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.
→ 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.
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.
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 (αobserved) 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.
→ 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,
→ 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,
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.
→ 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 Cn, 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.
→ 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.
For example, Benzene. Ethene etc., has centre of symmetry.
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.
→ 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 :
→ 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.
→ 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 :
→ Symmetrical Molecules (Meso Compounds) : In this group, tartaric acid can be taken as example,
→ Using the 2n formula, there should be maximum of 22 = 4 stereoisomers. These can be represented as :
→ 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.
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.
→ 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
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.
→ l(-) Tartane Acid can rotates the plane of polarized light towards left .
→ 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.
→ 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.