Q.10 For the given compound $\ce{X}$, the total number of optically active stereoisomers is ____.

enter image description here

This is from JEE Advanced 2018 second question paper.

I thought the answer to be at least 32 because of exactly 5 stereocentres present. One of the double bond cannot show geometrical isomerism possibly.

But the answer given in the answer key is 7:

enter image description here

I did not find any plane of symmetry because of the different kind of double bonds.

  • 3
    $\begingroup$ Two issues. (1) The image is not clear enough; it's hard to tell exactly which bonds are wavy versus wedge. (2) You're misreading the question. The wedge bonds are fixed. You can change configuration only for the wavy bonds. $\endgroup$
    – Zhe
    May 22 '18 at 17:21
  • $\begingroup$ I'm so sorry for the picture quality. I can clarify it, The long chain on the bottom of the cyclopentane system contain the 2 wavy bond S, all others are in dark bond. $\endgroup$
    – Sarah jane
    May 22 '18 at 17:40
  • 1
    $\begingroup$ Well, then there are 3 configurations to worry about, for a maximum of $2^{3}=8$. It can't be 32, but 7 is certainly reasonable. I'll let you figure out why it's 7 and not 8. $\endgroup$
    – Zhe
    May 22 '18 at 17:43
  • $\begingroup$ Okay, but where did you get the third stereogenic centre there? Those two in wavy and then? Does it show synanti about that sigma bond in cyclopentane. If it is 8 then it will be easy because there must be a plane of symmetry. $\endgroup$
    – Sarah jane
    May 22 '18 at 17:54
  • $\begingroup$ There's no third stereogenic center. But if the double bond isomerism that generates a separate isomer, that isomer can be chiral. If it is chiral, it is optically active... $\endgroup$
    – Zhe
    May 22 '18 at 18:07

The question is tricky, because

  • the two ends of the wavy bond are both stereogenic units (chiral cyclopentane ring carbon atom, and the double bond), so there is some additional unintentional ambuguity, but it's not a big deal, because the legend at right explains the situation somewhat;
  • it in fact asks “How many … stereoisomers does this structure represent?” (because of some stereogenic units with unspecified configuration) instead of “how many … stereoisomers does it have?”.

The unspecified-configuration stereogenic units are two chiral carbons and one double bond, which give total 8 stereoisomers.

Fig.1 - stereoisomers

The first one (top left) has a plane of symmetry, the other 7 stereoisomers are optically active.

  • $\begingroup$ (Just note that the question has been changed afterwards, original one did not include the answer key link/quotation :-| ) $\endgroup$
    – mykhal
    Jan 16 '19 at 15:54

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.