Question

unsubsituted biphenyl is non-planar but so differently meta substituted biphenyl are also non-planar by that logic and should show optical activity but ONLY ortho differently substituted biphenyl are optically active why so?

Answer 1

Dear Student,

Optical activity of ortho substituted biphenyl is because the free rotation of the central bond in restricted due to steric hindrance of the bulkyl groups at the ortho position. In case of meta substituted biphenyl, the groups are far from each other and so no steric hindrance occurs and as a results free rotation of bond takes place. Therefore, meta substituted biphenyl are optically inactive.