While going through my book, I came across this question and got really confused. Here's what the problem was:

In a pair of diastereomers,

  1. Both can be laevo-rotatory or both can be dextro-rotatory
  2. Both can be optically inactive
  3. One can be optically active other can be optically inactive
  4. Presence of two chiral carbons is must in both of the isomer

As you can see, this is a multiple correct problem.

I tried going with the definition, Diastereomers are stereoisomers whose molecules are not mirror images of each other

Also, to satisfy this, I believe there should at least be two chiral carbons. So option (4) seems right. But I'm really not able to figure out the others. How can we determine this?

  • $\begingroup$ You can have chirality around heteroatoms e.g. Esomeprazole $\endgroup$ – Beerhunter Apr 4 at 8:37
  • $\begingroup$ This might be easiest to think about if you picture a polyol, like glycerol or xylitol, but even longer. Every carbon except the ones on the end has a chiral center. But can you make more than one stereoisomer with a mirror plane that is optically inactive? Can they rotate light the same direction? It should be easy to think about from there. $\endgroup$ – Mike Serfas Apr 5 at 14:26
  • $\begingroup$ I think all the options are correct. The definition directly leads to all of them. $\endgroup$ – S R Maiti Apr 5 at 17:13
  • 1
    $\begingroup$ They’re all true except for 4. You don’t need chiral carbons for optical activity. Other heteroatoms will work. And even molecules with restricted rotation around a signal bond can show optical activity. $\endgroup$ – Josh Mitchell Apr 6 at 0:13

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