Prasanna – Page 2 – Class 12 Chemistry

Racemic Mixture and Racemisation Preparation and Properties

October 7, 2021September 17, 2021 by Prasanna

Racemic Mixture and Racemisation Preparation and Properties

→ When equal amounts of dextro rotatory and laevo rotatory isomers are present, the resulting mixture becomes optically inactîve because the optical activities of each isomer cancel each other. Such a mixture is called a racemic or dl mixture or racemic modification.

→ Racemic mixtures can be symbolized by a — or (±) prefix in front of the substance’s name. Since eriantiomers have equal and opposite specific rotations, a racemic mixture exhibits no optical activity.

→ For example,if racemic mixture of lactic acid is taken, it is written as dl-lactic acid. Thus, it is optically inactive and cannot rotate plane of polarised light in any direction, Infact, the amount of light rotated by half part i.e., d-lactic acid is neutralised by another half part i.e., i-lactic acid. Thus, both the parts combine together to form a racemic mixture. This is known as external compensation.
For example, following reactions give racemic mixtures,

Racemic Mixture and Racemisation Preparation and Properties 1

→ The process by which a racemic mixture is formed from chiral materials is called racemisation. Thus, above reactions are racemisation reactions.

→ Such reactions which occur by unimolecular nucleophilic substitution always form racemic mixture. In these reactions, a highly reactive carbocation is formed on which nucleophile can attack by both sides in the next fast step. Thus, mixture of 50% d and 50% ¡ isomer is formed. Example, 2-Chloropropanoic acid on alkaline hydrolysis gives dl racemic mixture. The reaction can be represented as follows:

Racemic Mixture and Racemisation Preparation and Properties 2

Racemisation can be affected by following reasons:

By heating at high temperature
By heating with water
By adding base
By adding sulphuric acid
By auto racemisation

Chemistry Notes

Relative and Absolute Configurations or D-L Nomenclature Preparation and Properties

October 7, 2021September 17, 2021 by Prasanna

Relative and Absolute Configurations or D-L Nomenclature Preparation and Properties

→ Two configurations are possible for a compound having one asymmetric carbon atom which represents two optical isomers. One of them is dextro rotatory and other one is laevo rotatory. E. Fischer suggested that optical activity is an experimental property.

→ In other words, we need a system of nomenclature that indicates the configuration (arrangement) of the atoms or groups about the asymmetric carbon. Some rules are given by Fischer to determine the configuration of asymmetric carbon atom.

→ This is a system of assigning relative configuration to a molecule by establishing configurational relationship among the members of a family of compounds (i.e., carbohydrates and amino acids). Glyceraldehyde is arbitrarily chosen as standard.

→ Most of the sugars are genetically related to either D or L-glyceraldehyde and thus their configurations are defined by D or L According to Fischer projection formula, the main carbon chain of glyceraldehyde is kept vertically in which —CHO group is at the top and CH₂OH group is at the bottom.

→ Now, if —OH group of the chiral carbon atom is placed right side, it is D-configuration and if —OH is placed on the left side, it is L – configuration. Note that D and L have no relation with optical rotation of a molecule.

Relative and Absolute Configurations or D-L Nomenclature Preparation and Properties 1

Similarly, end of sugars can be shown by D and L configurations.

Relative and Absolute Configurations or D-L Nomenclature Preparation and Properties 2

→ D and L nomenclature is referred as the relative configuration because the molecule to be assigned D- or L-notation is being related to another molecule D- or L-glyceraldehydes accepted as a standard.

→ The compounds which can be obtained by D-glyceraldehyde or can be converted to D-glyceraldehyde are referred to as compounds having D-configuration whereas the compounds which can be obtained by L-glyceraldehyde or can be converted to L-glyceraldehyde are referred to as compounds having L-configuration. This is relative configuration. For example, the compounds obtained by D-glyceraldehyde have D-configuration as :

Relative and Absolute Configurations or D-L Nomenclature Preparation and Properties 3

→ The D-notation may be given to both dextro rotatory [d or (+)] or laevo rotatory molecule [l or (-)]. Same is true for L-notation. Hence, in the carbohydrates, to avoid the confusion (in the spoken language) between D- and d (+) and L- and l -; the most commonly used term to denote dextro rotatory compounds is (+) and not d. Simi1rly, most commonly used term to denote laevo rotatory compounds is (-) and not l.

→ In 1950, when X-ray crystallographic techniques are developed then Bijvoet found that Rosen hoff was considered that form of glyceraldehyde as D-configuration volumterically, it was actually D-form. Some other examples:

Relative and Absolute Configurations or D-L Nomenclature Preparation and Properties 4

Chemistry Notes

Fischer Projection Formula Preparation and Properties

October 7, 2021September 17, 2021 by Prasanna

Fischer Projection Formula Preparation and Properties

Fischer projection is the two dimensional representation of a three dimensional molecule. The main points of Fischer Projection Formula is:

→ For the compounds with one chiral carbon at centre, Fischer projection formula is drawn as intersecting horizontal and vertical lines. The chiral carbon is considered to lie on the point of intersection and the lines originating from this point are considered as four bonds joined to that chiral carbon (chiral C atom is the one which is attached to four different atoms or groups).

→ The horizontal lines represent the bonds projecting out of the plane of paper and the vertical lines represent the bonds projecting away (or behind) the plane of paper.

The rest two substituents are written on left and right side of horizontal line. It is clear from the following examples:

Fischer Projection Formula Preparation and Properties 1

Following points should be kept in mind while writing Fischer Formula:

→ Projection formula should never be move out of plane or should not be rotated.

