mixed with water, the result of chemical reaction produces a powerful
Germicide called Hypochlorous acid. The presence of chlorine in the bleaching powder produces disinfection
action, kills germs and purifies the drinking water effectively.
CaOCl2+H2O → Ca(OH)2+Cl2
H2O+Cl2→HCl+HOCl
HOCl+ germs → germs are killed → water purified.
(c) Chlorination:-
Chlorination is the process of purifying the drinking water by producing a powerful
Germicide like hypochlorous acid. When this chlorine is mixed with water it produces
Hypochlorous acid which kills the Germs present in water.
H2O+Cl2→ HOCl+HCl
Chlorine is basic (means PH value is more than 7) disinfectant and is much effective over
the germs. Hence chlorine is widely used all over the world as a powerful disinfectant.
Chlorinator is an apparatus, which is used to purity the water by chlorination process.
(d) Ozonisation:-
Ozone is powerful disinfectant and is readily dissolved in water. Ozone being unstable
decomposes by giving nascent oxygen which is capable of destroying the Bacteria.This nascent
oxygen removes the colour and taste of water and oxidizes the organic matter present in water.
O3 → O2+ [O]
Biological oxygen demand (bod):
Biochemical oxygen demand (BOD) represents the amount of oxygen consumed by bacteria and
other microorganisms while they decompose organic matter under aerobic (oxygen is present)
conditions at a specified temperature. When you look at water in a lake the one thing you don't see
is oxygen.
Formula:
Numerical problem:
Chemical oxygen demand(COD):
The chemical oxygen demand (COD) is a measure of water and wastewater quality. The COD
test is often used to monitor water treatment plant efficiency. This test is based on the fact that a
strong oxidizing agent, under acidic conditions, can fully oxidize almost any organic compound
to carbon dioxide.
Formula:
Numerical problem:
Desalination:
The removal of dissolve solids (NaCl) from water is known as desalination process. It can be carried out
by:
(1) Reverse osmosis and (2) electro dialysis.
Reverse osmosis process:-
The membrane process used in the water purification system has been of much use now a days.
Electro dialysis and reverse osmosis are part of the membrane process. In osmosis, if a semi-
permeable membrane separates two solutions, solvent from the lower concentration passes to the
higher concentration to equalize the concentration of both. But in the reverse osmosis, pressure
higher than osmotic pressure is applied from the higher concentration side so that the path of the
solvent is reversed, i.e. from higher concentration to lower concentration.
This method is applicable mainly for the desalination of sea water. Sea water and pure water are
separated by a semi-permeable membrane made up of cellulose a cetate fitted on both sides of a perforated tube.
Inventions are in progress to search for better membrane Polymethylmethacrylate and polyamides have been
proved to be better membranes.
The process is very easy. It is used to make pure water. It removes the ionic and non ionic
substances in the water. It also can remove suspended colloidal particles. The life of a membrane is
nearly 2 years and it should be replaced after this period. By this process, sea water is made to fit
for drinking water obtained after being treated by this process is used in boilers.
UNIT: INTERMOLECULAR FORCES
SR
TOPICS
NO.
1.
INTRODUCTION
2.
ION INTERACTION
3.
VAN DER WAAL’S INTERACTION
4.
DIPOLAR INTERACTION
5.
HYDROGEN BONDING
6.
ION-DIPOLE FORCE
7.
EQUATIONS OF STATE OF REAL GASES
AND CRITICAL PHENOMENA
INTRODUCTION:
Intermolecular forces are the forces which mediate interaction between the molecules, including forces
of attraction or repulsion which act between the molecules and other types of neighboring particles, e.g.
atoms or ions. Intermolecular forces are weak relative to intramolecular forces (the forces which hold a
molecule together)
E.g. the covalent bond, involving sharing electron pairs between atoms, is much stronger than the
forces present between neighboring molecules.
IONIC INTERACTIONS:
Ionic bonds are formed due to transfer of one or more electrons from one atom to the other between a
metal and non-metal atom. The metallic atom loses its electron present in its valence shell and converts
into a cation. The non-metallic atom gains electrons and converts into an anion. The electrostatic force
of attraction holds the oppositely charged ions together. The number of electrons that an atom gains or
loses while forming an ionic bond is called its electrovalency. The atom which loses electrons is called
electropositive and the one which gains electrons is called electronegative atom. Since ionic bond is the
strongest bond, it takes a lot of energy to break it. The greater the charge difference, the stronger the
attraction.
VAN DER WAAL’S INTERACTIONS:
This is the weak attractive intermolecular force present in all molecules and atoms. It is known as Van
der Waals forces or dispersion forces or London forces. They occur between the molecules.
•Van der Waals forces explain the condensation of gases and freezing of liquids on cooling.
•High molecular weight indicates more electrons and more powerful attractive forces than Van der Waals forces.
This is the explanation for why high molecular weight compounds tend to be solids or liquids and low
molecular weight compounds tend to be gases.
•At the boiling point of a liquid, the amount of molecular agitation is enough to overcome Van der Waals force
of attraction. Hence, boiling point is the measure of these forces. This explains the periodic trend in boiling
points for the noble gases
e.g. Ne boils at much lower temperature than Xe. Since Ne is having lesser
electrons than Xe, its Van der Waals forces are more easily overcome by thermal
motion.
