In identifying stereogenic centers, any quaternary or tertiary carbons would be a good place to look. There are no quaternary carbons, and all the tertiary carbons are part of a benzene ring structure.

In the below given diagram of trans-1,3-dichlorohexane the chlorines cannot be positioned diaxial or diequatorial but can only be axial and equatorial or vice versa. Additionally, if they were diastereomers or structural isomers, then they could be separated by physical means.

See as the compounds who shows optical isomerism have a chiral carbon so an unsymmetrical alkane can show the optical isomerism. 6 carbon and 8 carbon alkane is not the answer. So, the smallest alkane is 7 carbon alkane is 3 methyl hexane which have 3rd carbon as chiral carbon.

3–Methylhexane shows optical activity as shown below:

Both molecules have the same molecular formula (C₉H₁₆BrCl) and the same connectivity. Each molecule also has three stereocenters, marked above with an asterisk, and does not contain a plane of symmetry. Therefore, both molecules are chiral. Using one of the methods outlined in this tutorial, it can be determined that these molecules are superimposable. Therefore, the molecules are identical.