→ The single most important rule regarding rotating a Fischer projection formula is that 900 or 270° rotations are disallowed. Such type of rotation will form image form of original compound.

→ Projection formula can be rotated at an angle of 180° because due to this, there is no change in three dimensional form of compound.

Fischer Projection Formula Preparation and Properties 3

→ Configuration is not changed by inter change of two groups twice on chiral carbon.

Fischer Projection Formula Preparation and Properties 4

Chemistry Notes

Optical Isomerism Preparation and Properties

October 7, 2021September 17, 2021 by Prasanna

Optical Isomerism Preparation and Properties

→ The concept of optical isomerism was first of all given by Louis Pasteur in 1848. He found that sodium ammonium tartarate is found in two crystalline forms which are mirror images of each other.

Optical Isomerism Preparation and Properties 21

→ Louis Pasteur separated these crystals and then by X-ray diffraction. It is found that these crystals are mirror image of each other which cannot be superimposed to one another. Such compounds can rotate plane of polarised light. Therefore, these isomers are known as optical isomers and this phenomenon is called optical isomerism.

→ Those compounds which can rotate the plane of polarised light are called optically active compounds and this property is known as optical activity. The isomer which can rotate the plane of polarised light towards right or clockwise direction is called dextro rotatory whereas which can rotate the plane of polarised light towards left or anticlockwise direction is called laevo rotatory. Dextro rotatory isomer is represented by d or (+) and laevo rotatory isomer is represented by or (-).

→ For example, d-lactic acid and i-lactic acid. The dextro rotatory and laevo rotatory compounds are called optically active compounds.

Optical Isomerism Preparation and Properties 22

Plane Polarised Light

→ Ordinary light can be considered as an electromagnetic wave, which has electric and magnetic vibrations in all the directions perpendicular to the path of propagation. Ordinary light is made of light wave of different wavelengths.

→ By using prism or diffraction grating, light of a single wavelength, known as monochromatic light is obtained. If the light waves pass through a polarizer, which is made of Nicol prism, then only electric vibrations emerge in one plane. Such a beam of light is called plane polarised light. Plane polarised light is produced by an instrumet called polarimeter.

Optical Isomerism Preparation and Properties 1

Polanmeter and Polarity (Optical Activity)

→ The angle of rotation by which the plane polarised light is rotated, can be measured by an analyzer in a polarimeter.

→ A polarimeter consists of a light source, two Nicol prisms and a sample tube to hold the substance. The sample tube is placed between two Nicol prisms. The prism placed near the source of light is called polarizer while the other placed near the eye is called analyzer.

→ The aqueous solution of the substance is placed between the polariser and analyser. The analyser can be rotated by certain angle to compensate for the rotation of the plane polarized light by the optically active sample. The observed rotation (α_observed) is expressed in degree.

→ When monochromatic, plane-polarised light passes through certain organic compounds, the plane of polarisation is changed. These compounds are referred to as optically active compounds. If the substance rotates plane polarised light to the right (clockwise), it is called dextro rotatory (Greek for right rotation) or the d-form and it is indicated by placing a (+) sign before the degree of rotation. If light is rotated towards left (anticlockwise), the substance is said to be laevo rotatory (Greek for left rotating) or the 1-form and negative (-) sign is placed before the degree of rotation.

Specific Rotation

→ It is the number of degree of rotation observed if a 1 dm long tube is used, and the compound being examined is present to the extent of 1 g/mL. This is usually calculated from observations with tubes of other lengths and at different concentrations by means of the equation,

Optical Isomerism Preparation and Properties 2

→ Where l represents length of the polarimeter tube (in dm) and C represents concentration of a solution or density for a pure liquid in g/mL (or g/cm³). t is the temperature and λ is the wavelength of the light used. The specific rotation for an active substance in any light in specific as other physical constants like melting point, boiling point, density, or refractive index etc.

Elements of Symmetry

→ A substance has three elements of symmetry. If any of these symmetry element is present in a molecule then the molecule does not show optical activity.

→ Plane of Symmetry : It represents the plane bisecting the molecule such that each half of the molecule is the mirror image of the other half. For example : Following letters of english alphabets have plane of symmetry,

Optical Isomerism Preparation and Properties 3

Tetrafluoroethane has vertical plane of symmetry and Tartaric acid has vertical plane of symmetry as shown below:

→ These compounds do not show optical activity i.e.,these are optically inactive.

Optical Isomerism Preparation and Properties 4

→ Axis of Symmetry : It represents a n-fold axis of symmetry such that when molecule possessing such an axis is rotated through an angle of 360°/n about this axis and then reflected across a plane perpendicular to this axis, an identical structure results. The notation for a symmetry axis is C_n, where a is an integer chosen so that rotation about the axis by 360° returns the object to a position indistinguishable from where it started. For example, water has C₂ axis of symmetry because water on rotating by 180° returns back to its original position.

Optical Isomerism Preparation and Properties 5

→ Similarly, Ammonia has C₃ axis of symmetry. The compounds in which axis of symmetry is found are optically inactive.

→ Centre of Symmetry : A molecule is said to have a centre of symmetry if any line drawn from the centre of the molecule meets identical atoms at equal distances from the centre. This is also called centre of inversion. For example, S letter of english alphabet.

Optical Isomerism Preparation and Properties 6

For example, Benzene. Ethene etc., has centre of symmetry.

Optical Isomerism Preparation and Properties 7

Chirality Centre and chiral compounds or Stereo Centre

→ If in any molecule, elements of symmetry are absent then the molecule will be asymmetric and able to rotate the plane of polarised light i.e., it shows optical activity. Such types of molecules have this property due to lack of symmetry, such molecules are not superimposable on their mirror images. An object, which is non.superimposable on its mirror image, is said to be chiral.

