Prasanna – Class 12 Chemistry

Advanced Materials Preparation and Properties

October 7, 2021September 18, 2021 by Prasanna

Advanced Materials Preparation and Properties

Some of the advanced materials are :

Carbon Fibres

→ The long chain cyclic structure of carbon is known as carbon fibres. These are stronger than steel, harder than titanium and lighter than aluminium. Due to less density and high tensile strength, these are of great importance.

→ Carbon fibres are formed by polymerisation of certain materials like rayon, polyacrylonitrile, gases such as methane, benzene etc. The structure of carbon fibres is as follows :

Advanced Materials Preparation and Properties 1

Uses of Carbon Fibres

Carbon fibres are of great importance in many fields. In Biology, as a component of bone plate, in artificial heart transplant etc. CFRP and CFRC are used in sports items like tennis and badminton racket. In frame of fast cyclers and cars etc. It is also used in protective materials and spacecrafts.

Chemistry Notes

Rocket Propellents Preparation and Properties

October 7, 2021September 18, 2021 by Prasanna

Rocket Propellents Preparation and Properties

→ Substances used for launching rockets are called rocket propellants. These are the combination of an oxidiser and a fuel.

→ Rocket propellents are mixture of oxidismg substance and fuel. This fuels release hot gases after combustion in the engine. These gases escape out of nozzle which gives sufficient energy to rocket to move up.

Characteristics of Rocket Propellents

Rocket propellants possess unique properties, such as:

Rocket propellent should be in solid state or liquid state which takes less space to store.
The mixture of fuel and an oxidiser should be prepared quickly.
The mixture should be highly inflammable which can give sufficient energy to speed up the rocket.
It should not give any residue on combustion.
Their chemical reaction should be fast.

Types of Rocket Propellents

Rocket propellents can be classified on the basis of their physical state:

Solid Propellents
Liquid Propellents
Mixed or Hybrid Propellents

Solid Propellents : In this type of propellents, both the oxjdjser and fuel are in solid state. These are of two types:

Combined and
Dibasic.

→ Combined solid propellents are most common and are generally used. These are made up of fuel, oxidiser and combining material. Polyurethane or polybutandiene is used as a fuel, Ammonium sulphate is used as oxidiser and finely divided magnesium is used as combining material.

→ Dibasic propellents are made up of nitroglycerine and nitrocellulose.

→Liquid Propellents : In these, both oxidiser and fuel are in liquid state. These are most commonly used because these have more thrušting power as compared to solid propellents. These can be controlled. These are of two types :

mono and
bi-propellents.

→ Mono propellents are the propellents in which both fuel and oxidiser works. These release large amounts of hot gases on combustion. For example hydrazine, nitromethane, hydrogen peroxide etc. Bi-propellents are those which contains mixture of two liquids in which one liquid acts as fuel and other as oxidiser. Kerosene oil, alcohol, hydrazine etc are used as fuelland liquid oxygen, liquid nitrogen etc. are used as oxidiser.

→ Mixed or Hybrid Propellents are mixture of solid and liqiid propellents. For example, mixture of acrylic rubber (as a fuel) and liquid nitrogen tetroxide as oxidiser.

Some Commonly Used Propellents

In America, mixture of kerosene oil and liquid oxygen is used j.n saturn booster rocket initially but later mixture of liquid hydrogen and liquid oxygen was used.
Mixture of kerosene oil and liquid oxygen is used in rocket Proton in Russia.
In India, solid propellents are used in SLV and ASLV.

Chemistry Notes

Insect Repellants Preparation and Properties

October 7, 2021September 18, 2021 by Prasanna

Insect Repellants Preparation and Properties

→ Those chemical substances which are used to kill and move away insects are called insect repeallants. Mostly, insect repellents mixes in air and affect respiratory system of insects.

→ Insect repellants are found in all three states : solid, liquid and gas. Some substances like neem leaves, tobacco leaves, sulphur etc. are used as natural insect repellants.

→ Many liquids insect repellants are sold by commercial names. Some of them are finit, bagon, endosuiphon, phenocol, phenyl etc. These are used in homes and fields as insect repellants. Some of the synthetic insect repellants are harmful for humans also. Now, herbal insect repellants like naphthalene balls, odonil, odomos etc. are used in homes.

Pheromones of Insect Repellants

→ A chemical substance produced and released into the environment by air animal, especially a mammal or an insect, affecting the behaviour or physiology of others of its species is called pheromone. Pheromones have a specific odour which is helpful in identification.

→ Pheromones can be of many types like sex pheromones, indicators etc. The main function of pheromone is sexual attraction. From ancient times, sex pheromones are secreted by females which attract males towards her. But in case of musk deer, musk is secreted in navel of male deer which attract female towards him. It is known as muskon. The structure of Pheromones can be given as:

Insect Repellants Preparation and Properties 1
→ Sex attracting pheromones are used for harmful insects control. The smell of pheromone is very intense. Its small quantity is detected by animals. If small amount of sex pheromone is kept at a place then opposite sex is attracted and come to that place and can be killed or sterilised. Thus, their reproduction rate is reduced and pests can be controlled. As it is not harmful for other species and is not sprayed, it does not cause any pollution.

Chemistry Notes

Cleansing Agents Preparation and Properties

October 7, 2021September 18, 2021 by Prasanna

Cleansing Agents Preparation and Properties

Soaps :

→ Soaps are sodium or potassium salts of higher fatty acids (containing 15-18 carbon atoms) such as stearic acid, oleic acid and palmitic acid. Sodium salts of fatty acids are known as hard soaps while the potassium salts of fatty acids are known as soft soaps.

→ Hard soaps are prepared by cheaper oil and NaOH while soft soaps are prepared by oil of good quality and KOH.

→ The soft soaps do not contain free alkali, produce more lather and are used as toilet soaps, shaving soaps and shampoos. The process of formation of soap is known as saponification. In our country, soaps are obtained from coconut oil, mustard oil, soyabean oil etc.

→ Oils and fats are structurally similar but carbon chains in oils are unsaturated but saturated in case of fats. For this reason, van der Waal’s forces in fats are comparatively stronger. This is the reason that oil, are liquid and fats are solid at room temperature. For example,

Cleansing Agents Preparation and Properties 1

→ It is clear from the above reaction that 3 moles of NaOH react with 1 mole of oil or fat and gives 3 moles of soap. A mixture of soap and glycerine is obtained which is known as ‘lye’. Soaps and glycerol are separated by adding salt. Soap is insoluble in salt solution and thus precipitates out due to common ion effect and glycerol remains in the solution.

Types of Soap :

→ Hard Soap : These are prepared by t.he reaction of NaOH with cheap oils. These are used for’ the washing clothes.

→ Soft Soap : It is prepared by the reaction of KOH with high quality oils. It is used for manufacturing both soap, shaving cream and shampoos, colour and perfumes are used in them for making attractive.

→ Transparent Soap : After evaporation at solution of bathing soap in alcohol transparent soaps are prepared. In this glycerol is used.

