Optical Rotation

What is Optical Rotation? 

If you observe, you will find that your left hand is the mirror image of your right hand. Now keep your hand in a way so that the palms are facing the same way. Now place your palms over one another. What do you notice? Are you able to place your left thumb over your right thumb? No! Well, your observation is correct. You can never place your thumb on one another while keeping the palms over one another. 

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Thus, our hands are non - superimposable mirror images of one another. In the same way there are few organic molecules in organic chemistry which possess the same non – superimposable mirror images of one another. They possess the same molecular formula or same sequence of atoms and bonds but different 3-D structures. These are known as optical isomers or enantiomers. 

Optical activity definition - Optically active substances or molecules rotate plane of polarization of plane polarized light either in clockwise or in anticlockwise direction. This is called optical rotation and this phenomenon is called optical activity. 

Optical isomers or enantiomers show optical rotation. This is the reason even if they have the same sequence of atoms and bonds still have different 3-D shapes. They possess the same physical properties. It means they have the same melting point, boiling point and density etc. 

Optical activity can only be shown by those molecules which show chirality. If a molecule cannot be superimposed on its mirror image through any combination of rotations is called a chiral molecule and this geometric property is called chirality. 

In 1811, French physicist Francois Jean Dominique Arago discovered the orientation of plane polarized light in quartz. After him in 1820 an English astronomer Sir John F.W. Herschel discovered that those quartz crystals whose crystalline structures are mirror images of each other rotate plane polarized light in equal amounts but in opposite directions. In 1849, while researching on the nature of tartaric acid, French biologist and chemist Louis Pasteur concluded that some molecules have two asymmetric crystal forms which are mirror images of one another. Solution of one form of crystal rotates the light in clockwise direction while the solution of another form of crystal in anticlockwise direction and an equal mixture of the two shows no optical activity. It means an equal mixture of the two doesn’t rotate the plane of polarized light. Louis Pasteur was the 1st one who showed the existence of chirality in organic molecules. 

Rotation of Light 

When oscillating waves (in a certain direction) pass through the solution of an optically active material then these waves interact with the solution of the optically active material and their plane of polarization gets rotated either in clockwise or anticlockwise direction. 

An enantiomer that rotates the plane of polarized light in clockwise or positive direction is called dextrorotatory. It is denoted by ‘+’ or ‘d’. 

An enantiomer that rotates the plane of polarized light in anticlockwise or negative direction is called levorotatory. It is denoted by ‘-’ or ‘l’. 

When, in a mixture of dextrorotatory and levorotatory isomers of an optically active molecule are present in equal amounts then it is called a racemic mixture. It doesn’t rotate plane of polarized light in any direction. 

Measuring Optical Activity 

Optical activity of optically active materials is studied by the polarimeter. Polarimetry is the measurement of rotation of plane polarized light by an optically active substance. The instrumentation of polarimeter is given below –

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Generally, light is unpolarized which means each light wave oscillates randomly. In the polarimeter first this unpolarized light is converted into polarized light by using polarizing filters or Nicol prism polarizer. Now this polarized light is passed through the polarimeter tube in which the sample (optically active) is kept. This polarized light  gets rotated and gives the result on a Nicol prism analyzer. 

The rotation of plane polarized light is given in degrees. It can either be positive (or dextrorotatory) or negative (or levorotatory). The rotation of light depends on sample concentration, temperature, length of polarimeter tube or sample tube and wavelength of the light passing through the tube. 

Specific Optical Rotation – For a specific chemical compound when angle of rotation of plane of polarized light is measured at a path length of one decimeter and concentration of one gram per ml is called specific optical rotation of that compound. For a pure substance specific optical rotation is an intrinsic property. Formula of specific rotation is (for a solution) –

\[[ɑ]_{λ}^{T}\] = ɑ/l.c

Where [ɑ] = specific rotation (Unit – degrees cm3 dm-1 g-1)

λ = wavelength in nanometer

ɑ = measured angle of rotation 

T = temperature in degrees

I = path length in dm

c = concentration of sample in g/ml

Application of Optical Rotation 

This property of optically active substances is very useful in many areas. For example, if you know the specific rotation of a pure substance then its concentration can be calculated. In this way polarimeters become a tool of great value for those traders who are using sugar syrups in bulk. 

It can also be used to measure the amounts of enantiomers in a mixture. Biologically, different enantiomers show different actions. So, which enantiomer is suitable for a specific biological action can be known by distinguishing them. For example, d and l ethambutol shows different actions biologically. d-ethambutol is used for treatment of tuberculosis while l -ethambutol causes blindness. 

Purity and concentrations of the sample is a major concern while preparing a drug in medicinal chemistry. Polarimeters can be useful to calculate the purity and concentrations of the samples which show specific rotations. It is used while preparing steroids, antibiotics, analgesics, narcotics, polymers etc. 

This ends our coverage on the topic “Optical Rotation”. We hope you enjoyed learning and were able to grasp the concepts. We hope after reading this article you will be able to solve problems based on the topic. If you are looking for solutions of NCERT Textbook problems based on this topic, then log on to Vedantu website or download Vedantu Learning App. By doing so, you will be able to access free PDFs of NCERT Solutions as well as Revision notes, Mock Tests and much more.