DIPOLAR INTERACTIONS:
The dipole-dipole force exists in all polar molecules. Polar molecules have permanent dipoles that
interact with the permanent dipoles of neighboring molecules. The positive end of one permanent
dipole is attracted to the negative end of another permanent dipole. This attraction is the dipole-dipole
force.
The distance between the two dipoles and their orientation determines the dipoledipole interaction. All
molecules including the polar ones have dispersion forces. In addition to dispersion forces, polar
molecules have dipole-dipole forces. These additional forces raise the melting and boiling points of the
polar molecules. Non-polar molecules having same molecular weight and shape but lacking the presence
the dipole movement has relatively low melting and boiling points. The same is illustrated by comparing
Formaldehyde and Ethane.
Formaldehyde being polar, it shows higher melting point and boiling point than nonpolar ethane despite
their molecular weights being almost the same. The polarity of molecules also helps in determining the
liquids miscibility i.e. its ability to mix without separating into two phases. Polar liquids are miscible with
other polar liquids, but are not miscible with non-polar liquids e.g. water, a polar liquid does not mix
with the oil, a non-polar liquid.
HYDROGEN BONDING:
Polar molecules containing hydrogen atoms bonded directly to fluorine, oxygen or nitrogen exhibit an
additional intermolecular force called a hydrogen bond e.g. HF, NH3 and H2O show hydrogen bonding.
The large electronegativity difference between hydrogen and these electronegative elements, as well as
the small size of these atoms gives rise to a strong attraction between the hydrogen in each of these
molecules and the F, O or N on neighboring molecules. This attraction between a hydrogen atom and
an electronegative atom is the hydrogen bond.
The example:
Similarly, the hydrogen atom in each water molecule is hydrogen bonded to the oxygen in four other
water molecules
Intramolecular Hydrogen Bond:
The hydrogen bond formed between hydrogen and an electronegative atom (F, O & N) within the same
molecule is an intramolecular hydrogen bond. It results in the cyclization of the molecules and prevents
their association. It does not affect physical properties of the compound. The intramolecular hydrogen
bonds present in different molecules.
The examples :
The intramolecular hydrogen bonding is important in biological molecules as the shapes of proteins and
nucleic acids are largely influenced by it e.g. the two strands of the double helix in DNA are held
together by hydrogen bond between hydrogen atoms of one strand with the lone pairs on the nitrogen
or oxygen on the other strand.
Intermolecular Hydrogen Bond:
The hydrogen bond formed between the hydrogen atom of one molecule and an electronegative atom
of the other molecule is called Intermolecular hydrogen bond e.g. Hydrogen Chloride, Water, Ammonia,
Alcohol etc. Effect of intermolecular hydrogen bonding is reflected in many properties of the
compounds such as increase in the melting point, boiling point, solubility etc.
Effects of hydrogen bonding:
1. Solubility : Solubility of substances in certain solvents is influenced by hydrogen bonding. Covalent
compounds generally do not dissolve in water. But those which form a hydrogen bond with water
readily dissolve in it e.g. Ammonia, Amines, Ethanol, Lower Aldehydes and Ketones are soluble in water
due to the formation of hydrogen bonds between hydrogen atom of water molecule and the
electronegative atom of these molecules.
2. Physical State : Intermolecular hydrogen bonding causes two or more molecules of a compound to
exist as associated molecules. This results in an increase in the size and molecular mass of the
compound which gets reflected in the physical state of the substance e.g. H2O is liquid and H2S is a gas,
although oxygen and sulphur belong to the same group. In water, oxygen is highly electronegative and
forms intermolecular hydrogen bonds. It results in getting water molecules associated due to which
molecular mass is increased. Hence, water exists as liquid at room temperature. On the other hand,
electronegativity difference of hydrogen and sulphur is less and there is negligible hydrogen bonding in
H2S. They are not associated and hence H2S exists as a gas at room temperature. The same applies for
existence of HF as liquid and HCL as a gas at room temperature.
3. Melting and Boiling Point : Hydrogen bonds are very strong intermolecular forces. Due to hydrogen
bonding and a consequent association of molecules, larger energy is required to separate these
molecules before they can melt or boil. Therefore these compounds usually show elevated melting and
boiling points e.g. methanol and ethane as shown in the Table 3.2
ION-DIPOLE FORCE:
The ion-dipole force occurs in the mixtures of ionic compounds and polar compounds. It is very
important in aqueous solutions of ionic compounds. e.g. When NaCl is mixed with water, the sodium
and chloride ions interact with water molecules via. ion-dipole forces. Ion-dipole forces are the
strongest of all types of intermolecular forces. They are responsible for the ability of ionic substances to
form solutions with water.
COMPARISON OF ALL FORCES:
EQUATIONS OF STATE OF REAL GASES AND CRITICAL PHENOMENA:
Critical Phenomena: The essential condition for the liquefaction of the gas is described by the study of
critical temperature, critical pressure and critical volume and their inter relationships.
UNIT:POLYMER
SR
TOPICS
NO.
1.
INTRODUCTION-CLASSIFICATION
2.
MOLECULAR WEIGHT-NUMERICALS
3.
GLASS TRANSITION TEMPERATURE,VISCOELASTICITY
4.
FABRICATION OF PLASTICS
5.