→ Medicated Soaps : When medicated substances are mixed in bathing soap medicated soap are prepared, e.g., Carboylic soap, soap of neem.

→ Sea’s Soap : These soaps gives lather in sea’s water also, e.g., soap which are made by coconut oil.

→ Shaving Soap : Soap which are used for shaving are prepared by using gum of glycerol and resin. They give proper lather.

Cleansing Action of Soap : A molecule of soap has two parts.

A long chain o hydrocarbon tail in soluble in water and
Water soluble polar head,

Cleansing Agents Preparation and Properties 2

→ When soap is added to some oily or greasy part of the cloth, the stearate ions arrange themselves around it in such a way that hydrophobic parts of the stearate ions are in the droplets.

→ As hydrophilic part is polar, these polar groups can interact with the water molecule present around the oil droplet. As a result, the oil droplet is pulled away from the surface of the clothing water to form ionic micelle which is then washed away with excess of water. In fact, the stearate ions of the soap molecules helps in making a stable emulsion of oil with water which is then washed away with water. A sheath of negative charge is formed around the globule which prevents them to come together and form aggregate.

Cleansing Agents Preparation and Properties 3

Detergents :

→ Synthetic detergents contain either sodium salts of alkyl hydrogen suiphates or sodium salts of long chain alkyl benzene suiphonic acid. Synthetic detergents have all the properties of soaps but actually does not contain any soap, so they are known as “soapless soaps”.

→ Straight chain alkyl group containing detergents are biodegradable whereas branched chain alkyl group containing detergents are non-biodegradable. Unlike soaps, synthetic detergents can be used in both soft and hard water. This is due to the reason that calcium and magnesium salts of detergents like their sodium salts are also soluble in water. Examples,

Cleansing Agents Preparation and Properties 4

→ Sodium p(1,3,5,7)-tetra methyl octyl benzene suiphonate. These molecules has one ionic end (hydrophillic) and other one is long hydrocarbon chain (hydrophobic). Hydrophobic end is known as tail which is soluble in dirt and grease. Their structure is similar to detergents.

Cleansing Agents Preparation and Properties 5

→ Their cleansing action is same as soaps. Ionic impurities are attached to hydrophilic part and oily impurities are attached to hydrophobic part. Then the dirt is removed by rubbing with hand or by machine.

→ Differences between Soaps and Detergents Both soaps and detergents are cleansing agents but detergents are superior to soaps because of the following reasons:

→ Soaps cannot be used in hard water since calcium magnesium ions present in hard water produce curdy precipitate of calcium and magnesium salts. These insoluble salts separate as scum in water and causes hinderance to washing because the precipitate adheres on to the fibre of the cloth as gummy mass. Thus, a Iot of soap is wasted if water is hard. On the other hand, synthetic detergents can he used even in case of hard water.

Cleansing Agents Preparation and Properties 6

→ Soaps cannot be used in acidic solutions since acids precipitate the insoluble free fatty acids which adhere to the fabrics and thus, reduce the ability of soaps to remove oil and grease from fabrics whereas synthetic detergents can be used in the acidic medium.

→ Synthetic detergents are more soluble in water and hence, form better lather than soaps.

→ Synthetic detergents have a stronger cleansing action than soaps.

→ Detergent maintains its cleaning power better than soap over a long period of time. For household managers who are looking to economize, this means they can purchase the larger size of detergent at a lower unit cost and not have to worry that its cleaning ability will subside while in storage.

→ It consumes fewer natural resources in the manufacturing process than soaps. Detergents are primarily made from synthetic materials while soap is comprised of vegetable oils and fatty acids. Synthetic detergents are maily classified into three categories:

→ Anionic Detergents : These are sodium salts of suiphonated long chain alcohols or hydrocarbons. They have anions at the soluble ends of the chain.

→ Alkyl hydrogen suiphates formed by treating long chain alcohols with concentrated sulphuric acids are neutralised with alkali to form anionic detergents.

→ Aikyl benzene suiphonates are obtained by neutralising alkyl benzene suiphonic acids with alkali.

Cleansing Agents Preparation and Properties 7

→ In such detergents, the anionic part of the molecule is involved in the cleansing action. They are mostly used for household work and in tooth paste.

→ Cationic Detergents : These are quaternary ammonium salts of amines with acetates, chlorides or bromides as anion. They have cation at the soluble ends of the chain. Cationic detergents are used in hair conditioners. They have germicidal properties but are expensive therefore, these are of limited use.

Cleansing Agents Preparation and Properties 8

→ Non-ionic Detergents : They have hydrogen bonding at the soluble ends of the chain. They are either monoesters of polyhydric alcohols or polyethers derived from ethylene oxide. Such detergents does not contain any ion in their constitution.

→ One such detergent can be obtained by reaction of stearic acid and polyethylene glycol. Liquid dish washing detergents are non-ionic type, Mechanism of cleansing action of this type of detergents is the same as that of soaps. Example is pentaerythrìtol stearate.

Cleansing Agents Preparation and Properties 10

Chemistry Notes

Chemical in Food Preparation and Properties

October 7, 2021September 18, 2021 by Prasanna

Chemical in Food Preparation and Properties

Chemicals are added to food for their preservation, enhancing their appeal and adding nutritive value in them. Main categories of food additives are as follows:

Food Preservatives

→ These are the chemical substances added to food to prevent their spoilage due to microbial growth (Bacteria, Yeasts and Moulds) and to retain their nutritive value for longer periods.

→ Thus, those chemical substances which prevents the spoilage of food done by bacteria and thereby destroy them are known as food preservatives.

An artificial preservatives should have the following characteristics :

Effective in small amounts.
Effective for long time.
Should not affect the quality of food.
Should not have harmful effects on health.

Following chemicals are used as preservatives.

→ Sodium Benzoate : This is the most commonly used preservative. Its 0.06% to 0.1% solution is used as preservative in fruit juices. jam, jelly, pickels etc.

→ Sorbates : These are salt of sorbic acid. These are used for preservation of food materials made of milk and cheese.
Potassium Metabisuiphite : Potassium meta-bisuiphite when mixed in food material evolued sulphur dioxide which helps in preservation of food.

K₂S₂O₅ → K₂SO₃ + SO₂

→ Parabean : It is parahydroxy benzoate. It is used in preservation of tomato, puree and sauce.

→ Propionates : These are ethyl and phenyl esters of propionic acid. They are used to preserve papadum and biscuits etc.

Artifidal Sweeteners

→ Organic substances which have been synthesized in lab are known to be many times sweeter than cane sugar. Such compounds are known as artificial sweetening agents or artificial sweetners.

→ Saccharin : Saceharin discovered by Johns Hopkins in 1879 (University of USA). It is the most popular artificial sweetener, it is 600 times as sweet as cane sugar. It is insoluble in water, but its Sodium salt is soluble in water. So, it is sold in the market as its soluble sodium salt. It is non-biodegradable so excreted from the body in urine (unchanged). Its use is of great value for diabetic persons and people who need to control intake of calories.

