Question

Explain the optical activity of 2-Chlorobutane

Answer 1

Optical activity:

• Optical activity is the ability of a substance to rotate the plane of polarization of a beam of light that is passed through it.
• A compound to be said as optically active is only if it has to have an asymmetric chiral carbon centre (the carbon is said to be chiral if they have different substituents attached).
• The optically active substances that rotate the plane of polarization of the light towards the right are known as right-handed or dextrorotatory. It can be Indicated by including (+) or ‘D’ before the compound name.
• Substances that rotate the plane of polarization of the light toward the left are known as left-handed or laevorotatory. It can be Indicated by including (-) or ‘L’ before the compound name
• The optical isomers that bear the relationship between an object and its mirror image, are also known as enantiomers.
• A mixture of equal quantities of enantiomers is called a racemic mixture.
• A racemic mixture is optically inactive because the rotation caused by molecules of one isomer is cancelled out by an equal and opposite rotation caused by an equal number of molecules of its enantiomers.

Optical activity of 2-Chlorobutane;

• 2-Chlorobutane is an optically active molecule having the molecular formula ${\mathrm{CH}}_3{\mathrm{CHClCH}}_2{\mathrm{CH}}_3$
• Here, to the second carbon atom, one Ethyl group$\left({\mathrm{C}}_2{\mathrm{H}}_5\right)$, Methyl group$\left({\mathrm{CH}}_3\right)$, one Hydrogen atom, and one Chlorine atom is attached,
• Since this second carbon has different substituents attached to it, it is a chiral carbon atom.

• It gives rise to enantiomers which are non-superimposable mirror images by rotating the plane of polarized light in different directions.
• These mirror images are referred to as optical isomers, one isomer will be dextrorotatory and the other will be laevorotatory.
• The structure of 2-Chlorobutane and its mirror images can be represented as
• The maximum number of optical isomers is given by the formula $2^{\mathrm{n}}$, where $\mathrm{n}$ is the number of chiral carbons.
• Therefore, the number of optical isomers possible for 2-Chlorobutane is $2^1=2$.

Therefore, 2-Chlorobutane is an optically active molecule with one chiral carbon atom and it forms dextro and levo isomers.