PREPARATION,PROPERTIES & USES OF PMMA, KEVLAR
6.
PREPARATION,PROPERTIES & USES OF PHENOL FORMALDEHYDE, UF
7.
CONDUCTING POLYMERS
8.
POLYMERS IN MEDICINE
Introduction:
• Polymer= Poly (Many), Mer (unit)
• When many monomers are linked together to form a chain, a polymer molecule is formed
Useful polymeric properties:
• High tensile strength
• Toughness
• Impact resistance: High force and shock- life and durability
• Melt viscosity
• High melting temperature
Classification (also examples of each type):
• Origin: Natural, Derived, Synthetic
Natural Polymers: found in animals and plants.
Eg. Cellulose, starch, proteins, resins etc.
Semi-synthetic/Derived Polymers : Chemically modified natural polymers
Eg. Cellulose derivatives: cellulose nitrate, rayon, cellophane etc.
Synthetic Polymers: Man-made polymers.
Eg. Polythene, Polystyrene, PET, Nylon, Buna-S etc.
• Ultimate forces: Plastic, Fibres, Elastomers, Resins
Thermoplastic: Weak interaction force, low m.p., can be moulded easily.
Eg. Polyethylene, Polypropylene, PVC etc
Thermosetting: Semi-fluid, cross-linking between polymer chains (on heating), hard and infusible. This
reaction is irreversible in nature.
Eg. Bakelite
Fibres: high length/diameter ratio. Have strong intermolecular forces.
Eg. Terylene, Nylon, polyesters etc
Elastomers: Possess elasticity. Have weak intermolecular forces. They retract to its original position
after being stretched.
Eg. rubber.
Monomers used: Homopolymer, Copolymer/Condensation polymer. Homopolymers:
One type of monomer
Eg. Polyethylene, PVC, Polypropylene etc.
Copolymer: Different type of monomers used
Eg. Buns S rubber (Butadiene and styrene),
Nylon 66 (hexamethylenediamine, adipic acid)
PET (Ethylene glycol, terephthalic acid)
Polymerization: Addition, Condensation polymers
Addition Polymers: Formed by the repeated addition of monomers containing double or triple
bonds. Can be both homo or copolymers.
Eg. Polyethylene, PVC, Poly propelene, Buns S
Condensation polymer: formed by the condensation of different monomeric units. It involves the
elimination of water, alcohol, etc.
Eg. Nylon 6,6.
PET
Backbone chain: Organic and Inorganic
• Organic:
Where backbone chain is made up of C atoms.
Eg. nylon, PET, Poly enes etc
• Inorganic: Where backbone involves inorganic atom
Eg. Silicon rubber, Poly phosphates
Branching: Linear, Branched, Crosslinked
• Linear Polymers:
Straight chain polymers which are formed when monomers are joined end to end.
Eg. Polythene, polyvinyl chloride etc.
• Branched chain Polymers:
Linear polymers containing branches. It occurs by replacement
of any substituents such as hydrogen atom, monomer subunit etc.
Eg. Low density polythene.
• Crosslinked Polymers:
Combination of bi-functional and tri-functional monomers
or cross-linking between different polymer chains.
Eg. Bakelite, melamine etc.
Molecular Weight determination (Numerical)
• In linear polymers the individual polymer chains rarely have exactly the same degree of
polymerization and molar mass, and there is always a distribution around an average value
• The molar mass distribution (or molecular weight distribution) describes the relationship
between the number of moles of each polymer species (Ni) and the molar mass (Mi) of that
species
Different average values can be defined, depending on the statistical method applied.
In practice two averages are used
• Number average molar mass or Mn (Number Average Molecular Weight (NAMW)):
• Weighted- average molar mass or Mw (Weight Average Molecular Weight (WAMW)):
Number average molar mass
• The number average molecular mass is the ordinary arithmetic mean or average of the
molecular masses of the individual macromolecules.
• It is determined by measuring the molecular mass of N polymer molecules, summing the
masses, and dividing by N.
Weighted- average molar mass
Some properties are dependent on molecular size, so a larger molecule will have a larger contribution
than a smaller molecule. The mass average molar mass is calculated by
• Where Ni is number of molecules and Mi is molecular mass
Dispersity Index: The ratio of Mw to Mn.
If greater than 1: Polymer is Polydisperse
If equal to than 1: Polymer is Monodisperse
Numerical:
Glass transition temperature/Viscoelasticity
• Different range of viscosities with variation in temperature: Viscoelasticity
• Polymers are amorphous; they do not have fixed melting point but softening range.
• ‘The temperature below which a polymer becomes hard, glass like and brittle or above which
it becomes soft and rubbery is known as Glass transition temperature (Tg)’.
• Stiffness is related to Strength of Van der Waals forces.
• At temperature immediately above Tg, Some unrestricted micro Brownian motion occurs
because of which polymer which was glass or brittle now becomes ductile.
Factors affecting Tg
• Double bond, Aromatic chain: Stiffen molecular backbone: High Tg
• Presence of bulky group: eg. PP(Tg=-18OC), PS(Tg=100OC) :High Tg
• Polar side groups (Cl, OH, CN) leads to intermolecular boundary forces. Making the molecule stif : High Tg
• High density branches or crosslinking reduces mobility : High Tg
• Linear polymers : low Tg,
• branched polymers : High Tg
• thermoset/cross linked polymer : No Tg.
