optical rotatory dispersion


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Seminar on Optical Rotatory Dispersion Presented by Mr. ABDUL RAZZAQ M. Pharm. Department -: Pharmaceutical Chemistry Luqman College of Pharmacy Gulbarga 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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Introduction -: ORD refers to the change in optical rotation with the change in wavelength of light source. i.e. applied only in optically active compounds. Optical rotation caused by compound changed with wavelength of light was first noted by Biot in 1817. 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.

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E.g.. ORD curves have been used to locate the position of carbonyl groups in steroid molecules. Djerassi & Klyne have suggested that rotatory dispersion curves (i.e. plot of optical rotation against wavelength.) should be classified in to two main types. Plain curves Cotton effect curves.

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OPTICAL ACTIVITY -: Certain organic compounds when placed in the path of plane polarized light, the plane of polarization is rotated, the property of compounds to rotate plane polarized light is 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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Enantiomers are optically active Optically active molecules have different refractive indices, and different extinction coefficients for L and R circularly polarised light

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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 3600/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 – tetramethyl 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 El ER 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 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 & apposite 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.

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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 photoelectricspectropolarimeters, known as ORD curves or Cotton effects.

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Cotton discovered a relation between RP and light absorption in optically active compounds. As one approaches certain optically active absorption bands in a compound from long wavelength side the rotatory at first increases strongly then falls off and changes sign. This is known as Cotton effect and the curves describing such effect is called Cotton effect curves. 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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Plain curves (Normal smooth curves or single curves.) Cotton effect is not seen for compounds which absorbs in far UV well below 220nm, because it occurs only near absorption maximum. The curves obtained do not contain any peak or inflections and that the curve do not cross the zero rotation line. The increase in magnitude of optically active is proportional to the decreases in wavelength, so that plot of Ф against λ is a plain curve. These curves shows no minimum i.e. they are smooth E.g. Alcohol's and Hydrocarbons.

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[Φ] * 102 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.

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If the trough is reached first and then the peak is called a negative cotton effect curves. 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:-


REFERENCE:- 1)Stereochemistry of Organic Compounds by Ernest L. Eliel & Samuel H. Wilen 2) Instrumental method of Chemical Analysis by Chatwal G.R. and Anand S.K. 3) Instrumental method of Analysis by Willard H.H.,Merritt L.L., Dean J.A., Settle F.A., 6th edition. 4) Instrumental methods of Chemical Analysis by B.K. Sharma

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