→ Chiral means asymmetrical. The word chiral is derived from the Greek word “kh&r” which means, “hand” and property of being chiral is called chirality or handedness. One mirror image rotates the plane of polarised lighi towards the right known as dextro rotatory which is denoted by d, whereas other mirror image towards the left known as laevo rotatory. It is denoted by l. Chirality results from an asymmetry in the molecule.

→ When four different atoms or groups are present around a carbon atom then that carbon atom is called chiral carbon or asymmetric carbon. Presence of a chiral carbon or asymmetric carbon generally leads to assymetry in a molecule and molecule will be optically active. For example : In lactic acid.

Optical Isomerism Preparation and Properties 8

→ The starred carbon is asymmetric carbon atom which is known as chiral centre. Chiral molecules and their mirror images are known as optical isomers just as in above example lactic acid and its mirror images are optical isomers of each other. Here, chiral centre is also known as ‘Stereogenic Centre’ because it is responsible for optical isomerism. Some other examples which show optical isomerism are :

Optical Isomerism Preparation and Properties 9

→ It must be noted that enantiomers possess identical physical and chemical properties but differ in the direction (sign) of rotation of the plane polarized light.

→ Magnitude of rotation, however, is same for the two isomers. One of the isomers is dextro rotatory (+) enantiomer while the other is laeveo rotatory (-).

Compounds with to Stergeic Centers

There are certain molecules, which contain more than one asymmetric carbon atoms. In that case, two conditions are possible :

Non-symmetrical molecules of the type : Cabe. Cxyz (or Cabc.Cabd).
Symmetrical molecules of the type : Cabc. Cabc

→ Non-symmetric Molecules : If an optical isomer has n chiral atoms, then the number of optical isomers will be 2ⁿ. For example, 2, 3-dichioro butanoic acid has two chiral atoms which contains different atoms or groups. Using the 2ⁿ formula there should be maximum of 2² = 4 stereoisomers.

Optical Isomerism Preparation and Properties 10

→ Structures I and II and structures III and IV are mirror images of each other. These are known as enantiomers. On the contrary, structure I is not mirror images of structure III or IV and structure II is also not a mirror image of either III or IV.

→ They are referred to as stereoisomers but not enantiomers as they are not mirror images. Stereoisomers that are non-superimposable and not mirror images of each other are called diastereoisomers.

→ The pairs of diastereoisoniers in this case are : I and III; II and III; I and IV: II and IV. It should be noted here that diastereoisomers have similar chemical properties, since they have same structural formula. However, their chemical properties are not identical.

→ Diastereoisomers have different physical properties, they have different melting points, boiling points, solubilities in a given solvent, densities, refractive indexes and so on. Diastereoisomers differ in specific rotation they may have the same or opposite signs of rotation, or some may be inactive.

→ As a result of differences in boiling point and solubility, diastereoisorners can be separated from each other by fractional distillation or fractional crystallization. Due to difference in their molecular shapes and polarity, they differ in adsorption and can be separated by chromatography.

→ In the Fischer projection formula, the erythro isomer has two identical substituents on the same side and the threo isomer has them on opposite sides. The terms erythro and threo are derived from erythrose and threose sugars respectively. The structure can be given as :

Optical Isomerism Preparation and Properties 11

→ Symmetrical Molecules (Meso Compounds) : In this group, tartaric acid can be taken as example,

Optical Isomerism Preparation and Properties 12

→ Using the 2ⁿ formula, there should be maximum of 2² = 4 stereoisomers. These can be represented as :

Optical Isomerism Preparation and Properties 13

→ I and II i.e., threo forms are mirror images of each other and are enantiomers. But in erythro form, plane of symmetry is present. Thus, it does not show optical isomerism. Therefore, tartaric acid exists only in three isomeric forms out of which one is meso (optically inactive) and the other two are optically active i.e., d or (+) and I or (-).

→ There are certain molecules which do not contain chiral carbon atom but they are optically active such as biphenyls, allenes.etc. Actually, in these compounds, no element of symmetry is present. Due to this, molecules are not superimposible on their mirror images. Such molecules have molecular chirality.

→ Internal CompensatIon : Erythro group (III) and (IV) of tartane acid is called mesotartaric acid. In this, half part is dextrorotatory (d) and half is laeveo rotatory (l). So, in the plane of monochromic light, resultant displacement is zero. Due to these reasons, it is optically in active.

Optical Isomerism Preparation and Properties 14

Examples of optically active compounds :

→ Lactic acid and maleic acid : Lactic acid and maleic acid are the examples of optically active compounds. which have one asymmetrical carbon atom.

Optical Isomerism Preparation and Properties 15

→ These two acids are found in three forms, out of which two forms are d and ¡ form and third form is optically inactive because it is racemic mixture of two optically active forms. Third form is originated only when two forms (d-and i-form) are mixed in equimolar amounts. This third form is known as dl form

Optical Isomerism Preparation and Properties 16

So lactic acids have three forms:

d (+) lactic acid → This rotates the plane of polarized light towards right.
l (-) lactic acid → This rotates the plane of polarized light towards left.
dl (±) lactic acid → It can not rotate plane polaresed light in any side.

Tartaric Acid : These contains two asymmetrical carbon atoms which has four isomers :

→ d(+) Tartaric Acid it can rotate the plane of polarized light towards right.