• Significance :Gives information about transforming temperature of a polymer. Help in choosing right
material.
• Polymers can be customized as per use
Plastics
• Lab synthesized polymers: rarely used in its polymeric form
• Polymers + Additives = Plastic (Practical product)
• ‘All Plastics are polymers, All polymers are not plastic’
Plastics gained importance due to
• Light weight
• Thermal and electrical insulatin
• Easy workability: molding and design
• Eye appealing: shine, finish
• Corrosion resistant• Transparent for optical useso
• Impermeable to water
• Good shock absorption etc.
Compounding of plastics
• Mixing of 4-10 additives to polymeric material: Compounding
• Main compounding ingredients are:
1. Binders: Thermoplastic, Thermosetting
2. Plasticizers
3. Fillers
4. Lubricants
5. Catalyst
6. Stabilizers
7. Pigments
1. Binders (Difference between thermoplastic and thermosetting)
• They are binding materials called Resins.
• Two types: Thermoplastic and Thermosetting• Range 50%-100% (100% = Pure Polymer)
• It has flowability and viscosity: helps in binding, molding, solidify
Thermoplastics
• Soften on heating and become plastic
• Molded in desired shape
• Temporary hardness/softness
• Addition polymerization
• Linear/branch structure
• Molecular weight does not change on heating/cooling
• Secondry bonds break on heating
• Eg. Poly ‘enes’, Buna S, acrylics, cellulose derivatives, Polyamides
Thermosettings
• Once set upon heating cannot be reformed
• Cannot be molded again and again• Permanent hardness
• Condensation polymerization
• 3D/ cross linked structure
• Very high Molecular weight
• Secondry bonds do not break, on heating they are hard and brittle
• Eg. Phenol formaldehyde, Epoxy, urea formaldehyde, polyesters, Polyurethane
2. Plasticizers
• Added to increase plasticity and flexibility
• Reduces molding temperature and pressure
• Neutralizes intermolecular forces between molecules, Impart freedom of movement: more
flexibility and Plasticity.
• Mostly for thermosetting, Upto 60%
• Eg. Vegetable oils, camphor, esters of fatty acid, tributyl phosphates and tri phenyl phosphates.
3. Fillers
• Better hardness and Tensile strength to final plastic.
• Reduce cost
• Reduce shrinkage, Upto 50%
• Eg. Mica, talc, chalk, wood powder, clay, cotton fiber, paper pup etc.
4. Lubricants
• To make molding easy: avoid sticking to the mold
• Provide glossy finish
• Eg. Waxes, soaps, oils etc.
5. Catalyst
• Only to thermosetting plastic
• To catalyse crosslinking.
• Eg. H2O2, benzoyl peroxide, ammonium salts, metal oxides.
6. Stabilizers
• Prevent decomposition and discoloration at molding temperature.
Eg. Lead salts (white Pb, red Pb, lead silicate, stearate of Pb, Cd, Ba).
7. Pigments
• To impart colour
Zn Oxide: White, CaCO3: White, Ultramarine: Blue, Carbon black: Black
Ferric oxide: Red, Chromium trioxide: Green etc.
Fabrication of plastic
• Fabrication process depends upon shape of the final product and type of material used.
Fabrication methods
1. Compression Molding
2. Injection Molding
3. Transfer Molding
4. Extrusion Molding
1. Compression Molding (Diagram from Book)
• High pressure molding process
• Low setup cost
• Applicable for thermoplastics and thermosetting resins.
• Plastic material is squeezed directly into the mold cavity.
• Temperature : 100-200oC
• Pressure: 100-500Kg/cm2
is applied for the desired shape.
• For Thermosets: Mold remains hot throughout the cycle. As soon as molded part is ejected, a
new charge of molding powder is added.
• For thermoplastics: Mold is cooled to harden, before molded article is ejected. Therefore,
compression molding is slow for thermoplastics.
• Common for thermosetting plastics.
Applications:
• Electronic Equipments
• Dinnerware plates, tray
• Bottle caps
• Large containers
• Automotive parts
• Flat article
• Handles: cookware, mirror, brush, doors, electrical iron
Advantages:
• Low initial setup cost
• Good surface finish of the molded parts
Limitations:
• Low production rate
• Limited largely to flat or moderately curved parts
2. Injection Molding (Diagram from Book)
• Most common for thermoplastic.
• Wide range of domestic and industrial articles are produced.
• Plastic material is melted, injected into the mold.
• It is cooled and finally finished article is ejected
• Plastic material (powder or pallets) are fed from Hooper into injection chamber.
• Reciprocating screw arrangement is used to forward the material towards mold.
• The temperature near nozzle: 130-260oC.
• The pressure near nozzle: 1758 kg/cm2
• Mold is then cooled and article is taken out.
Applications:
• Thin walled open containers like buckets.
• Household appliances,
• Automotive dashboards etc.
• Articles with intricate designs,
• toothbrush,
• toys,
• medical devices like syringes etc.
Advantages:
• High speed/ production rate
• Process cycle is 2-60 seconds.
• Complex geometry can be produced
• Minimum material wastage.