Chemical in Food Preparation and Properties 1

Some of the artificial sweeteners used commercially are given in table below:

Chemical in Food Preparation and Properties 2

Anti-oxidants

A substance that inhibits oxidation, especially one used to counteract the deterioration of stored food products is called anti-oxidant. These are active towards oxygen. These destroy themselves and prevent food spoilage. Its structure can be given as:

Chemical in Food Preparation and Properties 3

Food Colour

The chemicals used to make food more attractive and colourful are knoyn as food colours. Food colours are generally dyes.
Example
Tetrazine : It is of yellow colour.
1 4-Di-p-toluedino Anthraquinone : It is of green colour.
Generally, food colours are harmful for children and asthma patients.

Chemistry Notes

Dyes and Pigments Preparation and Properties

October 7, 2021September 18, 2021 by Prasanna

Dyes and Pigments Preparation and Properties

General Characteristics of dyes :

→ The natural or synthetic organic compounds impart colour to fabric are called dyes. In ancient times, for colouring fibres and fabrics, dyes are obtained from plants and other living substances.

→ There is no difference in usage of dyes and pigments. The main difference between the two is that dyes are the substances which are soluble in water and other solvents whereas pigments are the substances which are insoluble in water and other solvents.

→ Pigments colour substance by coagulation thereby forming a layer on the substance. In other words, dyes functions by absorbing from the solution of the substance whereas pigments works by forming layer on the substance. The main difference between dyes and pigments are as follows:

Dyes and Pigments Preparation and Properties 1

A dye have following characteristics:

It must have a suitable colour.
It can be fixed on the fabric either directly or with the help of mordant.
It, must be resistant to ‘he action of water, acid and alkalies. The groups, responsible for colour, are called chromophore.
These should be unaffected by light.

→ The colour of a substance depends on the dye present in it and the light falling on it. When white light is incident on a substance, then it is reflected and absorbed. When all the light falling on a surface gets reflected. the substance appears white in colour. When all the part of incident light gets absorbed, the substance appears black in colour.

→ But if some part of light is absorbed and some part is reflected, then a particular colour is observed. Visible light is mainly made up of seven colours : Violet, Indigo, Blue, Green, Yellow. Orange and Red. The wavelength of light increases from 400 nm to 750 nm as we move from violet to red. The colour of reflected light is complementary to the colour of absorbed light. For example, if a substance absorbs green colour then the substance is violet in colour. This means that violet colour is complementary to green colour.

→ Following table shows colour of absorbed light and their complementary colour in increasing order of wavelengths.

Wavelength Absorbed (nm)	Color Absorbed	Color Observed
400 – 435	Violet	Yellow – Green
435 – 480	Blue	Yellow
480 – 490	Green-Blue	Orange
490 – 500	Blue-Green	Red
500 – 560	Green	Purple
560 – 580	Yellow-Green	Violet
580 – 595	Yellow	Blue
595 – 605	Orange	Green – Blue
605 – 700	Red	Blue – Green

→ Thousands of substances are known which can be used as dye but only about 1500 dyes are known which are chemically important and are synthesised commercially.

→ First useful dye was synthesised in 1856 by W.H. Parking who was only 18 years old at that time. He prepared violet dye by impure aniline which is basically 6-phenyl phenosafranine and its homologue.

Dyes and Pigments Preparation and Properties 3

Structural Features of Dyes

→ Like the physical and chemical properties of organic compounds, there is a definite relationship between the colour and constitution. For example. benzene is colourless whereas its isomer fulvene is coloured compound. Grabby and Liebemian first of all explain the concept of colour and chemical structure.

→ In 1876, a German Scientist Otto Witt put forward a theory known as ‘Chromophore Auxochrome theory” or Witt Theory. Some of the important points of this theory are:

→ The colour of a substance is mainly due to the presence of an unsaturated group known as chromophore (Greek chroma = color, and phore = bearing). The important chromophores are:

Dyes and Pigments Preparation and Properties 4

→ The compounds possessing chromophores are referred to as chromogens. The higher the number of chromophores in a chromogen higher is the colour bearing capacity. The chromophoric groups are of the following two types.

→ When a single chromophore is sufficient to impart colour to the compound. Examples are : NO, NO₂, N = N, = = N = N N, N = N → O, p-quinonoidetc.

→ When more than one chromophore is required to impart the colour, e.g, > C = O, > C = C < etc. This can be exemplified by various examples. Acetone (having one > C = O as chromophore) is colourless, whereas biacetyl (with two > C = O) is yellow.

→ Certain groups, while not producing colour themselves, when present along with a chromophore in an organic substance, intensify the colour. Such colour assisting groups are called auxochromes (Greek word, Auxanien = to increase; Chrome = colour), i.e., they make the colour deep and fast and fix the dye to the fabric. The auxochromes are acidic or basic functional groups. The important auxochromes are,

Dyes and Pigments Preparation and Properties 5

→ It can be explained with the help of an example, azobenzene is a colourless compound but when NH₂ group is introduced in it. p.aminobenzene is obtained which is a yellow colour dye.

Here, —N = N— is a chromophore whereas —NH₂ is a auxochrome.

Dyes and Pigments Preparation and Properties 6

→ According to Modern theories, structural properties of dyes can be explained on the basis of Valence Bond Theory and Molecular Orbital Theory. These theories are based on quantum mechanics which will be studied in higher classes.

Classification of dyes on the basis of Application

Dyes are used to colour fibres, fabrics, paper, leather. walls, food etc. On the basis of application, dyes are of following types:

vDirect Dyes : The fibres are soaked in hot solution of dye and then dries after taking out. These dyes applied directly to fibre and are more useful to the fabrics containing H-bonding like cotton, rayon, wool, silk and nylon, e.g., martius yellow, congo red etc.

Dyes and Pigments Preparation and Properties 7

Dyes and Pigments Preparation and Properties 8

→ Acid Dyes : These dyes are used in slightly acidic medium and are generally suiphoriic acid and its derivatives. These are water soluble and contain polar acidic groups which interact with the basic group of e.g., orange-I, congo red, methyl orange, etc. These dyes does not have affinity for cotton but are used for silk, wool etc.

Dyes and Pigments Preparation and Properties 9

→ Basic Dyes : These dyes contain basic group (like NH₂ group) and react with anionic sites present on the fabric. These are used tu dye nylon and polyester, e.g.. butter yellow, magenta (rosaniline). aniline yellow. etc.

Dyes and Pigments Preparation and Properties 10

→ Disperse Dyes : Disperse dyes are water insoluble. These dyes are finely grind and are available as a paste or a powder that gets dispersed in water. These particles dissolve in the fibres and impart colour to them. These dyes were originally developed for the dyeing of cellulose acetate but now they are used to dye nylon, cellulose triacetate and acrylic fibres too.