Optical Isomerism Preparation and Properties 17

→ l(-) Tartane Acid can rotates the plane of polarized light towards left .

Optical Isomerism Preparation and Properties 18

→ Racemic Tartaric Acid or (dl) Tartaric Acid It is optically inactive due to ixternal compensation. It is a equimolar mixture of d and l-forms.

Optical Isomerism Preparation and Properties 19

→ Mesotartaric Acid : In this isomer, one asymmetric C-atom rotates plane of polarised light towards right and other asymmetric C-atom rotates plane of polarised light towards left. So, whole molecule becomes optically inactive. It can not be dissociated into d(+) and l (-) forms.

Optical Isomerism Preparation and Properties 20

Chemistry Notes

Types of Configurational Isomerism Preparation and Properties

October 7, 2021September 17, 2021 by Prasanna

Types of Configurational Isomerism Preparation and Properties

Geometrical Isomerism :

→ It includes theres isomers which can be converted into one another at normal temperature. The essential energy for free roEation is 5-15 kcal/mol, which can be easily available at room temperature. Free rotation about carbon-carbon double bond is not possible.

→ Restricted rotation can be represented by nail model. Two card boards are coloured by some colour on one side and a nail is kept in between them as shown in figure. The card boards can be rotated freely with respect to the nail. This shows free rotation. But if two nails are placed parallel then free rotation is not possible to change the position of card boards, it is necessary to remove nails.

→ This is restricted rotation. Compounds having double bonds and cyclic compounds show same behaviour which is responsible for geometrical isomerism.

Types of Configurational Isomerism Preparation and Properties 1

→ Geometrical isomerism is a type of steroisomerism, which is represented by organic compoundš having C = C bond in which two atoms or groups attached with C = C bond are different. This isomerism is obtained due to rotation around C = C bond. Such type of isomerism is called geometrical isomerism or cis-trans isomersm and such isomers are called geometrical isomers. This type of isomerism is found mainly in following type, of compounds :

In alkenes
In oximes
In alicyclic compounds

The compounds having geometrical isomerism should satisfy following two conditions:

Bond having restricted rotation should be present.
The substituents around the bond having restricted rotation should be different.

→ Geometrical Isomerism In Alkenes : In Alkenes, due to presence of carbon-carbon double bond, free rotation is not possible. If two carbon atoms containing double bond are attached to different substituents, then two stable configurations are possible in space.

→ The isomer of the type (a), in which two identical atoms or groups lie on the same side of the double bond is called cis-isomer and the other isomer of the type (b), in which identical atoms or groups lie on the opposite side of the double bond is called trans-isomer.

→ Thus cis and trans isomers have the same structure but have different configuration (arrangement of atoms or groups in space)

Types of Configurational Isomerism Preparation and Properties 2

→ It should be noted that if any two substituents on the carbon atoms containing double bonds are same, then geometrical isomerism is not possible in such compounds. For example:

Types of Configurational Isomerism Preparation and Properties 3

→ Above compounds do not show geometrical isomerism because on changing the position of substituents. no new configuration is obtained.

→ Geometrical Isomerism in Oximes : The reaction of aldehydes and ketones with amines gives aldoximes and ketoximes as:

Types of Configurational Isomerism Preparation and Properties 4

→ It is clear that in C = N, C and N are in sp<sup>2</sup> hybridisation. Due to presence of double bond, free rotation is not possible. If OH group on N atom is present on side of hydrogen attached to carbon or to small group, then cis and if on opposite side, then trans isomer is possible. For example : Acetaldoxime

Types of Configurational Isomerism Preparation and Properties 5

→ Geometrical Isomerism ¡n Alicylic Compounds: Disubstituted cycloalkane derivatives like cyclopropane, cyclobutane, cyclopentane etc. have restricted rotation due to cyclic structure. These compounds also show cis and trans geometrical isomerism. If both the substituents are present on same side of plane, it is cis isomer and if they are present in opposite side, it is isomer. Some exampks of geometrical isomerism in alicyclic compounds are:

Types of Configurational Isomerism Preparation and Properties 6

Charaderistics of Geometrical Isomeis

The spatial arrangement of geometrical isomers is different. Due to this, they have different physical and chemical properties. Some of the characteristics are:

→ Dipole Moment : Generally, trans isomers have no dipole moment because they cancel electronic effect of each other because they are placed opposite to each other. Example, trans isomer of both 2-butene and 1.2-dichioroethene have zero dipole moment whereas cis isomers have some dipole moment as these are polar. The dipole moments are 1.85 D and 1.4 D.

Types of Configurational Isomerism Preparation and Properties 7

→ If the isomers are such that substituents present on both the carbon atoms have opposite electronic effects then dipole moment of-1.is isomer will be relatively low. For example: 1-chioropropene

Types of Configurational Isomerism Preparation and Properties 8

→ Melting and Boiling Points : The melting and boiling points are also effected due to geometrical isomerism. Generally, van der Waal’s forces in trans isomer are stronger than cis isomer. Hence, their melting and boiling points are also higher. For example:

Types of Configurational Isomerism Preparation and Properties 9

→ It should be noted here that if such substituents are present which have opposite effects, then melting point of cis isomer will be high.

Types of Configurational Isomerism Preparation and Properties 10

→ Chemical Properties : The functional groups in cis isomers are on one side which can easily undergo cyclisation but it is not possible in trans isomer. For example, maleic acid undergoes dehydration to give maleic anhydride on heating at 140°C but its trans isomer, Fumaric acid, does not show such reaction under the same reaction conditions.

→ This is because the later has two —COOH groups farther away and hence it undergoes dehydration to give maleic anhydride on heating at 270°C be cause it. is first converted into maleic acid and then it is dehydrated.

