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Are diasteriomers always optically active? If not then why? Though they are chiral then also.

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    $\begingroup$ @Mansi Jain because they might have elements of symmetry. $\endgroup$ – user14857 Jan 28 '16 at 14:35
  • $\begingroup$ If you carefully studied your book, you would have found an answer. $\endgroup$ – Quark Jan 28 '16 at 20:18
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Some definitions (by IUPAC):

  • Diastereomer: A stereoisomer that isn't an enantiomer.
  • Stereoisomer: An isomer that has the same bond connectivity, but arises due to differences in the spatial arrangements of atoms.
  • Enantiomer: Non-superimposable mirror image.

So, diastereomers do not necessarily have to be optically active. Consider cis- and trans-1,2-difluoroethene.

Difluoroethene

They are stereoisomers (connectivity is the same, but the fluorines are on opposite sides) which are not enantiomers (they aren't mirror images of each other), which makes them diastereomers.

Neither are chiral because they are superimposable on their respective mirror images. Equivalently, the presence of the plane of symmetry (which is the plane of the molecule) makes both isomers achiral.

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  • $\begingroup$ I don't think this is correct. They are stereoisomers, which are not enantiomers, but that does not necessarily make them diastereomers. Usually a diastereomer would exhibit two chiral centres or more, but doesn't necessarily have to be an enantiomer e.g. mesotartaric acid $\endgroup$ – Beerhunter Jan 28 '16 at 20:10
  • $\begingroup$ A diastereomer is just the same compound with one group twisted about a stereocentre,right? That does not necessarily mean diastereomers are those stereoisomers which are not enantiomers? This confused me. $\endgroup$ – Quark Jan 28 '16 at 20:16
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    $\begingroup$ The IUPAC definition of a diastereomer is any stereoisomer that is not an enantiomer. goldbook.iupac.org/D01679.html @Beerhunter I could use meso-tartaric acid as an example too but the alkenes were simply easier. I may edit if I have the time later. $\endgroup$ – orthocresol Jan 28 '16 at 21:04
  • $\begingroup$ cc @Quark comment above Your definition is narrower than the correct definition. Compounds which differ in the configuration of some of their chiral centres are diastereomers, but diastereomerism is not restricted to that $\endgroup$ – orthocresol Jan 28 '16 at 21:06
  • $\begingroup$ @orthocresol Which other compounds are diastereomers then? $\endgroup$ – Quark Jan 29 '16 at 5:46

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