Dyes and Pigments Preparation and Properties 11

→ Reactive Oyes : Reactive dyes react with fibres molecules to form a chemical compound. These dyes, are either applied from alkaline solution or from neutral solutions which are then alkalized in a separate process.

→ Sometimes heat treatment is also used for developing different shades. After dyeing, the fabric is washed well with soap so as to remove any unfixed dye. Reactive dyes were originally used for cellulose fibers only but now their various types ar used for wool, silk, nylon, acrylics and their blends as well. Example. prussian red.

→ Ingrain Dyes : These dyes are produced in dye solution during the process of dyeing. Ingrain dyes are those which are synthesized directly on the fabric.

→ Examples of this type of dyes are azo dyes. The fabric is immersed in the solution of coupling reagent (usually a phenol or naphthot). Then it is dipped in the solution of suitable diazonium salt. Both react to form the dye whose molecules are adsorbed on the surface of fabric.

→ The ingrain dyeing is particularly suitable for cotton fabrics. For example, if the fibres soaked in phenol or anthill are put in diazonium salts, then coupling reaction takes place at surface of fibre and insoluble azo dyes are adsorbed at the surface of fibres. Cotton, silk. polyester, nylon etc., are coloured by this process. These dyes are also known as ‘ice colours’ because they react at low temperatures.

→ Vat Dyes : These dyes are known from ancient times. Vat dyes are insoluble in water and cannot dye fibres directly. However, they can be made soluble by reduction in alkaline solution which allows them to affix to the textile fibres. Subsequent oxidation or exposure to air restore the dyé to its insoluble form. Indigo is the original vat dye. These dyes are the fastest dyes for cotton, linen and rayon. They are used with mordants to dye other fabrics such as wool, nylon, polyesters, acrylics and modacrylics.

Dyes and Pigments Preparation and Properties 12

→ Mordaflt Dyes : These are applied with the help of a binding material (e.g., metal ion, tannic acid or metal hydroxide) called mordant. Depending upon the metal ion used, the same dye can give different colours. The mordant or chrome dyes are acidic In character. Sodium or potassium bichromate is used with them in the dyebath or after the process of dyeing is completed. This is done for getting the binding action of the chrome. They are mostly used for wool which gets a good colour fastness after treatment with mordant dyes. They are also used for cotton, linen, silk, rayon and nylon but are

Dyes and Pigments Preparation and Properties 13

→ less effective for them. Alizarin is an important example of such dye. It gives pink colour with aluminium ions and blue colour with beryllium ions.

Dyes and Pigments Preparation and Properties 14

Classification of dyes on the basis of Structure

→ Classification of dyes on the basis of structure is more accurate than on the basis of chemical nature. It gives way to discover more dyes. On the basis of structure, dyes can be classified as:

→ Nitro and Nitroso Dyes : These are ancient known dyes. These dyes contain nitro or nitroso groups as the chromphores and -—OH as auxochrome. A few examples are,

Dyes and Pigments Preparation and Properties 15

→ Diphenyl Methane Dyes : These dyes have basic structure of diphenyl methane. For example, auramine —O is an important dye of this group which is used to colour silk, wool, paper, jute etc.

Dyes and Pigments Preparation and Properties 16

→ Triphenyl Methane Dyes : These dyes are amino derivatives of triphenyl methane. Many dyes are included in this grup. For example, malachite green, an important dyes which is used to colour wool and silk directly.

Dyes and Pigments Preparation and Properties 17

→ Phthalein and Xanthe Dyes : The compounds formed by combination of pthalic anhydride and phenolic compounds are known as phthalein. Xanthene ring structure can also be included in this group. For example, phenolphthalein has phthalein as a ring structure and fluorocene is a xanthene derivative.

Dyes and Pigments Preparation and Properties 18

→ Azo Dyes: It is the largest group of synthesised dyes which contains almost all the colours. Dyes characterised by the presence of an azo group (—N = N—) as the chromophore, whereas group like —NH₂ —NHR, —NR₂, —OH etc. are present as auxochromes. Azo dyes are found in many of the synthetic dye classes. Almost all are fast dyes. Examples are methyl orange, aniline yellow, sudan —I etc.

→ Indigo Dyes : This is the most important group of natural dyes. The dye stuff is extracted from Indigofera tinctorki, a bush pea family. The dye was used pre historically in India, where it probably originated. The word is derived from “Indican”.

Dyes and Pigments Preparation and Properties 19

→ Anthraquinone Dyes : These dyes have anthraquinone as their base and the carbonyl group (>C = O) as the chromophore. Anthraquinone-based dyes are found in most of the synthetic dye classes. Some of the most important red dyes are based on the anthraquinone structure. They are obtained both from plants and insects.

→ These dyes are characterized by good fastness to light. They form complexes with metal salts and the resultant metalcomplex dyes have good wash fastness. This dye can be used with mordant dye where it gives different colours with different ions. Alizarin is an important member of this group.

Dyes and Pigments Preparation and Properties 20

→ Heterocyclic Dyes : In these dyes. at least one odd cyclic ring should be present. It is also a large group and new dyes are discovered. Example, acriflavine dye is used in calico printing, dyeing, medicine etc.

Dyes and Pigments Preparation and Properties 21

Chemistry Notes

Chemtstry in Human Health Preparation and Properties

October 7, 2021September 18, 2021 by Prasanna

Chemtstry in Human Health Preparation and Properties

→ Chemistry influences almost every aspect of human life. The principles of chemistry have been used for the benefit of mankind. Of course, sickness and diseases remind us of medicines-again chemicals. Explosives, fuels, rocket propallents, building and electronic materials, etc., are all chemicals. Air pollution, hereditary disorders, accidents, bacterial infections etc. are reason for diseases in human beings. Various medicines are used for treatment of these diseases.

→ Medicines used in different fields of medical sciences like Ayurvedic, Unani, Allopathy etc.. are either naturally occurring or synthesised materials by che mists. Use of chemicals for therapeutic effect is called chemotherapy.

→ Drugs are chemicals of low molecular masses (∼100-500u). These interact with biomolecules like carbohydrates, proteins. lipids, nucleic acid etc. that take part in various biochemical reactions and produce a biological response. These are used in diagnosis, prevention and treatment of diseases.

→ When the biological response is therapeutic and useful, these chemicals are called medicines. A Doctor select suitable medicine according to the symptoms of the disease. Medicines can be classified on various basis like pharmacological effect, chemical structure, target molecules etc. If these are consumed in doses higher than the recommended ones, they may cause havoc. They may act as poisons and may even prove to be fatal.

→ A doctor prescribed medicine according to symptoms of diseases. Medicines are classified on the basis following:

On the basis of pharmalogical effect.
On the basis of function of medicines.
On the basis of chemical structure.
On the basis of t.raget molecules.

→ Now we will discuss functions, structure and medicinal effect of various drugs.