Limitations:
• High tooling and setup cost
• Minor cavities may not get filled by injection
3. Transfer molding (Diagram from Book)
• The method combines features of both Compression Molding (hydraulic pressing and the same
molding materials - thermosets) and Injection Molding (Plunger and filling the mold through a
sprue).
• The method is used primarily for molding thermosetting resins (thermosets), but
some thermoplastic parts may also be produced by Transfer Molding.
• Molding powder is placed in a heating chamber, maintained at minimum temperature to just
melt polymer.
• With the help of plunger, material is injected to orifice.
• Friction at orifice increase end temperature of material.
• Molding material now flows quickly into preheated mold.
Curing (heat treatment for chemical bonding) is completed within mold
Application:
• This process is widely used to encapsulate items such as integrated circuits, plugs, connectors,
pins etc.
• Transfer molding is also used for manufacturing radio and television cabinets and car body
Shells.
Advantages:
• Large production rate than Compression Molding.• The method is capable to produce more complicated
shapes than Compression Molding.
• Plastic parts with metal inserts can be made.
• Design flexibility, Uniform thickness of parts.
• No extra pressure is required.
Limitations:
• Wastage of material.
• Production rate lower than injection molding.
Air can be trapped in the mold
4. Extrusion Molding (Diagram from Book)
• High volume manufacturing process
• Good for thermoplastics of uniform cross section.
• Plastic material is melted and extruded through die into desired shape.
• A cylindrical rotating screw is placed inside the barrel which forces out molten plastic through a die.
• Finished product is extruded out, cooled by atmospheric exposure, air blow or spraying water.
Applications:
• Insulated electric cables
• Tubes
• Strips
• Other continuous articles with uniform cross section.
Limitation:
• Overheating in a heating chamber may cause degradation of material.
• Speed of the extruder screw should be uniform
Poly(methyl methacrylate): PMMA
• Also named as Acrylic glass, Plexiglass, Lucite and Perspex.
• Is a transparent thermoplastic.
• Used in sheet form as a lightweight or shatter-resistant alternative to glass, therefore also
termed as Safety glass.
Preparation:
Properties (PMMA):
1. Tg of PMMA is 105 °C, commercial grades range from 85 to 165 °C
2. PMMA transmits up to 96% of visible light , gives reflection of about 4% (refractive index is
1.49).
3. It filters ultraviolet (UV) light at wavelengths below about 300 nm
4. Low chemical resistance: PMMA swells and dissolves in many organic solvents, acids and alkali
(easily hydrolysed ester groups.)
Uses (PMMA):
• Being transparent and durable, PMMA is a versatile material.
• Vehicle instrument cluster.
• Rear-lights
• Lenses for glasses
• Decorative items and Jewellery
• Shatter resistant panels: building windows, skylights, bulletproof security barriers, signs &
displays,, LCD screens
• PMMA is naturally compatible with human tissue :Contact lenses, dentures, bone replacement.
Kevlar
(Poly-paraphenylene terephthalamide )
• Kevlar is a heat-resistant and strong synthetic fiber
• A type of polyamide with NH-CO linkage. (eg. Nylons)
Preparation:
Properties:
• High tensile strength to weight ratio.
• Inter-chain Hydrogen bonds between the carbonyl groups and NH centers owes its high
strength.
• Additional strength is derived from high electron density (aromatic stacking
interactions) between adjacent strands
• 5 times stronger than steel,
• 10 times stronger than aluminium
Uses:
• It was first commercially used in the early 1970s as a replacement for steel in racing tires.
• Spun into ropes or fabric sheets.
• Used in marching drumheads as it can withstand high impact.
• Bulletproof vests, halmet, face mask
• High heat resistance uniforms worn by firefighters, body armour worn by police officers,
security
Kevlar is used to manufacture gloves, sleeves, jackets and other articles of clothing.
Phenol Formaldehyde
• These are phenolic resins also named as Bakelite.
There are two main production methods.
• One reacts phenol and formaldehyde directly to produce a thermosetting network polymer,
while
• The other restricts the formaldehyde to produce a prepolymer known as novolac which can be moulded and
then cured with the addition of more formaldehyde and heat.
Preparation: Blackboard notes: 4 step process
• Methylolation
• Novolac formation
• Resole formation
• Phenol formaldehyde crosslinked structure
Properties:
• Rigid, hard, infusible
• Scratch and water resistant
• Resistant to acids, salt, organic solvents
• Can withstand high temperature: missile nose cone
• Excellent electric insulating character
• Low thermal conductivity
• Adhesive and bonding property
Uses:
• Electric insulating parts
• Molded articles like telephone
• TV and radio cabinets
• Billiard balls
• As adhesive
• As hydrogen exchanger resin in water softening
• In paints and varnishes.
Urea Formaldehyde (UF)
• Thermosetting resin
• Also known as amino resins or urea-methanol
• It is produced from urea and formaldehyde.
• is a non-transparent thermosetting polymer
Properties:
• High tensile strength, flexural modulus• Good electrical insulation
• Chemical resistance
• Abrasion resistance
• Low water absorption
• Low mould shrinkage
• High surface hardness
• Urea-formaldehyde is pervasive therefore spreads easily.
Uses:
These resins are used in adhesives
• It is also used to glue wood together. particle board, medium-density fibreboard (MDF), etc.
• Urea formaldehyde was commonly used when producing electrical appliances casing (e.g. desk
lamps).
