When I was reading about crystalline nature of ionic compounds, I came across the statement that ionic compound doesn't show stereoisomerism. What does that mean and can anybody explain the reason with an example?

  • $\begingroup$ If any, it will not even called stereoisomerism. Rather one would say that the crystals have opposite light polarizing properties. Isomerism refesr to molecular compounds, because is where it has sense to be discussed. $\endgroup$
    – Alchimista
    Jun 9 '19 at 11:49

They can, and they do. The secret is to have a chiral cation and separately a chiral anion. Letting $D^+$ be the dextrorotatory form of the cation, $L^+$ be the levorotatory form, and anologously for the anion, we then have four isomeric salts:





The first and second are diastereomers because only the anion is mirror-reflected; the cation is not mirror-reflected. Similarly for three of the other five possible pairs; only two of the six possible pairs are enantiomeric.

Being diastereomers instead of enantiomers, the first and second salts, for instance, may have different physical properties such as solubility in a given solvent. This may be used for separating different enantiomers of the anion (or, with a different pair such as first and third, separating different enantiomers of the cation). And it's actually done. See here for a brief summary and here for an example of diastereomeric salt formation.

  • $\begingroup$ Interesting! Can you suggest me a source for further reading on stereoisomerism in ionic compounds? $\endgroup$ Jun 10 '19 at 5:45
  • 2
    $\begingroup$ This goes into more detail and provides a textbook reference. $\endgroup$ Jun 10 '19 at 8:10

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