Question

When is a compound optically inactive?

Answer 1

Optical activity is defined as the ability of a compound to rotate the plane polarized light towards the right side or left side. The compounds which do not contain a chiral carbon and contain a symmetry element cannot be able to rotate the plane polarized light.

Complete answer:
Chemical compounds containing the carbon attached to the four different carbons are said to have achiral carbon. If the compound consists of more than one carbon, and containing at least one chiral carbon makes the compound optically active.
Though the compounds contain a chiral carbon, along with the chiral carbon, if any symmetrical element was there it leads to the compound to be optically inactive.
Symmetrical elements are center of symmetry, plane of symmetry, and axis of symmetry. If any molecule tends to possess the same configurations when rotated through an imaginary axis then it has an axis of symmetry. When the compound has equivalent structures when rotated through an imaginary plane, then the molecule has a plane of symmetry.
Thus, when the compounds contain any element of symmetry mentioned above refers to optical inactivity.
The compound will be optically inactive, if it contains all achiral carbons and any element of symmetry.

Note:
Optically active compounds rotate the plane polarized light towards the right side and left side, the right-side form is known as d-form which is known as dextro-form, and the left side form is known as l-form which is known as Leave form. But optically inactive compounds do not have two forms.