Fundamental Principle of Optical Rotatory Dispersion

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Seminar on Fundamental Principle of Optical Rotatory Dispersion Presented by KUNAL C MEHTA DEPARTMENT OF QUALITY ASSURANCE MORDEN PHARMACEUTICAL ANALYSIS

Optical Rotatory dispersion -::

Optical Rotatory dispersion -: Definition -: ORD is defined as the rate of change of specific rotation or rotatory power with change in wavelength. Light is an electromagnetic radiation and consist of vibrating electric and magnetic vector perpendicular to each other. The branch of physics dealing with nature, sources, properties and effects of light is called optics.

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PRINCIPLE:- There are 2 types of plane polarized light[PPL], 1) Right circularly plane polarized light (RCPL) 2) Left circularly plane polarized light (LCPL) They are equal and opposite direction. When a PPL is pass through the optical active compound due to it’s CIRCULAR BIREFRIGENCE results unequal rate of propagation of left and right circularly polarized rays. This unequal rate of propagation of both RCPL and LCPL deviates the PPL from its original direction and it is called to be ‘OPTICAL ROTATION’

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In the same way unequal absorption co-efficient of the substance for the RCPL and LCPL is also observed which changes CPL to elliptically polarized light and this phenomenon is called as CIRCULAR DICROISM. The combination of ‘circular birefringence’ and ‘circular dicroism is called as ‘COTTON EFFECT’ Curves obtain from cotton effect are called ‘COTTON CURVES’ and these are the main fundamental evidence for the structural elucidation, absolute configuration and conformational studies for the organic compound.

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ORD curves in recent years are made use in structural determination by comparing the curve obtain from compound believed to have related structures particularly applied to carbonyl compounds. E.g.. ORD curves have been used to locate the position of carbonyl groups in steroid molecules .

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OPTICAL ACTIVITY -: Certain organic compounds when placed in the path of plane polarized light, they rotate plane of polarized light due to their chirality (asymmetric character) this phenomenon is known as ‘optical activity’ and the substance possessing this property are ‘optical active’ When a ray of monochromatic polarized light strikes a solution, several phenomenon’s occurs like – 1. Reflection on the surface. 2. Refraction. 3. Rotation of plane polarization 4. Absorption.

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For a compound to be optically active it must be devoid of the following properties – 1. Plane of symmetry ( σ ) 2. Center of symmetry ( i ) 3. Alternating rotation – reflection axis of symmetry or An improper axis ( s )

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1. Plane of symmetry -: ( σ ) It is the plane, which divides a molecule such that one half forms the mirror images of other e.g. Mesotartaric acid 2. Center of symmetry -: ( i ) It is the point in the compound from which a line drawn from one side & extended equally in the opposite side reaches the same group. e.g. Diketo dimethyl piperazine

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3. Alternating rotation – reflection axis of symmetry or an improper axis ( s ):- A molecule possess n-fold-alternating axis of symmetry, if then rotated through an angle 360 0 /n about the axis followed by a reflection in a plane perpendicular to the axis, the molecule is same as original one. e.g. 1, 2, 3, 4 – tetra methyl cyclobutane. The isomer that rotates the plane polarized light to the left is called levoisomer (-) & to the right is called as dextroisomer (+)

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Rotation of plane polarized light (FRESNEL’S EXPLANATION) -: According to Fresnel, a plane polarized light may be considered as the combination of two circularly polarized light of which one is right circularly polarized light (RCPL) & other is left circularly polarized light (LCPL) which are in equal & opposite in nature. A circularly polarized light (CPL) is one whose plane of polarization rotates continuously & in the same sense around the axis of the polarization of the wave & it may be described as right handed screw or helix twisting around the direction of propagation, where LCPL wave describe the left handed screw.

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The figure below represents how the electric vector of RCPL (ER) & that of LCPL (EL) combined to give a plane polarized wave (E) E E l E R RCPL + LCPL = PPL Plane of polarization

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The two circularly polarized light vibrate in opposite direction with the same angular velocity if refractive index is same

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Zero resultant The two circularly polarized light vibrate in opposite direction with same angular velocity if refractive index is same.

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Specific rotation (Rotatory power) -: - It gives intensity of optical rotation - It is the rotation produced by a solution in 10 cm length tube having 1 gm of substance in 100 ml. Denoted by [ α ] The specific rotation depends on following factors:- Nature of substance. Length of the column. Conc. of the sol. Temp of the sol. Nature of the solvent. Wavelength of the light used.

