How would we determine the optical activity in a compound as such:

enter image description here

The biggest difficulty I'm having is determining the R-S configuration as that will determine whether the pair are enantiomers or diastereomers.

My attempt:

enter image description here

We know that the carbon having the tritium atom will always have a higher priority than the carbon having a regular hydrogen (as per CIP rule).

Since $C_1$ has the lowest priority group (hydrogen) at the line and not the dash, and the rotation is clockwise, it has S configuration.

Similarly for $C_2$, since it has hydrogen at the dash, and the rotation is anti-clockwise, it has S configuration.

For it's mirror image,

enter image description here

$C_3$ has hydrogen at the dash and the rotation is clockwise, so it has R configuration.

As for $C_4$, it has hydrogen at the line and not the dash, and rotation is anti-clockwise, so it has R configuration.

Since configuration of both carbon atoms changes in the mirror image, they should be enantiomers, however they are supposedly diastereomers.

Why are the pair of compounds diastereomers and could someone point out where I'm going wrong?

  • 2
    $\begingroup$ Where you're going wrong is to trust the source that tells you that these are diastereomers instead of trusting your own work. Just rotate the right structure by 180-degrees, and the mirror plane is pretty clear. $\endgroup$
    – Zhe
    Mar 25 at 14:45
  • $\begingroup$ @Mithoron Is that a justification for the downvote? $\endgroup$
    – Bongo Man
    Mar 25 at 15:43
  • $\begingroup$ Edited post can be downvoted. chemistry.meta.stackexchange.com/questions/57/… $\endgroup$
    – Mithoron
    Mar 25 at 16:16
  • $\begingroup$ @Mithoron If you feel it hinders the search features of Chem.SE or the "discoverability" of the question, feel free to make the suitable edits, I'll gladly approve them :) $\endgroup$
    – Bongo Man
    Mar 25 at 17:11
  • $\begingroup$ The down vote was probably because the first illustration simply switches the configurations not showing the mirror image. the diastereomer would be the cis isomer RS becoming SR $\endgroup$
    – jimchmst
    Mar 28 at 22:57

1 Answer 1


You are absolutely right! The compounds are enantiomers. The second compound is just rotated by a certain angle(Rotating a compound doesn't change its configuration). Most of the questions do this in order to confuse you.


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