Types of Configurational Isomerism Preparation and Properties 11

→ Hydrogen Bonding : Geometrical isomers also show hydrogen bonding which affect physical properties. For example. cis isomer of 1,4-dibutenoic acid has intramolecular hydrogen bonding whereas trans isomer has intermolecular hydrogen bonding. It is clear from their values of boiling points.

Types of Configurational Isomerism Preparation and Properties 12

Chemistry Notes

Stereoisomerism Preparation and Properties

October 7, 2021September 16, 2021 by Prasanna

Stereoisomerism Preparation and Properties

→ In stereoisomerism, the position of atoms in different isomers is different. The compounds that have the same composition and sequence of covalent bonds but differ in relative positions of their atoms or groups in space are called stereoisomers.

→ This special type of isomerism is called stereoisomerism and the isomers are called as stereoisomers.

The stereo isomers can he classified into two main groups:

Configurational isomerism
Conformational isomerism

→ Large amount of energy (chemical energy) is required for interconversion of these isomers as in this conversion, old bonds are broken and new bonds are formed. The energy required for the process should be more than loo kJ/mol. On the contrary, conformational isomers are interconvertible at room temperature, because their interconversion energy is very less, which can be obtained at room temperature only. Configuration isomers are not interconvertible at room temperature.

→ Configurational isomers are again classified into two groups:

Geometrical Isomerism
Optical Isomerism.

Chemistry Notes

Isomerism Preparation and Properties

October 7, 2021September 16, 2021 by Prasanna

Isomerism Preparation and Properties

→ First of all, isomerism was discovered by Liebig and Wohier in 1820. They observed that the chemical composition of silver fulminate and silver cyanate is same but show different in physical and chemical properties.

→ Similarly, Berzelius and Wohier observed that ammonium cyanate (NH₄ CNO) and urea (NH₂CONH₂) have saine molecular formula but show different physical and chemical their physical and chemical properties. Gay Lussac suggested that the reason for the difference in properties is that the bonding in these compounds is different.

Example : A compound with molecular formula C₂H₆O can be represented by two forms:

Isomerism Preparation and Properties 1

→ In this way, we cannot recognise a compound by molecular formula but its structural formula must be known. Berzelius gave the name ‘isomerism’ to this phenomenon. The compounds having same molecular formula but differ in both physical and chemical properties are called isomers and the phenomenon is called isomerism. The classification of isomerism can be shown by the flow chart:

Flow chart :

Isomerism Preparation and Properties 2

Chemistry Notes

Stereo Chemistry Preparation and Properties

October 7, 2021September 16, 2021 by Prasanna

Stereo Chemistry Preparation and Properties

Summary of the chapter and glossary :

→ Isomerism : The compounds which have the same molecular formula but different in physical and chemical properties are called isomers and the phenomenon is called isomerism.

→ Stereoisomerism : The compounds that have the same constitution and sequence of covalent bonds but differ in relative positions of their atoms or groups in space are called stereoisomers and the phenomenon is called stereoisomerism.

→ Geometrical Isomerism : The compounds in which two different configuration of different atoms or groups attached with C = C bond in space due to restricted rotation are called geometrical isomers and this phenomenon is called geometrical isomerism.

→ Optical Isomerism : Those compounds which can rotate the plane of polarised light are called optically active compounds or optical isomers and this phenomenon is called optical isomerism.

→ Dextro rotatory : The isomer which can rotate the plane of polarised light to the right or clockwise is called dextro rotatory. It is represented by d or (+).

→ Laevo rotatory : The isomer which can rotate the plane of polarised light towards left or anticlockwise is called laevo rotatory. This is represented by I or (—).

→ Colour Dispersion : When white monochromatic light is passed through Nicol prism and distributed in mixture of seven colours are called colour dispersion.

→ Plane of Polarised Light: If the light waves pass through a polarizer which is made up of Nicol prism then only electric vibrations emerge in one plane, such beam of light is called plane of pol an sed light.

→ Specific Rotation : The optical property of the compound can be expressed by specific rotation.

Stereo Chemistry Preparation and Properties 1

where α_obs observed angle of rotation
l = length of polarimeter
C = Concentration of solution

→ Plane of Symmetry : It represents the plane bisecting the molecules such that each half of the molecule is the mirror image of the other.

→ Axis of Symmetry: It represents a n-fold axis of symmetry such that when molecule possessing such an axis is rotated through an angle of 360°/n about this axis.

→ Centre of Symmetry : A molecule is said to have a centre of symmetry if any line drawn from the centre of the molecule meets identical atoms at equal distance from the centre.

→ Chiral Compounds : If the compounds in which elements of symmetry is not present, are called chiral compounds and these compounds are non-superimposable to their mirror images. This property is known as chiral.

→ Internal CompensatIon : In meso compounds, rotations are equal to upper and lower position due to plane of symmetry that’s why compounds are optically inactive and this property is called internal compensation.

→ Racernic Mixture : When equal amounts of dextrorotatory and laevo rotatory isomers are present in a compound then their type of mixture is called raemic mixture.

→ External Compensation : Racemic mixtures are optically inactive because half part rotates plane of polarised light towards right and other part rotates in opposite direction. So, they balanced each other, so it is called external compensation.

→ Optical Resolution : The process of separation of racemic mixture (dl) mixture into d and ¡ form is called optical resolution.

→ Auto Racemisation : Sorne compounds are auto racemised at room temperature, this phenomenon is called auto racemisation.