Analgesics of Chemtstry in Human Health

→ Medicines used to get relieve from pain are called analgesics. These reduce or abolish pain without causing impairment of consciousness, mental confusion, incoordination or paralysis or some other disturbances of nervous system. These are classified as follows:

→ Non-narcotic (Non-addictive) Analgesics These are non-habit forming chemicals which reduce mild to moderate pain such as headache, toothache, muscle and joint pain etc. These are also termed as non-addictive.

→ These drugs do not produce sleep, unconsciousness. Aspirin and paracetamol belong to the class of non-narcotic analgesics. Asprin is the most familiar example. These dris have many other effects such as reducing fever (antipyretic) and preventing platelet coagulation. Aspirin should not be taken empty stomach because on hydrolysis it forms salicylic acid which produces ulcers in walls of empty stomach.

Chemtstry in Human Health Preparation and Properties 1

→ Narcotic Drugs Drugs which produce sleep and unconsciousness are called narcotics. Morphine and many of its homologues, when administered in medicinal doses, relieve pain and produce sleep. These are habit forming drugs. For example, Morphine and Codeine. Morphine diacetate is commonly known as heroin.

Chemtstry in Human Health Preparation and Properties 2

Tranquilizers of Chemtstry in Human Health

→ Tranquilizers are a class of chemical compounds which influence the central nervous systeme and reduce anxiety and tension and thus are used to cure mental diseases. These are neurologically active drugs. These relieve anxiety, stress, irritability or excitement by inducing a sense of well-being. They form an essential component of sleeping pills. There are various types of tranquilizers.

→ They function by different mechanisms. They are the common constituents of sleeping pills. Iproniazid and pheneizine are two such drugs. Some tranquilizers namely, chiordiazepoxide and meprobamate, are relatively mild tranquilizers suitable for relieving tension. Equanil is used in controlling depression and hypertension.

→ Barbiturates Derivatives of barbituric acid viz., Veronal, Amytal, Nembutal, Luminal and Seconal constitute an important class of tranquilizers. These derivatives are called barbiturates. Barbiturates are hypnotic, i.e.. sleep producing agents. Some other substances used as tranquilizers are valium and serotonin.

Chemtstry in Human Health Preparation and Properties 4

Antimicrobials of Chemtstry in Human Health

→ An antimicrobial tends to kill or prevent development of microbes. It inhibits the pathogenic action of microbes such as bacteria, fungi and virus selectively. Diseases in human beings and animals may be caused by a variety of micro-organisms such as bacteria, virus, fungi and other pathogens.

→ An antimicrobial tends to destroy or prevent development or inhibit the pathogenic action of microbes such as bacteria (antibacterial drugs), fungi (antifungal agents), virus (antiviral agents), or other parasites (antiparasitic drugs) selectively. Antibiotics, antiseptics and disinfectants are antimicrobial drugs.

→ Micro organisms are very small in size. These can be observed under microscope only. Many substances are secreted in body which destroys these micro organisms but there is some problem in secration of these substances, then these micro organisms causes diseases by infecting tissues. The diseases caused by these micro organisms can be controlled by three methods:

→ Use of such medicines which destroys micro organisms in the body. These medicines are called bactericidal.
Use of such medicines which stops the growth of the micro organisms. These are known as bacteriostatic.
By increasing immunity in the body.

Antibiotics of Chemtstry in Human Health

→ Antibiotics are the drugs produced by micro organisms and are used to kill or prevent the growth of other micro organisms by intervening in their metabolic processes.

→ These are the substances (produced wholly or partially by chemical synthesis) which in low concentrations inhibit the growth of micro.organisms or destroy them by intervening in their metabolic processes. Antibiotics are very specific in their nature.

→ Paul Edrlich. a German Bacteriologist, investigated arsenic based structures in order to produce less toxic substances for the treatment of syphilis. He developed the medicine, Arsphenamine. known as Salvarsan.

→ Paul Edrlich got Nobel prize for Medicine in 1908 for this discovery. It was the first effective treatment discovered for syphilis. In 1932, he succeeded in preparing the first effective antibacterial agent. Prontosil, which resembles in structure to the compound, salvarsan. Soon it was discovered that in the body prontosil is converted to a compound called suiphanilamide, which is the real active compound thus the suipha drugs were discovered.

→ A large range of sulphonamide analogues was synthesised. One of the most effective is suiphapyridine.

Chemtstry in Human Health Preparation and Properties 6

→ Penicillin was the first antibiotic discovered by Alexander Fleming in 1929. It is a narrow-spectrum antibiotic. Alexander Fleming was given Nobel Prize for the discovery of penicillin in 1945.

Antibiotics are of two Types

→ Bactericidal Antibiotics : Bactericidal antibiotics have cidal (killing) effect on microbes. For example, Penicillin, Ofloxacin, Aminoglycosides, etc.

→ Bacteriostatic AntibiotIcs : Bacteriostatic antibiotics have a static (inhibitory) effect on microbes.
For example, Erythromycìn, Tetracycline, Chioramphenicol, etc.

→ Bacteria can be Gram positive or Gram negative. The range of bacteria or other micro-organisms that are affected by a certain antibiotic is expressed as its spectrum of action.

→ Broad Spectrum Antibiotics : Antibiotics which kill or inhibit a wide range of Gram-positive and Gram negative bacteria are said to be broad spectrum antibiotics.

→ Narrow Spectrum Antibiotics : Those effective mainly against Gram-positive or Gram-negative bacteria are called narrow spectrum antibiotics.

→ Limited Spectrum Antibiotics : If effective against a single organism or disease, they are referred to as limited spectrum antibiotics.

→ With the help of antibiotics, many infectious diseases can be cured. These are highly specific in nature. Their small amounts effective against micro organisms. Some of the important antiboitics are:

→ Penicillin : Total six naturally occurring penicillin are separated. Penicillin G has a narrow spectrum and is mostly used. Ampicillin and amoxycillin are synthetic modifications of penicillins. These have broad spectrum.

→ It is absolutely essential to test the patients for sensitivity (allergy) to penicillin before it is administered. In India, penicillin is manufactured at the Hindustan Antibiotics in Pimpri and in private sector industry.

Chemtstry in Human Health Preparation and Properties 7

→ Chloramphenicol : It is also known as chioromycetin. Chioramphenicol is used in treatment of acute fever, dysentery, typhoid, meningitis, pneumonia etc. Streptomycin is hacteriostatic in low concentration and bactericidal at high concentration. It is given for tuberculosis.

Chemtstry in Human Health Preparation and Properties 8

→ Streptomycin : ¡t is used in the treatment of tuberculosis. It is a broad spectrum antibiotic. It is also used in mental diseases and pneumonia.

→ Tetracycliries : The main antibiotics of this group is iromycine. teramycine. Iromycine is used in treatment of eye infection and tetramycine is used in treatment of typhoid.

→ Suipha Drugs : These are suiphonamides and their derivatives. These are used in treatment of diseases caused by cocci infection. Examples are : Sulfadiazine, Sulfaguanidine. Sulfapvridine. Sulfathizole etc.