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CIRCULAR BIREFRENGENCE -: The two equal & opposite beams of CPL of a PPL when passes through an optically active compound it result in characteristic phenomenon is called circular birefringence

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The angle of rotation per unit path length is, α = (nL – nR ) π / λ Where, λ = wavelength of incident light n = refractive index If RCPL travels faster α is positive & the medium is dextrorotatory, If LCPL travels faster then α is negative & the medium is levorotatory.


CIRCULAR DICHROISM Whenever circular dichroism occur the two circularly polarized components of PPL are absorbed differentially i.e. one is absorbed more intensively than the other. So when the component emerges out there is an imbalance in their strength and the resultant two will not be linearly polarized but elliptically polarized and this phenomenon known as CIRCULAR DICHROISM. The ellipticity is define as the angle whose tangent is the ratio of major to minor axis of the ellipse and is given by: θ = ¼ ( aL – aR ) d Where a= absorption co-efficient d= optical path length aL - aR = circular dichroism of the medium

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COTTON EFFECT: The combination of circular dichroism and circular birefringence is known as cotton effect . Which may be studied by observing the change of optical rotation with the wavelength so called ORD. It was discovered by a French physicist A. COTTON . The curves obtained by plotting optical rotation v/s wavelength down to about 220nm using photoelectric spectropolarimeters, known as ORD curves or Cotton effects.

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They are of two types: 1. Plain curves 2. Anomalous curves (a) Single cotton effect curves (b) Multiple cotton effect curves

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1. Plain curves (Normal smooth curves or single curves.) - These are called to be “Normal curves” - The curves obtained do not contain any peak or inflections and that the curve do not cross the zero rotation line. This type of curve obtained for compound which do not have absorption in the wavelength region where optical activity being examined or when compound does not have chromophore in it. e.g.- alcohol , acids etc. The plain curve again divided into +ve and –ve curves according to the rotation of the compound.

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[Φ] * 10 2 λ

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2. Anomalous curves: - These curves on the other hand shows a number of extreme peaks and troughs depending on the number of absorbing groups and therefore known as Anomalous dispersion of optical rotation. - This type of curves is obtained for the compounds which contain an asymmetric carbon atom and also contain chromophore, which absorb near the UV region.

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Single cotton effect curves : - These are anomalous dispersion curves which shows maximum and minimum both of them occurring in the region of maximum absorption. - If in approaching the region of cotton effect from the long wavelength, one passes first through maximum (peak) and then a minimum (trough), the cotton effect is said to be positive. - If the trough is reached first and then the peak is called a negative cotton effect curves.

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- The vertical distance between the peak and trough is called the amplitude “a” and it is conventionally expressed in hundreds of degrees. Molecular amplitude, a = Ф1 – Ф2 / 100 Where, Ф2 = molar rotation of extreme peak or trough from large wavelength Ф1 = molar rotation of trough or peak from shorter wavelength

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(b) Multiple cotton effect curves: In this type of ORD curves two or more peaks and troughs are obtained. E.g. Ketosteroids, camphor etc. Graphs:-


OCTANT RULE It is the rule that establishes the absolute stereochemistry from the sign and intensity of cotton effect. Conversely the sign of the cotton effect may be deduced from the stereochemistry of the molecule. OCTANT RULE FOR CARBONYL CHROMOPHORES The compound to be oriented in a three- dimentional co-ordinate axis. The co-ordinate system divides the space around the carbonyl group into 8 sector or 8 octant. The compound is 3 mutually perpendicular planes

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OCTANT RULE STATE THAT Substituent lying in the co-ordinate planes make no contribution to the rotatory dispersion. Substituent lying in the upper left and back lower makes +ve contribution. Substituent in the upper right and back lower makes negative contribution. (+) UL (-)UR (-)LL (+)LR


APPLICATION:- In recognition of optical activity In quantitative analysis In qualitative analysis In conformational studies In structural elucidation Recognition of functional group Detection of location and position of functional group in compound


REFERENCE:- 1) Instrumental method of Analysis by Willard H.H.,Merritt L.L., Dean J.A., Settle F.A., 6 th edition. 2) Instrumental method of Chemical Analysis by Chatwal G.R. and Anand S.K. 3) Instrumental methods of Chemical Analysis by B.K. Sharma

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