→ Chemical Racemisation : When any other chemical is mixed with d and 1 enantiomers to prepare a racemic mixture than this process is called chemical racemisation.

→ ConformatIon : The free rotation of one carbon with respect to another gives rise to various arrangements of the atoms differing in relative position 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 conformations.

Chemistry Notes

Biodegradable and Non-biodegradable Polymers Preparation and Properties

October 7, 2021September 16, 2021 by Prasanna

Biodegradable and Non-biodegradable Polymers Preparation and Properties

→ Bio-polymers : Many polymers which are present in plants and animals such as polysaecharides (starch. cellulose), proteins and nucleic acids etc. which control various life processes in plants and animals are called biopolymers. Examples are proteins, Carbohydrates, Nucleic acids, Lipids etc.

→ Biodegradable Polymers : These can be natural or synthetic both. These are degraded to small molecules by bacteria. Examples are PHBV, Nylon 2,6 etc.

→ Non-biodegradable Polymers : These are synthetic polymers and cannot be biodegraded. Examples are polythene, bakelite, teflon, PVC, terviene etc, Non-biodegradable polymers are very useful in daily life.

→ As these are non-biodegradable, their disposal is a big problem. Management of polymer waste is a very serious problem. Therefore, such polymers are synthesised which can be degraded. Some of them are:

→ Polylactic Acid (PLA) : It is an aliphatic polyester. It is obtained by polymerisation of the dimer of lactic acid (HO—CH(CH₃) COOH) or by micro biological syithesis of lactic acid followed by the polycondensation and removal of water by evaporation.

→ Use : Mainly it is used in me4icine for making stitches, dialysis etc. It is also used in preparation of bio.plastic.

Biodegradable and Non-biodegradable Polymers Preparation and Properties 1

→ Polycaprolactone (PCL) : It is a low melting point polyester and is obtained by chain polymerization of the lactone of 6-hydroxy hexanoic acid.

Biodegradable and Non-biodegradable Polymers Preparation and Properties 2

→ Use : It is used in Control Drug Release for making capsules. These capsules when enters in human body then ester chain of PCL is degraded and drugs is released slowly in the body.

→ PGA (Polyglycolic Acid) : It is a linear aliphatic polyester. It is obtained by the chain polymerization of dimer of glycolic acid, H O—CH₂COOH.

Biodegradable and Non-biodegradable Polymers Preparation and Properties 3

→ Uses : PGA are very strong fibres and are soluble in water mainly they are used in stiching wounds.

→ PHBV (Poly-β-Hydroxybutyrate-Co-β-Hydroxy Valerate) : PHBV is a copolymer of 3.hydroxy hutanoic acid, and (3.Hydroxypentanoic Acid), in which, the monomer units are connected by ester linkages.

Biodegradable and Non-biodegradable Polymers Preparation and Properties 4

→ Nylon-2-Nylon-6 : It is an alternating polyamide copolymer of glycine (H₂N—CH₂—COOH) and Amino caproic acid [H₂N(CH₂)₅COOH] and is biodegradable.

Biodegradable and Non-biodegradable Polymers Preparation and Properties 5

Chemistry Notes

Some Commercially Important Polymers Preparation and Properties

October 7, 2021September 16, 2021 by Prasanna

Some Commercially Important Polymers Preparation and Properties

Some important commerciafly important polymers are as follow.

Polyethylene or PE

Monomer : Ethene or ethylene (CH₂ = CH₂)

→ Method of Polymerisation : This polymer is first of all prepared in England in 1933. It is a polymer of Ethene. Polythene is formed by addition polymerisation of Ethene monomer units. It is mainly of two types

Low Density Polymer
High Density Polymer.

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→ The low density polythene (LDP) obtained through the free radical addition and H-atom abstraction has highly branched structure. Whereas high density polythene is formed by addition polymerisation of Ethene in the presence of a catalyst such as Triethylaluminium and Titanium tetrachiori de (Ziegler- Natta catalyst).

Uses :

→ Low density polythene is chemically inert and tough but flexible and a poor conductor of electricity. Hence, it is used in the insulation of electricity carrying wires and manufacture of squeeze bottles, toys and flexible pipe.

→ High density polythene is used for manufacturing buckets, du sthins, bottles, pipes etc.

Properties :

→ LDPE has melting point of i loo to 125°C. It is not soluble in any solvent at room temperature. But at high temperature, it is soluble in carbon tetrachioride, toluene, xylene etc.

→ The melting point of HDPE is 144° to 150°C. As compared to LDPE. it is more crystalline. The tensile strength, rigidity and resistance to solvents of HDPE is higher than LDPE.

Polyvinyl Chloride (PVC)

Monomer-Vinyl Chloride

Some Commercially Important Polymers Preparation and Properties 2

→ Process of Polymerisation : The monomer units are vinyl chloride molecules. PVC is prepared by heating vinyl chloride in an inert solvent in the presence of dibenzoyl peroxide.

Some Commercially Important Polymers Preparation and Properties 3

Properties :

pvc is a rigid, bright and hard polymer.
It is insoluble in, alcohols, water and organic solvents. It is resistant to chemicals as well as heat.
It is soluble in solvent made by mixing ketone and carbon disuiphide.

Uses:

→ PVC is cheap and mostly used plastic. Mainly it is used for making raincoats, handbags, toys hose- pipes, gramophone records, electrical insulation and floor covering.

→ PVC is dissolved in suitable solvents like chorobenzene and chlorination is done. It is known as chlorinated PVC. Chlorinated PVC is used in making adhesives and fibres.

