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Our teacher has taught us today about optical isomers . And at the end given a formula for finding number of optical isomers of a symmetrical molecule( n is number of chiral atoms and the n is odd for this formula ). Which is Total = Active ( referring to enantiomers ) + Meso. And Total = 2^n-1

I am a bit confused here because I have never imagined meso compound as optical isomers for they are optically inactive and show no deflection for polarised light. So are they optical isomers. Is my definition of optical isomers flawed. Please enlighten me . Thanks in advance. I know that they are stereo isomers for their spatial arrangement in 3D. Please help me in thinking in the right direction.

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    $\begingroup$ In my understanding, meso isomers can be considered as a special case of optical isomers, where parts of the molecule are optically active, but due the molecule symmetry their optical activity cancel each other for the whole molecule. // When in doubts about statements of lectures or textbooks, always try first to verify the facts in independent available offline or online sources, before you decide to ask. This way you prevent answering the answered. $\endgroup$
    – Poutnik
    Feb 8 at 9:15
  • $\begingroup$ Minor note: Some compounds can be meso at higher temp and have enantiomers at lower: "At room temperature, most 1,2-disubstituted cyclohexanes undergo rapid ring flipping..., and as a result, the two cis stereoisomers behave chemically identically with chiral reagents At low temperatures, however, this is not the case, as the activation energy for the ring-flip cannot be overcome, and they therefore behave like enantiomers." en.wikipedia.org/wiki/Meso_compound $\endgroup$
    – DrMoishe Pippik
    Feb 10 at 0:12

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