Optical Activity- Definition and Explanation, Classification, Measurement, Practice Problems and FAQs

Children love to go with parents driving a car. The inquisitive curiosity is great in children. WHile observing the road changing as the car moves on, they also keenly learn how the car is being operated. Now, you know about how to change the direction of the car. Absolutely! We need to rotate the steering wheel and the direction of motion will depend on the direction we rotate the steering wheel.

Similarly, in chemistry, there are some organic compounds that can rotate the direction of a plane polarized light with varying degrees when allowed to pass through it? Initially, optical phenomena were discovered by French scientist Jean-Baptiste Biot. He found that when plane-polarized light moved through such objects, the change in direction was basically a rotation of light with a molecular source.

Table of Content

• Definition and Explanation of the Optical Activity
• Measurement of Optical Activity
• Practice Problems
• Frequently Asked Questions- FAQ

Definition and Explanation of the Optical Activity

The property of an organic compound in aqueous solution to rotate the plane polarized light (made by passing ordinary light through a Nicol prism) is known as optical activity, and the compounds are known as optically active compounds.

Let us consider an ordinary light which will have oscillation in all directions and is known as unpolarized light. This light which is a form of an electromagnetic wave contains an electric field vector.

When this ordinary light is allowed to incident on a Nicol prism, its oscillation in all the planes except one are canceled out and hence oscillate only in a single plane and is known as plane-polarized light. When this plane polarized light is next passed through an optically active organic compound dissolved in water, present in a polarimeter tube, it will either rotate the direction of the plane-polarized light in a clockwise or anti-clockwise direction or left undeflected.

If an organic compound rotates the plane of polarized light in any direction clockwise or anticlockwise it is said to be an optically active compound. But if it does not rotate the plane-polarized light and moves undeflected then it is said to optically inactive compound.

A polarimeter gives a quantitative measure of the optically activity of the material by measuring the rotation of the plane-polarized light.The angle by which the plane-polarized light is rotated is measured by an instrument called a polarimeter.

Here,

2 is the normal light with electromagnetic waves in all directions

3 is the Nickel prism for filtering all vibrations except in one direction

4 is the collimator for the lane polarized light

5 is the polarimeter

6 is the change of direction by the optically active compound

7 is the measurement of the rotated plane polarized light

Classification of Optically Active Compounds:

On the basis of the direction in which the plane-polarized light is rotated, the optically active compounds are classified into two types:

• Dextrorotatory Optically Active Compounds: Compounds that move the direction of the plane polarized light in the clockwise direction or in right-handed direction are notified as dextrorotatory optically active compound or the d-form. Such compounds are written with either a plus (+) symbol or ‘d-’ preceding the names.

Or

Dextrorotatory chemicals rotate plane-polarized light to the right or clockwise direction.

• Laevorotatory Optically Active Compounds: It is the optically active compound which rotates the plane polarized light towards the left or in an anticlockwise direction, it is called laevorotatory optically active compound.or the l-form. Such laevorotatory compounds are written with a negative (–) sign or ‘l-’ preceding their name..

• Any optically active compound exists in both dextrorotatory and laevorotatory forms. They are isomers specifically called enantiomers. The ‘d- and l-’ forms of enantiomers rotate the plane polarized light to the same degree ut in opposite directions. For example, dextrorotatory optical rotation of thyroxine rotates the plane polarized light by (+30) degree. Then (-30) degree will be the optical rotation of the laevorotatory of thyroxine.

Note:

• Such (+) and (–) isomers of optically active compounds are called optical isomers and the phenomenon is known as optical isomerism.
• A polarimeter can show nil rotation under two cases. 1. The compound itself may be optically inactive or incapable of rotating the plane polarized light. Or 2. There could be an equal amount of both ‘d- and l-’ forms of the enantiomers in the mixture, such that ‘d-’form clock wise rotation cancel the ‘l-’ forms anticlockwise rotation.

Measurement of Optical Activity

• Measurement of angle of rotation is done in terms of specific rotation.
• Specific rotation is defined as the rotation of the plane polarized light by the optically active compound of unit concentration( present in a polarimeter tube of length 1 decimeter for a given wavelength of light at specific temperature.
• The angle of rotation( of the plane polarized light depends on several factors:

1. Concentration the solution ( in : It was observed that the angle of rotation is directly proportional to the concentration of solution. Mathematically,

Angle of rotation

Where,

‘ represent the concentration of the solution in

1. Wavelength of the light used: It was observed that the angle of rotation is inversely proportional to the square of wavelength of the light used. Mathematical,

Angle of rotation

Where,

’ represents the wavelength of the light used.