Chemtstry in Human Health Preparation and Properties 9

Antiseptics of Chemtstry in Human Health

→ Drugs which are used either to kill or stop the growth of micro organisms are called antiseptics. Antiseptics are applied to the living tissues such as wounds, cuts, ulcers and skin diseases in the form of antiseptic creams like Furacin and Soframycin. Some important examples of antiseptics are dettol which is a mixture of chioroxylenol and terpineol. Bithional is added to soaps to impart antiseptic properties to reduce the odours produced by bacterial decay of organic matter on the skin.

→ Tincture of iodine is a 2-3% solution of iodine in alcohol, which is a powerful antiseptic for wounds. lodoforni (CHI₃) is also used as an antiseptic for wounds.

Chemtstry in Human Health Preparation and Properties 10

→ Antihistomines or Antiallergic Drugs The chemicals which are used for treatment of allergy are known as antihistamines or anti-allergic drugs. The allergy is due to a chemical called histamine. The drugs which interfere with the natural action of histamine for binding sites of receptor where histamines exerts effects are called antihistamines.

→ Histamine is a potent vasodilator. ¡t has various functions. It contracts the smooth muscles in the bronchi and gut and relaxes other muscles, such as those in the walls of fine blood vessels.

→ Histamine is also responsible for the nasal congestion associated with common cold and allergic response to pollen. Histamine is produced by decarboxylation of alpha amino acid. histidine.

Chemtstry in Human Health Preparation and Properties 11

→ In case of allergies such as the common cold, the histamines bind to the receptors in the nasal cavity and cause blood vessels to swell and secrete excess fluid, causing sneezing and a running nose. Antihistamine drugs provide relief from the allergic effects of histamines by attaching themselves to the receptors and thus preventing histamines from binding to receptors. Some of the examples are given below:

→ There are some side effects of taking antihistamines like unconsciousness, sleep etc. Therefore, these drugs should be taken in adequate amount as prescribed by Doctor.

Chemtstry in Human Health Preparation and Properties 12

Anti Fertility Drugs of Chemtstry in Human Health

→ Chemical substances which are used to check pregnancy in women are called anti-fertility drugs. These control the female menstrual cycle and ovulation. All the drugs contain chemicals related to female sex hormones have a steroid ring structure.

→ The increased population has caused many social problems in terms of food resources, environmental issues, employment, etc. To control these problems, population is required to be controlled. This has lead to the concept of family planning.

→ Antifertility drugs are of use in this direction. Birth control pills essentially contain a mixture of synthetic estrogen and progesterone derivatives. Both of these compounds are hormones. It is known that progesterone suppresses ovulation.

→ Estrogen controls the menstrual cycle while progesterone suppresses ovulation. Some of the examples of antifertility drugs are Novestrol. Mifepristone etc.

→ Mifepristone is used as ‘morning after pill”. It works after a women is pregnant and fertiised egg has attached to the wall of the uterus. It causes the uterus to

→ expel the egg thereby terminating the pregnancy. Norethindrone is an example of synthetic progesterone derivative most widely used as antifertility drug. The estrogen derivative which is used in combination with progesterone derivative is ethynylestradiol (Novestrol).

→ Fluoro derivatives of aromatic hydrocarbons are used as antifertility drugs in laboratory. Antifertility chemicals are also found in Soybeans, Peas etc. For example, Metaxylohydroquinone is found in oil of peas.

Chemtstry in Human Health Preparation and Properties 13

Antacids of Chemtstry in Human Health

→ The chemical substances which remove the excess acid in gastric juice and raise the pH to an appropriate level in stomach are called antacids. The most commonly used antacids are weak bases such as Sodium bicarbonate [Sodium Hydrogen Carbonate, NaHCO₃], magnesium hydroxide [Mg(OH)₂] and aluminium hydroxide [Al(OH)₃].

→ Generally liquid antacids are more effective than tablets because they have more surface area available for interaction and neutralisation of acid. Milk is a weak antacid. Antacids are commonly taken to relieve heart burn, which is a common symptom of gastritis or acidity. The stomach produces gastric acid to aid digestion.

→ This acid is mainly composed of hydrochloric acid. Excessive secretion of the acid can lead to stomach disorders such as gastritis, gastric ulcers and peptic ulcers disease. These disorders can be treated by antacids, which reduce the amount of acid in the stomach. Antacids contain weak bases that neutralize the acid in the stomach by reacting with them chemically to produce salt and water.

→ For example sodium hydrogen carbonate reacts with hydrochloric acid to give sodium chloride and water. However, antacids provide only temporary relief as they act on the symptoms alone and not on the cause of gastritis. Hence, their effect can wear off quickly and they have to be taken continually to obtain relief.

→ Excessive hydrogen carbonate can make the stomach alkaline and trigger the production of even more acid. Thus, two antacids drugs are designed namely cimetidine and ranitidine which are helpful in treatment of acidity. Recently, omaprazole and lansoprazole are also synthesised. Structure of some antacids are as follows:

Chemtstry in Human Health Preparation and Properties 14

Chemistry Notes

Chemistry in Daily Life Preparation and Properties

October 7, 2021September 17, 2021 by Prasanna

Chemistry in Daily Life Preparation and Properties

Summary of the chapter and glossary :

→ Chemotherapy: Use of chemicals in treatment of diseases are called chemotherapy.

→ AnalgesIcs : Drugs which reduce or abolish pain without causing impairment of consciousness, mental confusion, incoordination or paralysis or some other disturbances of nervous system are called analgesics.

→ Non-narcotic Analgesics : These are non-addictive. These drugs are effective in relieving skelatal pain such as that due to arthritis. e.g., aspirin and paracetomol.

→ Narcotic Analgesics : These drugs are addictive. These are the drugs which when administered in small amount releive pain and produce sleep. e.g., morphine, codeine and heroin.

→ Tranquilizers : Tranquilizers are a class of chemical compounds used for the treatment of stress, fatigue and mild or even servere mental diseases. e.g., epronizid, phenalgene etc.

→ Barbiturates : These are the derivative of barbituric acid e.g., veronal, amytal, nembutal, luminal and sleep producing agents.

→ AntimIcrobial : An antimicrobial tends to destroy the pathogenic action of microbes such as bacteria, fungi etc.

→ Antibiotics : These are the chemical substances produces wholly or partially by chemical synthesis which in low concentration inhibit the growth or destroy microorganisms by intervening in their metabolic process. e.g., suiphapyridine, sulpanylamide etc.

→ Antlseptics : These are the chemical substances which prevent the growth of microorganisms or kill them. These are applied to wounds, cuts, ulcers etc. e.g., furacin, soframycin etc.

→ Disinfectants : These are chemical substances which kill microorganisms or stop their growth but are harmfull to living tissues. These are used to kill the microorganism present in floors, drains, toilets etc. e.g., 1% solution of phenol etc.

→ Antihitamines or Antiallergic drugs : The drugs which are used in treatment of allergy are called antihistamines. e.g., bromphenaramine, diphenyl hydramine, chioripheniramine etc.