→ It is used as plasticiser in ester and makes ester soft. It is also used as vinyl leather. This vinyl leather is used in manufacturing of rain-coat, floor polishing, handbags. curtain clothes etc.

→ Above 200°C, PVC degrades and releases HCl. To prevent this, suitable stabilisers are used.

Polyester

Monomer :

Dicarboxylic acid and diol monomers.
Diester and diol.
dithydroxylic acids (OH — R — COOH)

→ Process of Polymerisation : Polyesters are formed by condensation polymerisation.

→ Condensation Polymerisation of dicarboxylic acid and diol.

Some Commercially Important Polymers Preparation and Properties 4

→ Condensation Polymerisation of diester and diol.

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→ Uses : The polyester textile fibres made from the polymer are marketed under the trade name Terelene or dacron. It is resistant to the action of most of the common chemicals and biological substances and also to abrasion. It has a low moisture absorbing power.

→ As such it is widely used in making wash and wear fabrics. It is also blended with cotton and wool in clothing. It is also used in seat belts and sails. The polymer is also used in the preparation of films, magnetic recording tapes and for packing food. Dacron (and Teflon) tubes are good substitutes for human blood vessels in heart by pass operation.

→ If polyster is made up of aliphatic monomer units, it is known as aliphatic polyester and if aromatic monomers are present, aromatic polyester is formed. As compared to aromatic polyesters commercial use of aliphatic polyesters arè very less.

→ Terylene or PET (Polyethylene Terphthalate) It is an unsaturJed polyester. It is also known as dacron.

→ Monomer : Ethylene Glycol and Terephthalic Acid.

→ Process of Polymerisation : It is a polymer obtained by the condensation reaction between ethylene glycol and terephthalic acid.

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→ Uses : These polymers are mainly used in textile industry. This polymer is also used in the preparation of films, magnetic recording tapes and for packing food.

Nylon – 6

→ It is a polymide. Its polymer chain contains amide group
(—NH—C = O).

Some Commercially Important Polymers Preparation and Properties 7

→ Process of Polymerisation : Caprolactum has a ring structure. Nylon-6 is formed by anionic addition reactions by caprolactum. Here, monomer contains 6 Carbon atons therefore it is known as Nylon-6.

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→ Uses : Nylon-6 is a synthetic fibres having high tensile strength. It is generally white in colour and is resistant to acids and bases. Mainly, it is used in gears of vehicles. Apart from this, it is also used in toothbrushes, bristles, musical instruments wires such as violin, sitar etc. Also used in making threads, ropes, filaments, nets etc.

Nylon6, 6

It is also polyamide.

→ Monomer : Adipic acid (Tetra Methelene dicarboxylic Acid) and Hexamethylene diamine.
Process of Polymerisation : It is prepared by the condensation polymerisation of hexamethylene diamine with adipic acid under high pressure and at high temperature.

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→ Uses: Here diamine and dicarboxylic acid both have 6 Carbon atoms, therefore it is called Nylon-6, 6. First 6 indicates carbon atoms of diamine and second 6 indicates carbon atoms of dicarhoxylic acid.

→ Uses : Nylon-6, 6 is a polymer of high tensile strength. They are tough and resistant to abrasion. They are also somewhat elastic in nature. These are used both in fibres and plastic form. Nylon finds use in making bristles and brushes, carpets and fabrics in textile industry, elastic hosiery in the form of crinkled nylon.

Bakelite :

→ It is a thermosetting resin made of phenol and formaldehyde.

→ Monomers : The reaction of phenol with formaldehyde is done in the presence of either an acid or a base. The reaction starts with the initial formation of o-andlor p-hydroxymethylphenol derivatives, which further react as monomers to form bakelite.

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→ Process of Polymerisation : phenol and formaldehyde react in two ways:

→ The condeisation reaction of phenol with formaldehyde in the presence of either an acid or a base catalyst initial product which is a linear product – novolac used in paints. The ratio of phenol and formaldehyde is less than one during the formation of novolac.

→ Novolac contains many free —CH₂OH groups. Taking ratio of formaldehyde more, when heated at appropriate temperature and pressure, then cross links are introduced and forms three dimensional network which is known as ‘bakelite’. This means that the product obtained by heating phenol and formaldehyde at high temperature and pressure is bakelite.

→ The ratio of phenol and formaldehyde is taken more than 1 and the product Resol is obtained.

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→ Uses : It is a linear crystalline polymer. It is not soluble in strong acids and is stable at high temperatures. Also do not react with organic solvents and corrosive alkalies. Therefore, it is used in electric resistance, non-stick cookware s.

Teflon :

Polytetrafluoro ethylene (PTFE)
Monomer : Tetrafluoro ethylene (CF₂ = CF₂)
Method of Polymerisation → Teflon is homopolymer of tetrafluoro ethylene.

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→ Uses : it is linear and crystalline polymer. It is insoluble in strong acid (for example HNO₃) and thermally stable at very high temperature. it does not react with corrosive alkalies and organic solvents so it is used in making insulators and non-stick cookware and as a lubricant.

Chemistry Notes

Polymers of Rubber Preparation and Properties

October 7, 2021September 16, 2021 by Prasanna

Polymers of Rubber Preparation and Properties

It is of two types : Natural rubber and Synthetic rubber.

→ Natural Rubber: Ruboer is a natural polymer and possesses elastic properties. It is also termed as elastomer and has a variety of used. It is manufactured from rubber latex which is a colloidal dispersion of rubber in water. This latex is obtained from the bark of rubber tree (Hevea brasiliensis), Gatla Percha and is found in India, Srilanka, Indonesia, Malaysia and South America.