1. Length of column tube or path length: It was observed that angle of rotation is proportional to the length off the column tube.Mathematically,

Angle of rotation

Where,

’ represent the length of column tube in

Note: Angle of rotation also depend on other factors like nature of solvent, nature of substances and temperature (in celsius scale) at which the angle of rotation is measured.

At a constant temperature for a fixed wavelength of a light using equation and angle of rotation can be represented as:

Angle of rotation

Removing the proportionality constant we get,

Here,

represent the specific rotation of light

represent the angle of rotation

represent the concentration of the solution

‘ length of the column tube

Note: Angle of rotation is measure by an optical device named polarimeter tube which measure angle of rotation in term of specific rotation of light.

Practice Problems

Q1. Select the correct option for the factors on which the angle of rotation depends:

1. Concentration of solution
2. Intensity of light
3. Wavelength of light
4. Both A and C

Answer: D)

Solution: Angle of rotation does not depend on intensity of the light.The angle of rotation( of the plane polarized light depends on factor which includes:

Concentration the solution ( in : It was observed that the angle of rotation is directly proportional to the concentration of solution. Mathematically,

Angle of rotation

Where,

‘ represent the concentration of the solution in

Wavelength of the light used: It was observed that the angle of rotation is inversely proportional to the square of wavelength of the light used. Mathematical,

Angle of rotation

Where,

’ represent the wavelength of the light used

Note: There are other factors as well which include nature of solution, nature of solvent, temperature at which angle of rotation is measured and path length.

Q2. Select the correct option with respect to the use of a polarimeter tube.

1. It helps in measuring the pH of a solution
2. It helps in measuring the angle of rotation of optically active compound
3. It helps in determining the wavelength of the light
4. It results in the formation of plane polarized light when allowed to pass through it.

Answer: (B)

Solution: Angle of rotation is measure by an optical device polarimeter tube which measure angle of rotation in term of specific rotation of light. Therefore option (B) is correct and all other option is incorrect.

Q3.A optically active solution of concentration has a observed angle of rotation of at with a light of wavelength in a tube . Select the correct option for the specific rotation of the compound.

Answer:

Solution:According to the given data,

Concentration of the solution =

Observed angle of rotation ( =

Length of a column tube = path length =

we know that specific rotation can be calculated using the formula,

Here,

represent the specific rotation of light

represent the angle of rotation

represent the concentration of the solution

‘ length of the column tube

Putting the value of the variable we get,

Q4. Select the correct option for what will be the effect on specific rotation if the concentration of the solution is doubled?

1. Specific rotation gets doubled.
2. Specific rotation is halved
3. Specific rotation becomes four times
4. Specific rotation becomes 8 times

Answer:

Solution:As we know that specific rotation can be calculated by using formula,

Here,

represent the specific rotation of light

represent the angle of rotation

represent the concentration of the solution

‘ length of the column tube

As we can observe from equation( that,

Specific rotation is inversely proportional to concentration of the solution.Therefore, when the concentration of the solution is doubled the specific rotation is halved.

Frequently Asked Questions-FAQs

Q1. What is the difference between optically inactive and optically active compounds?
Answer: Optically active compounds is a type of compound which can rotate the plane polarized light in either clockwise direction or anticlockwise direction when plane polarized light is passed through it in polarimeter tube. But compound in which when plane polarized light is passed it passes undeflected or unchanged is said to be optically inactive compound.

Q2. What is the different application of optical activity?
Answer: Specific rotation measured by the polarimeter is unique for individual compound and is directly proportional to the concentration of the optically active compound.

Some of the important application of optically activity includes:

• In relation to the maximum specific rotation expected of the substance, the percentage of optical active compound present in the solution can be calculated using the measured optical rotation.
• Sugar level in urine is due to the optically active glucose present in it. Comparing the measured optical rotation to the maximum specific rotation sugar level can be calculated.
• Optical rotation rotation decrease with increasing wavelength of the plane polarized light. The energy of the light brings about molecular changes and hence indirectly affects the rotation values. So the rotation measurement can be used to molecular changes and structures of the compound..

Q3. What is the relationship between the optical activity and chirality of a compound?
Answer:"An item that is asymmetric and cannot be placed over its mirror counterpart is known as chiral or stereocenter," according to the definition. Chirality is the name for this characteristic. For instance, our hands, legs, and other body parts. Chirality is one of the conditions to check if the compound is optically active or inactive but it is not the only condition.

Q4. What will happen when we have an equimolar concentration of two optically active substances but have opposite rotation is present in a mixture.
Answer: When the equimolar concentration of two optically active substances having opposite rotation is present in a mixture it results in the optically inactive compound because the optical rotation cancels out each other and due to internal compensation it is optically inactive.