→ Antifertility Drugs : The drugs which are used to prevent unwanted pregnancies. e.g., norethindrone, ethynylestradiol etc.

→ Antacids: Chemical substances which remove the excess acid in the stomach and raise the pH to appropriate level. e.g., sodium hydrogen carbonate, zentac, calcium hydroxide etc.

→ Dyes : The organic compound which are used in colouring of food, paper. walls and other substance are called dyes.

→ Artificial Sweetning Agents : These are the chemical substances which are sweet in taste but do not add any calories to our body. For example: saccharin, aspartame, sucrolose etc.

→ Food Preservatives: Chemical substances which are used to protect food against baëteria, yeasts and moulds are called food preservatives. For example: sodium meta bisulphite, sodium benzoate etc.

→ Cleansing Agents : The substances which are used for cleaning purpose are called cleansing agents.

→ Soaps : Soaps are sodium or potassium salts of fatty acids e.g.. sodium palmitate.

→ Detergents : These are the cleansing agents which have all properties of soap but actually do not contain any soap. These can be used both in soft and hard water as they give foam even in hard water.

→ Insect Repellents : Those chemical substances which are used to kill and move away insects are called insect repellents e.g., pheniL begon, andosulphon, phenocol, phenyl, odonyl, odomas etc.

→ Pheromenes – Sex-Attractants : A chemical substance produced and released into thè environment by an animal and affecting the behaviour or physiology of others of its species is called pheromone. e.g., muskon, sevoton etc.

→ Rocket Propellents : Substances used for launching rockets are called rocket propellents. These are the combination of an oxidiser and a fuel.

Other Important Facts

Treatment of diseases from chemical substances are called chemotherapy.
Sulpha medicines are called suiphonamides.
Antipyretics are used to bring down the body temperature during high fever.
Paracetomol is an example of antîpyretic.
Antibiotics are the chemical substances which prevent the growth of microorganisms or kill them hut are not harmful to living human tissues.
Alexender Flaming discovered the first antibiotic pencillin.
Antiseptics are the chemical substances which kill microorganisms or stop their growth but are harmful to living tissues. e.g., tincher of iodine, mercurochrome etc.
1% solution of phenol is disinfectant but 0.2% solution of phenol is antiseptic.
Mixture of terpineol and ethyl alcohol, chiorozylenol is called dettol.
The chemical compounds which aré used for treatment of stress, fatigue and several mental diseases are called tranquilizers.
These releive anxiety, stress, irritability or excitement by inducing a sense of well-being.
The compounds which decrease the sleep and mental stress are called tranquilizers.
Wavelength of visible light is 400 nm to 750 nm.
First useful dye is synthesised by W.H. Parkin in 1856.
German scientist Otto Witt gives Witt Principle for explain the relation between colour and structure or organic compounds in 1876.
Compounds which contains chromophores are called chromogen&.
Sodium benzoate is most useful preservative.
Elitam is 2000 times sweeter than sucrose.
Saccharine sweetner is used by diabetics patient.
Detergents can be used with hard and soft water both.
All insect repellants affect respiratory system of insects.
Muscon pheromon is present in Kastoori of Kastoori dear.
Social attracted pheromens are used in insect controll.
In solid propellants fuel and oxidiser both are in solid form.
Carbon fibres are long chain structures of carbon compounds. Carbon fibres are great importance in many fields.

Chemistry Notes

Importance of Stereochemistry Preparation and Properties

October 7, 2021September 17, 2021 by Prasanna

Importance of Stereochemistry Preparation and Properties

→ Many organic compounds have same structure and molecular formula but the atoms or groups present in the molecule have different positions in space. The study of such compounds is done in stereochemistry.

→ With the help of stereochemistry, we can discover new methods for synthesis of organic compounds and new researches are also performed. It is helpful in understanding of reaction kinetics.

Chemistry Notes

Conformational Isomerism Preparation and Properties

October 7, 2021September 17, 2021 by Prasanna

Conformational Isomerism Preparation and Properties

→ Consider a simple hydrocarbon ethane, the free rotation of one carbon with respect to another gives rise to various arrangements of the atoms differing in relative positions of hydrogen atoms attached to these carbon atoms. These different spatial arrangements of atoms in space resulting due to free rotation around a single bond are called conformation or conformers or rotamers.

→ The energy required to show such isomerism is 3-15 kcal per mole which can be easily obtained at room temperature. These can be isolated because free rotation of carbon-carbon single bond takes place automatically at room temperature.

Conformation in Ethane

→ Ethane molecule (C₂H₆)contains a carbon-carbon single bond with each carbon atom attached to three hydrogen atoms. Infinite number of spatial arrangements of hydrogen atoms áttached to one carbon atom with respect to the hydrogen atoms attached to the other carbon atom.

→ There are two extreme cases. One such conformation in which hydrogen atoms attached to two carbons are as closed together as possible is called eclipsed conformation and the other in which hydrogens are as far apart as possible is known as the staggered conformation. Any other intermediate conformation is called as skew conformation. It may be remembered that in all the conformations, the bond angles and the bond lengths remain the same. Eclipsed and the staggered conformations can be represented by-

Sawhorse formula and
Newman projection formula

→ In a Newman projection formula, the molecule is viewed along an axis containing two atoms bonded to each other and the bond between them, about which the molecule can rotate. In a Newman projection formula, the “substituents” of each atom composing the bond, be the hydrogens or functional groups. can then be viewed both in front of and behind the carbon-carbon bond.

→ Specifically, one can observe the angle between a substituent on the front atom and a substituent on the back atom in the Newman projection, which is called the dihedral angle or torsion angle. Three front and three rear, total six sigma bonds are present.

→ In ethane, these are bonded by hydrogen bonding. Now if front three carbon atoms totally overlap rear three carbon atoms, it is known as eclipsed from. In staggered form of ethane, the electron clouds of carbon-hydrogen bonds are as far apart as possible.

Conformational Isomerism Preparation and Properties 1

→ In ethane, if the dihedral angle is o° and the hydrogens on the first carbon line up with or eclipse the hydrogens on the second carbon. When the dihedral angle is 0° and the hydrogens line up perfectly, ethane has adopted the eclipsed conformation.

→ The other extreme occurs when the hydrogens on the first carbon are as far away as possible from those on the second carbon; this occurs at a dihedral angle of 60° and is called the staggered conformation.

→ In staggered form of ethane, the electron clouds of carbon-hydrogen bonds are as far apart as possible. Thus, there are minimum repulsive forces, minimum energy and maximum stability of the molecule. In eclipsed conformation, the C—H bonds on the front and back carbons are aligned with each other with dihedral angles as 0°.

→ The eclipsed conformation of ethane has three such C—H eclipsing interactions, so one can infer that each eclipsed C—H ‘costs’ roughly 4.2 kJ mol^-1 (1 kcallmol). Eclipsing interactions are an example of a general phenomenon called steric hindrance, which occurs whenever bulky portions of a molecule repel other parts of the same molecule. Because such hindrance causes resistance to rotation, it is also called torsional (or Pitzer) strain.