→ Method to Obtain Natural Rubber : To obtain natural rubber, first of all latex of rubber plant is obtained. The latex contains about 25.40% of rubber hydrocarbons dispersed in water along with stabilizer proteins and some fatty acids. Then acetic acid is mixed due to which rubber coagulates and precipitates out. It is separated by filtration.

This rubber is known as Crepe rubber. It is not so useful because of following reasons :

It has low elasticity.
Rubber is brittle at low temperature and becomes very soft at high temperatures. Thus it can be used only in the limited temperature range of 10-60°C
It is too soft to be used for heavy duty operation. Its tensile strength is only 200 kg/cm2.
Not resistant to mineral oils, organic solvents and even action of water. It has large water absorption capacity.
Vulcanisation is done to improve the physical properties of rubber.

Uses : Natural rubber is mainly used for shoes, rain coat, golf ball etc.

Structure and Composition : Natural Rubber may be considered as a linear polymer of isoprene

(2-methyl-1, 3-butadiene) and is also called as cis-1, 4-polyisoprene,

Polymers of Rubber Preparation and Properties 1

→ Two isomers cisand trans are obtained by combination of 1, 4 isoprenc units. Natural rubber is cis isomer. In these, monomer units are joined by weak van der Wall’s forces and form spring like structure. Due to this structure, rubber shows the property of elasticity. They can be stretched like springs.

Polymers of Rubber Preparation and Properties 2

→ The natural obtained from rubber tree, Gutta Percha is found in trans form.

Polymers of Rubber Preparation and Properties 3

trans—Polyisoprene (Natural Rubber Gutta Percha)

→ Vulcanisation of Rubber : The wide applications of rubber are due to its property called elasticity and that is why rubber is said to be an elastoplastic or elastomer. Accidentally, in 1893. Charles Goodyears discovered that addition of Sulphur to hot rubber cause changes that improve its physical properties in a spectacular manner. This process is called Vulcanisation. At the time, Thomas Hancock, performed vulcanisation and in 1843 takes patent of it.

→ Although natural rubber is thermoplastic substance in which there is no crosslink between the polymer chain and it on vulcanization, set into a given shape which is retained.

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→ The vulcanization process performed originally was slow. Now a days, same additives such as zinc oxide etc. are used to accelerate the rate of vulcanization. During vulcanization sulphuï crosslinks are formed the double bonds in the rubber molecule acts as reactive sites. The allylic —CH₂, alpha to double bond is also very reactive.

→ During vulcanization, sulphur forms crosslinks at these reactive sites. As a result rpbber gets stiffened and inter molecular movement of fubber springs is prevented resulting in physical character of rubber. The extent of stiffness of vulcanization rubber depends upon the amount of sulphur is used for making batter case rubber.

→ In a polymer. the chains are normally tanged up with each other. When the rubber is stretched, the chains straighten out to some extent. The chains can not slip past each other because of the polysuiphide bridges.

→ Thus, rubber can be stretched only to a limited extent. When the tension is removed, the chains try to coil up again and the rubber resumes its original shape.

Polymers of Rubber Preparation and Properties 5

Synthetic Rubber

→ Synthetic high polymers possessing similar physical properties as that of natural rubber are called synthetic rubber. Usually synthetic rubber is an improvement over natural rubber, specially with respect to its resistance to oils, gas, solvents, etc.

→ It also contain carbon-carbon double bonds. Synthetic rubber are vulcanisable rubber like polymer, which is capable of getting stretched to twice its length. However, it returns to its original sha1e and size as soon as the external stretching force is released.

→ Thus, synthetic rubbers are either homopolymers of 1,3-Butadiene derivatives or copolymers of 1, 3-Butadiene or its derivatives with another unsaturated monomer.

→ Styrene Butadiene Rubber or SBR or Buna-S : It is obtained by polymerisation of 1,3-butadiene and styrene in presence of Sodium metal

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→ In Buna-S, Bu stands for butadiene, Na for sodium and S stands for styrene. It is also called S.B.R. (Styrene Butadiene Rubber). It has slightly less tensile strength than natural rubber. Buna-S is used for making automobile tyres and rubber soles, belts and hoses etc.

Polyisobutylene or PIB : It is obtained by polymerisation of isobutylene.

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→ Uses : It is the only polymer which can be kept in air for long time i.e., it is impermeable to air. Therefore, these are used in making inner line for tyres and basketball.

→ Apart from this, it is also used as adhesive.

Note : Balloons made of simple rubber (polyisoprene) contracts after sometime because they are permeable to air.

→ Polybutadiene : It is obtained by polymerisation of butadiene monomer units.

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→ Uses : It is a synthetic rubber and shows similarity with natural rubber. When rubber is required to use at low temperatures, this rubber is used. It is used in preparing gaskets and other parts of vehicaler.

→ Polychioroprene or Neoprene Rubber : Neoprene or Polychloroprene is formed by the free radical polymerisation of chioroprene.

Polymers of Rubber Preparation and Properties 9

→ It has superior resistance to vegetable and mineral oils. It is used for manufacturing conveyor belts, gaskets and hoses.

→ Buna-N or Nitrile Rubber: Buna-N is obtained by the copolymerisation of 1, 3-butadiene and acrylonitrile in the presence of a peroxide catalyst.

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→ It is resistant to the action of petrol, lubricating oil and organic solvents. It is used in making oil seals, tank lining etc.

→ Acrilonitrile Butadiene-Styrene Rubber (ABS rubber) : It is formed by copolvmerisation of acrylonitrile (30%), 1, 3-butadiene (20%) and styrene (50%).