The meso-form of tartaric acid is optically inactive due to the plane of symmetry. But how are these forms able to show optical activity when the σ-bonds can freely rotate, which can change the configuration of the atoms?

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    $\begingroup$ Bond rotation doesn't change the fact that the molecule is still overall chiral and lacking in symmetry. $\endgroup$
    – Zhe
    Commented Dec 24, 2020 at 15:16
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    $\begingroup$ In other words, bond rotation can't change the configuration of the atoms. $\endgroup$ Commented Dec 24, 2020 at 16:04
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    $\begingroup$ @Nivedita Chowdhury: You are confused with conformation with configuration. There are no bond breaking with free rotation to change the configuration. yet, each isomer has infinite conformers due to rotation. $\endgroup$ Commented Dec 24, 2020 at 17:06
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    $\begingroup$ Consider the R,R-acid. Cut the central C-C bond with your magical chemical scissors. Now you have two tetrahedral objects that are identical in all respects. The fact that they happen to "dock" together is immaterial. After all, they are objects before they are chemical entities. The same holds for the S,S-acid. $\endgroup$
    – user55119
    Commented Dec 24, 2020 at 18:21
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    $\begingroup$ chemistry.stackexchange.com/a/38989/9961 $\endgroup$
    – Mithoron
    Commented Dec 25, 2020 at 0:33


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