• Decorative articles, plates etc.
• Foams have been used as artificial snow in movies.
CONDUCTING POLYMERS
POLYMERS IN MEDICINE AND SURGERY
•Biomaterials are materials that can be implanted in the body or used in diagnostic,
surgical and therapeutic applications without causing adverse effect on blood and other
tissues.
•They are developed from metals, ceramics and polymers.
• Use of biomaterials made from polymers is increasing day-by-day.
•Their appeal and acceptability is mainly
due to versatility and the fact that they can be modified at will to suit specific body
functions.
Characteristics of biomedical polymers:
(1) It should be bio compatible.
(2) It should be fabricated into the desired shape or form without being degraded.
(3) It should have purity and reproducibility.
(4) They can be easily sterilized with no alteration in properties.
(5) They should have optimum physical and chemical properties.
(6) They should not destroy cellular elements of blood, enzyme or produce toxic or allergic reactions.
(7) They should not deplete electrolytes in the body.
UNIT: ATOMIC AND MOLECULAR
STRUCTURE
SR
TOPICS
NO.
1.
ATOMIC ORBITAL
2.
ELECTRONIC CONFIGURATION
3.
MOT&LCAO
4.
SHAPES OF MOLECULAR ORBITALS
5.
MOLECULAR ORBITAL TREATMENT FOR HOMONUCLEAR
DIATOMIC MOLECULES
6.
MOLECULAR ORBITAL TREATMENT FOR
HETERONUCLEAR DIATOMIC MOLECULES
7.
EASY WAY TO FIND BO AND MAGNETIC
BEHAVIOUR
CONCEPT OF ATOMIC ORBITAL:
In atomic theory and quantum mechanics, an atomic orbital is a mathematical function describing the
location and wave-like behavior of an electron in an atom. This function can be used to calculate the
probability of finding any electron of an atom in any specific region around the atom's nucleus.
Determination of shapes of orbitals:
s-orbital: Boundary surface diagram for s orbital looks like a sphere having the nucleus as its
center which in two dimensions can be seen as a circle.
p-orbitals: Each p orbital consists of two sections known as lobes which lie on either side of the plane
passing through the nucleus.The three p orbitals differ in the way the lobes are oriented; whereas they
are identical in terms of size, shape and energy. As the lobes lie along one of the x, y or z-axis, these
three orbitals are given the designations 2px, 2py, and 2pz.
d-orbital: Magnetic orbital quantum number for d orbitals is given as (-2, -1, 0, 1, 2). Hence, we can
say that there are five d-orbitals. These orbitals are designated as dxy, dyz, dxz, dx 2 -y 2 and dz 2 .
Out of these five d orbitals, shapes of the first four d-orbitals are similar to each other, which is
different from the dz 2 orbital. However, the energy of all the five d orbitals is the same.
ELECTRONIC CONFIGURATION:
The distribution of electrons in various orbitals is known as electronic configuration of the atom. The
atom is built up by filling electrons in various orbitals one at a time, by placing it in the lowest energy
orbital. The atom is said to be in the ground state when it is in its lowest energy state. This is the most
stable state for the atom. The filling of orbitals by electrons in the ground state is determined by
following rules.
1.Aufbau principle:
The Aufbau principle states that in the ground state of an atom, the orbital with lower energy is filled
first, before the filling of the orbital with higher energy. In other words, the electrons enter the orbitals
in order of their increasing energies. The order in which the energies of the orbitals increase, and hence
the order in which the orbitals are filled is as follows. 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d,
6p, 7s,
2. Pauli’s exclusion principle:
3. Hund’s Rule of maximum multiplicity:
This rule deals with the filling of electrons into the orbitals belonging to the same subshell i.e. orbitals of
equal energy. It states “pairing of electrons in the orbitals belonging to the same subshell (p, d, f) does
not take place until each orbital belonging to that subshell has got one electron each i.e. it is singly
occupied”.
MOLECULAR ORBITAL THEORY (MOT):
In molecular orbital theory, electrons in a molecule are not assigned to individual chemical
bonds between atoms, but are treated as moving under the influence of the atomic nuclei in the whole
molecule. [1] Quantum mechanics describes the spatial and energetic properties of electrons as
molecular orbitals that surround two or more atoms in a molecule and contain valence
electrons between atoms.
We are going to discuss only one approximation which is rather simple, qualitative and easy to
understand, known as the linear combination of atomic orbitals, abbreviated as LCAO method.
The salient features of MOT are as follows :
1. When two atoms approach each other, their atomic orbitals lose their identity and mutually overlap to form
new orbitals called molecular orbitals.
2. The number of MO formed is equal to the number of overlapping atomic orbitals.
3. Maximum capacity of a MO is two electrons with opposite spins. MO is a polycentric region in space defined
by its size and shape, associated with two or more atoms in a molecule and each has a capacity of two electrons
with opposite spins.
4. Only atomic orbitals having comparable energies as well as proper orientations interact significantly. For
example, 1s atomic orbital can overlap with 1s atomicm orbital but not with 2s atomic orbital or 2s atomic
orbital can overlap with 2px atomic orbital.