Conformational Isomerism Preparation and Properties 2

→ Magnitude of torsional strain depends upon the angle of rotation about C—C bond. This angle is also called dihedral angle or torsional angle. Among all the conformations of thane, the staggered form has the least torsional strain and the eclipsed form has maximum torsional strain. Thus it may be inferred that rotation around C—C bond in ethane is not completely free. The energy difference between the two extreme forms is of the order of 12.5 kJ mol^-1 ,which is very small.

→ Even at ordinary temperature, the ethane molecule gains thermal or kinetic energy sufficient enough to overcome this energy barrier of 12.5 kJ mol^-1 through intermolecular collision.

Newman’s Projection Formula of n-butane

Conformational Isomerism Preparation and Properties 3

Stability : Fully staggered form > Gauche form > Eclipsed form > Fully eclipsed form
Energy : Fully eclipsed form > Eclipsed form > Gauche form > Fully staggered form

→ Conformation In Cyclic System : In this series, we can take the example of cyclohexane. In cyclohexane, all the carbons are sp³ hybridised and thus configured at an angle of 109.28’. Therefore, it should not have planar hexagonal shape. Infact, cyclohexane exist, either in the shape of chair or boat. in 1918, Mohr suggested that cyclohexane can exist in two forms chair and boat form and the energy difference between the two forms is 6-7 kcallmol.

→ Hence these forms can be intercoverted at room temperature. The chair conformation is the most stable conformation. Chair form is free from torsion strain. In the chair conformer of cyclohexane, all the bond angles are 1110, which is very close, the ideal tetrahedral bond angle of 109.5°, and all the adjacent bonds are staggered.

→ Cyclohexane can also exist in a boat conformation. Like the chair conformer, the boat conformer is free of angle strain. However, the boat conformer is not as stable as the chair conformer because some of the bonds in the boat conformer are eclipsed, giving it torsional strain.

→ The boat conformer is further destablized by the close proximity of the flagpole hydrogens (the hydrogens at the “bow” and “stern” of the boat), which causes steric strain. Torsional strain and flagpole interactions cause boat conformation to have considerably higher energy than chair conformation. The chair form is more stable than the boat form by 44 kJ mol^-1.

Conformational Isomerism Preparation and Properties 4

Newman’s Representation of Chair and Boat form of Cyclohexane.

Stability : Chair form > Boat form
Energy : Boat form > Chair form

Axial and Equatorial Hydrogen Bonds

→ In chair form of cyclohexane, there are two different types of C—H bonds, and thus two different types of hydrogen atoms as substituents. The C—H bonds, which point vertically upward or downward, are called axial hydrogen atoms (H_ax).

→ There are six of these, three upward and three downward bonds, and they alternate up/downlup, etc., around the ring. The other six bonds which radiate away from the “equator” of the ring, are called equatorial hydrogeñ atoms (H_eq). There are six of them, three of which are “slant up” and three of which are “slant down”, again alternating around the ring.

Conformational Isomerism Preparation and Properties 5

Chemistry Notes

Resolution of Racemic Mixture Preparation and Properties

October 7, 2021September 17, 2021 by Prasanna

Resolution of Racemic Mixture Preparation and Properties

→ The process of separation of racemic mixture into the enantiomers is called resolution. Most of naturally occurring compounds are optically active because the enzymes that bring about their formation and often the raw materials from which they are made are themselves optically active. But most of the synthetic organic reactions result in the formation of the racemic mixture.

→ Since, the enantiomers present in the racemic mixture have identical physical properties (like melting points, boiling points, zefractive indices, reactivities, and solubilities etc.), except for direction of rotation of polarized light, hence they cannot to be separated by the usual simple methods like fractional crystallization or fractional distillation, etc.

→ There are many methods for their separation, but the most widely used is by the chemical method in which racemic mixture is converted to diastereomeric mixture.

→ Mechanical Separation Method : This was the first method of separation of racemix mixture used by Louis Pasteur, and it is mainly of historical interest. This involved mechanical separation of the crystals of one enantiomer from the other in a racemic mixture based on differences in their shapes.

→ Louis Pasteur did so by growing the crystals of tartaric acid and they looked like mirror images. He then used a microscope and tweezers to separate the crystals from each other. But this method has limitations as it is very time consuming andonly used on crystalline solids.

→ Bio-chemical Separation Method : In this method, certain bacteria, yeast or mould are allowed to grow in a dilute solution of the racemic mixture. It reacts with or consumes only one of the enantiomers for its growth and give other small products while the other is left behind pure.

→ For example, when allowed to grow in racemic mixture of ammonium tartarate, it consumes (+) ammonium tartarate while pure (-) ammonium tartarate is obtained as the product. This method also has some limitations as only one isomer can be obtained.

→ Also the yield of that isomer is also very poor because only dilute solution can be used. Similarly, acetylation of dl-amino acids takes place in which amino group can be protected. Now, yeast is added to this racemic mixture, here, enzyme amylase destroys l-acetyl derivative selectively by hydrolysis. d-derivative is unaffected. By hydrolysis of d-acetyl derivatives d-amino acids can be obtained.

Resolution of Racemic Mixture Preparation and Properties 1

Some of the limitations of this method are:

Selection of suitable bacteria is very difficult.
Bacteria reproduces in dilute solutions. Hence, to obtain bacteria by dilute solutions is time taking.
Sometimes one isomer is destroyed by bacteria. Thus, it is very costly method.

→ Column Chromatographic Method : A racemic mixture can also be resolved with the help of column chromotography. The solution of the racemic mixture is prepared in a suitable solvent and is passed through a column packed with a suitable chiral adsorbent. One of the enantiomers is selectively adsorbed on the surface of adsorbent.

→ It is then diluted with a suitable solvent and the solution is collected at the bottom of the column, which contains pure enantiomer. For example, first of all. Henderson and Rule in 1939, resolve d and l isomers by adsorbing D-lactose for resolution of camphor derivative. (p.phenylene bis (imino) camphor).

→ Chemical Methods : Consider a racemic mixture of enantiomeric organic acids, (±)-HA. It can be separated into pure enantiomeric acids by reaction with an optically active base, say a laevo rotatory base, (-)-B.

→ The mixture of enantiomeric acids on reaction with an optically active base results in the formation of two diastereomeric salts as shown in figure. These diastereoisomeric salts have, of course, different physical properties, including solubility in a given solvent. They can, therefore, be separated by a simple technique, i.e., fractional crystallization.

Resolution of Racemic Mixture Preparation and Properties 3

→ The optically active acid can then be recovered from each salt by the addition of a strong mineral acid. The most commonly used optically active organic bases for this purpose are (-)-Brucine, (-)-Quinine, (-)-Strychnine. and (+)-Cinochonine. For example, resolution of a racemic mixture of 2-chioro propanoic acid by salt formation with (+) 1 -Phenylethyla mine.