LIGHT: It is an electromagnetic radiation consists of both vibrating electric and magnetic vectors perpendicular to each other & having the wavelength of 4000-7700 angstroms. Study of light is called Optics WAVE LENGTH: The distance between successive crests or peaks of a wave. It is denoted by “ λ ”. CHROMOPHORE: An atom or group whose presence is responsible for the colour of a compound
OPTICAL ROTATORY DISPERSION (ORD) DEFINITION: Rate of change of specific rotation with change in wave length is called as ORD. It is used for structural determination of carbonyl compounds. FUNDAMENTAL PRINCIPLES OF ORD: Plane polarized light. Optical activity Specific rotation. Circular Birefrengence. Optical Rotation.
Light from ordinary lamp consists of waves vibrating in many different planes. When it is passed through polaroid lense it is found to vibrate in one plane is said to be plane polarised light or polarised light.
The compounds which are having the ability to rotate the plane of polarised light are called optically active compounds. This property of compound is called optical activity. It is measured by Polarimeter. Compound which rotates plane of polarised light to right(clock wise) is called DEXTROROTATORY & is denoted by (+) sign.
If the compound rotates plane of polarised light towards left side (anti clock wise) is called LEVOROTATORY & is denoted by (-) sign. Image of Polarimeter .
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.
Enantiomers are optically active compounds. Optically active molecules have different refractive indices, and different extinction coefficients for L and R circularly polarised light.
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 symmetryor An improper axis ( s )
1. PLANE OF SYMMETRY -: (σ) It is the plane, which divides a molecule suchthat one half forms the mirror images of other Ex: Mesotartaric acid 2. CENTER OF SYMMETRY -: (i) It is the point in the compound from which aline drawn from one side & extended equally inthe opposite side reaches the same group. Ex: Diketo dimethyl piperazine
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. Ex. 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 (+)
According to Fresnel, a plane polarized light may beconsidered as the combination of two circularly polarizedlight of which one is right circularly polarized light (RCPL)& other is left circularly polarized light (LCPL) which are inequal & opposite in nature.A circularly polarized light (CPL) is one whose plane ofpolarization rotates continuously & in the same sensearound the axis of the polarization of the wave & it may bedescribed as right handed screw or helix twisting aroundthe direction of propagation, where LCPL wave describe theleft handed screw.
THE FIGURE BELOW REPRESENTS HOW THEELECTRIC VECTOR OF RCPL (ER) & THAT OF LCPL(EL) COMBINED TO GIVE A PLANE POLARIZEDWAVE (E) E E1 ERPlane of polarization RCPL+LCPL = PPL
If Refractive index is same for two circularlypolarized light then it vibrate in opposite directionwith the same angular velocity.In the below image we can observe the change of E
SPECIFIC ROTATION• It is defined as the rotation produced by a solution of length 10cm & unit concentration (1gm/ml) for given wavelength of light at the given temperature. It is denoted by α. IT DEPENDES ON VARIOUS FACTORS LIKE:• Nature of substance.• Length of the column.• Conc. of the sol.• Temp of the sol.• Nature of the solvent.• Wavelength of the light used.
If two equal & opposite beams of CPL & PPL passes through an optically active compound it result in characteristic phenomenon is called Circular Birefringence.
When a plane polarized light(PPL)t is passed through optically active compound due to it’s Circular Birefrigence results in unequal rate of propagation of left & right circularly polarized rays. This unequal rate of propagation of both left & right circularly polarized light deviates the PPL from it’s original direction & it is called as OPTICAL ROTATION. Optical rotation caused by compound changed with wavelength of light was first noted by Biot in 1817.
Circular Dichroism occurs when the two circularly polarized components of PPL are absorbed differentially. When the component emerges out there is an imbalance in their strength & the resultant two will not be linearly polarized but elliptically polarized & this phenomenon is called as CIRCULAR DICHROISM
COTTON CURVESCOTTON EFFECT: French physicist discovered it & he described it as, 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 & the curves obtained are called as cotton curves.Djerassi & Klyne suggested that rotatory dispersion curves (i.e. plot of optical rotation against wavelength.) should be classified in to two main types. 1. Plain curves 2. Cotton effect curves.
These are normal or plain curves. These curves occurs at absorption maximum. The curves obtained do not contain any peak or inflections and that the curve do not cross the zero rotation line. These curves obtained for compounds which don’t have any absorption in wavelength where optically active compounds are examined. Ex: Hydrocarbons, Alcohols etc.,
• According to compound rotation these curves are again divided into +ve & -ve curves.
ANOMALOUS CURVES• These curves will show the high peaks & troughs which depends on the absorbing groups. So they are called as Anomalous dispersion of optical rotation.• These curves will obtain for the compounds which are having asymmetric carbon & chromophore which absorbs near UV region.• These are again divided into 2 types, they are 1. Single cotton effect curves & 2. Multiple cotton effect curves.
SINGLE COTTON EFFECT CURVES• These single cotton curves will show both maximum & minimum curves at maximum absorption.• From the longer wavelength towards the cotton effect region if crest passes first through it then the trough it is called as +ve cotton effect.• In the same manner if it happens in opposite way it is called as –ve cotton effect.
The vertical distance between the crest and trough is called as amplitude “a” and it is expressed in degrees. Molecular amplitude, a = Ф1 – Ф2 / 100 Ф2 = molar rotation of extreme peak or trough from large wavelength Ф1 = molar rotation of trough or peak from shorter wavelength
MULTIPLE COTTON EFFECT CURVES• These are a little different from the single cotton effect curves.• Here more than two crests & troughs are obtained .• Ex: camphor etc.,
OCTANT RULE• It is an empirical rule that establishes an absolute configuration or stereochemistry from the sign & intensity of cotton effect.• It was developed by Djerrasi & coworkers.• It relates the sign & amplitude of cotton effect exhibited by an optically active saturated ketone to the spatial orientation of atoms about carbonyl functional group.
(+) (-) (-) (+)• The substituents lying in the co-ordinate planes make no contribution to the rotatory dispersion.• Substituents lying top left & down right make +ve contribution.• Substitution lying top right & down left makes –ve contribution.
Prediction of cotton effect. Recognition of optical activity. Study of conformational mobility Structural elucidation. Qualitative analysis. Quantitative analysis
Organic spectroscopy by William kemp 3rd edition page no 279-280. Instrumental method of Chemical Analysis by Chatwal G.R. and Anand S.K. Instrumental method of Analysis by Willard H.H.,Merritt L.L., Dean J.A., Settle F.A., 6th edition. Instrumental methods of Chemical Analysis by B.K. Sharma. Internet