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We were doing the topic "Isomerism" in our Organic Chem class and there's a question in our worksheet whose answer I haven't really understood, the question is as follows:

The racemization of optically active compounds is driven by? (a): Entropy (b): Enthalpy (c): Entropy and Enthalpy (d): Element of Symmetry

The worksheet answer key says the answer is (a): Entropy. But I wasn't so sure of this answer, although I could rule out that it isn't (d) for sure since, for an optically active compound, there shouldn't be an element of symmetry (or it would be Inactive). But I can't quite understand why the answer should be Entropy, I always thought entropy just means the degree of randomness like particles just vibrating.

It would be of great help if you could tell me if the answer is in fact Entropy; and if so, could you perhaps explain how Entropy would influence this optical activity.

Thank you for your time!

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    $\begingroup$ The two species are related only by mirror symmetry so have the same enthalpy. Starting with just one enantiomer racemization produces two types of molecules so has increased the entropy $\endgroup$
    – porphyrin
    Jun 2, 2022 at 12:22
  • $\begingroup$ See also: chemistry.stackexchange.com/questions/103915/… $\endgroup$
    – theorist
    Jun 5, 2022 at 5:36

1 Answer 1

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Where there is a mechanism to interconvert enantiomers, entropy will drive racemisation

There is some context that is helpful but omitted from the question. Not all chiral compounds racemise under normal conditions and you need an easily accessible mechanism for racemisation to occur.

If there is such a mechanism, then a pure chiral compound will racemise over time (as the thalidomide molecule does in typical biological conditions with tragic results). If there is no mechanism, it will not (there is no easy way to convert the two chiral forms of camphor). In the first case the driving process is entropy. The two isomers are indistinguishable thermodynamically so the racemisation is not driven by differences in the energy involved. The point is that the chemical process is random and can have no memory of the original optical isomer, so the result of the interconversion reaction is random and gives both isomers equally. This is one way of saying that the apparent direction of the reaction (from optical purity to racemic mixture) is purely entropy driven.

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