5. When two atomic orbital overlap, they interact to form two molecular orbitals, in the following two ways.
x When the atomic orbitals overlap in-phase it leads to an increase in the intensity of the negative charge in the
region of overlap. The molecular orbital thus formed has lower potential energy than the separate atomic
orbitals and is called bonding molecular orbital. The difference in energy between the combining atomic
orbitals and the bonding molecular orbital formed is called the stabilization energy. Thus bonding molecular
orbital stabilizes the molecule.
Characteristics of bonding molecular orbital:
(a) It possesses lower energy than that of the combining atomic orbitals.
(b) It imparts stability to the molecules.
(c) Every electron in it contributes to the attraction of two combining atoms.
(d) It possesses high electron density between the two nuclei.
(e) It is formed when the lobes of combining atomic orbitals have the same signs.
When the atomic orbitals overlap out-of-phase, it leads to a decrease in the intensity of the negative charge
between the nuclei and leads to higher potential energy. The molecular orbital of this type is called antibonding
molecular orbital. The difference in energy between the antibondingmolecular orbital and combining atomic
orbitals is called destabilization energy. Thus antibonding molecular orbital destabilizes the molecule.
Characteristics of antibonding molecular orbital:
(a) It possesses higher energy than that of the combining atomic orbitals.
(b) It imparts instability to the molecules.
(c) Every electron in it contributes to the repulsion of two combining atoms.
(d) It possesses low electron density between the two nuclei.
(e) It is formed when the lobes of combining atomic orbitals have the opposite signs.
6. The shape of MO formed depends on the type of combining atomic orbitals.
7. The bonding MO are represented by V, S, G etc., whereas antibonding
MO are represented by V*,S*,G*etc.
8. Inner orbital MO which do not take part in bond formation are called non bonding MO.
LINEAR COMBINATION OF ATOMIC ORBITALS (LCAO) METHOD
SHAPES OF MOLECULAR ORBITALS:
(a) s-s combination of orbitals (1s with 1s or 2s with 2s) : The two molecular orbitals
formed may be designated as bonding V (1s) or V (2s) and antibonding V* (1s) or V* (2s). This
indicates that the overlap is along the internuclear axis. V (1s) is formed by constructive
overlapping and V* (1s) is formed by destructive overlapping of the two s-orbitals.
(b) coaxial s-p combination of orbitals (s-orbital with px orbital) : An s-orbital may
combine with a p-orbital provided that the lobes of the p-orbital are pointing along the axis joining the
nuclei.
When the lobes which overlap have the same sign, results in a bonding MO with an increased
electron density between the nuclei. When the overlapping lobes have opposite signs, it gives an
antibonding MO with reduced electron density in between the nuclei.v
(c) axial p-p combination : Consider the combination of two p-orbitals which have lobes pointing
along the axis joining the nuclei. In this case, both a bonding MO and an antibonding MOs are produced
(d) Side-to-side p-p combination: Consider the combination of two orbitals which both have
lobes perpendicular to the axis joining the nucleus. Lateral overlap of orbitals results in the
formation of S bonding MO and S antibonding MO.
MOLECULAR ORBITAL TREATMENT FOR HETERONUCLEAR DIATOMIC
MOLECULES:
EASY METHOD TO FIND BOND ORDER AND MAGNETIC BEHAVIOUR
THROUGH MOT
TRIANGLE:
UNIT : PHASE RULE
SR
TOPICS
NO.
1.
INTRODUCTION
2.
GIBB’S PHASE RULE
3.
ONE COMPONENT PHASE RULE(WATER SYSTEM)
4.
REDUCED PHASE RULE
5.
LEAD-SILVER SYSTEM
6.
ADVANTAGES & DISADVANTAGES
7.
NUMERICALS
INTRODUCTION:
Phase equilibrium deals with the study of equilibrium conditions of heterogeneous systems. Substances
are present in different phases in heterogeneous systems. Such a system can be conveniently studied
with the help of a generalization called Phase Rule. It relates the conditions which must be specified to
describe the state of a system at equilibrium. The plot indicating the relationships between various
phases under different temperature, pressure and concentration is known as phase diagram. Phase rule,
with the help of a phase diagram, is useful in predicting the effects of temperature, pressure and
concentration on the equilibrium of heterogeneous systems.
GIBB’S PHASE RULE:
Gibb’s phase rule states that in every heterogeneous system in equilibrium, the sum of the number of
phases and degree of freedom is greater than the number of components by 2.
This is mathematically expressed as
P + F = C+ 2
Or
F = C - P + 2
Where,
P: Number of phase in equilibrium
C: Components of the system
F: Degree of freedom
APPLICATION OF PHASE RULE TO ONE COMPONENT SYSTEM:
The Water System: Water is the most common example of one component system. The water system
consists of three phases, namely, ice, water and water vapour.
Ice (s)
Water (l)
Water Vapour (g)
The three phases may occur in four possible combinations in equilibrium as follows :
(i) Liquid
Vapour
(ii) Liquid
Solid
(iii) Solid
Vapour
(iv) Solid
Liquid
Gas
As water (H2O) is the only compound involved in the system, therefore, it is single or one component
system. From the phase rule, when
C = 1,
F = C - P + 2
F = 1 - P + 2
F = 3 - P
The degree of freedom depends on the number of phases present at equilibrium. Three different cases
are possible.
(i) P = 1; F = 2 (bivariant system)
(ii) P = 1; F = 2 (bivariant system)
(iii) P = 3; F = 0 (invariant system)
DESCRIPTION ABOUT THE PHASE DIAGRAM:
REDUCED PHASE RULE:
THE LEAD SILVER SYSTEM:
Eutectic point:
Two or more solid substances capable of forming solid solutions with each other have the property of
lowering each other’s freezing point; and the minimum freezing point attainable corresponding to the
eutectic mixture, is termed the eutectic point (means lowest melting point).
ADVANTAGES OF PHASE RULE:
DISADVANTAGES OF PHASE RULE:
Numerical based on Phase Rule:
3.
4.
5.
Practice problems:
1) An alloy of Cd and Bi contains 20% of Cd. Find the mass of eutectic in 2 Kg of alloy, if the eutectic system
contains 50% Cd.
2) An alloy of tin and lead contain 80% tin. Find the mass of eutectic in 1Kg of solid alloy if the
eutectic contains 60% of tin.
3) An alloy of Cd and Bi contains 25% of Cd. Find the mass of eutectic in 1 Kg of alloy, if the eutectic system
contains 40% Cd.
4) An alloy AB of 10 g weight contains 25% of A. The molten AB on cooling gave out B and a
eutectic alloy with Aand B at equal percentage. What is the amount of B that has formed?
UNIT: AROMATIC SYSTEMS AND THEIR
MOLECULAR STRUCTURE
SR
TOPICS
NO.
1.
INTRODUCTION
2.
AROMATICITY
3.
HUCKEL’S RULE
4.
ATOMIC & MOLECULAR STRUCTURE OF BENZENE
5.
ATOMIC & MOLECULAR STRUCTURE OF PYRROLE
INTRODUCTION:
In earlier times, during the study of organic chemistry some sweet smelling compounds were obtained
from natural sources. These compounds showed different properties compared to aliphatic compounds.
They were called aromatic (Greek, aroma = pleasant smell) compounds. Further studies revealed that
these compounds contain benzene rings involving six carbon atoms in a ring. Later on, a large number of
aromatic compounds were discovered which lacked the sweet smell. Thus this word aroma lost it’s
significance.
AROMATICITY:
Aromaticity is a property of cyclic (ring-shaped), planar (flat) structures with pi
bonds in resonance (those containing delocalized electrons) that gives increased stability compared
to other geometric or connective arrangements with the same set of atoms. Aromatic rings are very
stable and do not break apart easily. Organic compounds that are not aromatic are classified
as aliphatic compounds—they might be cyclic, but only aromatic rings have enhanced stability.
Characteristics of aromatic compounds:
1. They are highly unsaturated as shown by the lesser number of hydrogen atoms in their
molecular formulae.
2. They are cyclic compounds with five, six or seven membered rings.
3. Their molecules are flat or nearly flat as shown by physical methods such as x-ray and electron
diffraction methods.
4. They undergo readily certain electrophilic substitution reactions such as nitration,
halogenation, sulphonation, Friedel-Crafts alkylation and acylation etc.
5. Although their molecular formulae suggest a high degree of unsaturation, yet they
do not respond to tests characteristics of unsaturated compounds. They fail to decolourize an aqueous
solution of potassium permanganate (Baeyer’s test).
6. They are associated with high thermodynamic stability as is indicated by their low heats of
combustion and hydrogenation.
HUCKEL’S RULE:
Examples:
Extra problems:
STRUCTURE AND BONDING OF BENZENE:
According to Kekule, six carbon atoms of benzene are linked to each other by alternate single and
double covalent bonds to form a hexagonal ring. Each carbon atom is linked to one hydrogen atom thus
conforming to its molecular formula.
Molecular orbital structure of benzene:
Structure of benzene can be best described by using the orbital concept. The orbital picture of benzene
shows that each carbon atom in benzene is sp2 hybridised. The C-H bonds in benzene are sp2 -s, V bond.
The C-C bonds are sp2 -sp2 , V bonds. The sp2 hybridisation of the carbon atoms indicates that all the
Carbon atoms of the ring are in the same plane with their bonds separated by angles of 120° .
The six hydrogen atoms radiate from the carbon atom in the same plane like the spokes of a wheel. The
whole molecule is planer. Each carbon atom has a pure p orbital with one electron. It overlaps laterally
with p-orbitals of the adjacent carbon atoms on either side. It results in the formation of three S bonds.
The 6S electrons of benzene are enough to fill all the bonding S- molecular orbitals. However, the p
orbitals may overlap simultaneously with both adjacent p orbitals
Evidence in support of orbital structure of benzene:
1. Unusual stability: Benzene molecule exhibits unusual stability and resists the
formation of addition products. This can easily be understood in terms of delocalization
of S-electrons which is responsible for aromaticity.
2. Isomer number: According to orbital concept, all the six carbons in benzene are
completely equivalent. Similarly, all the six hydrogen atoms also occupy identical
positions. Thus, benzene should form only one monosubstituted and three disubstituted
products. This has been found to be in actual practice.
3. Electrophilic substitution reactions: There are two continuous ring like Selectron clouds one above and
the other below the plane of carbon atoms. The S-electrons
are easily attacked by electrophiles. Hence, benzene undergoes electrophilic substitution
reactions.
STRUCTURE AND BONDING OF PYRROLE:
