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If I were to rotate the front carbon clockwise by $60^\circ$, it would become achiral. But my text book says that it's chiral. Is it that rotating an individual atom about a sigma bond changes its conformation is that why we can't rotate it?

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Gaurang Tandon
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Avyansh Katiyar
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    Related: Is (2R,3S)-butane-2,3-diol chiral? –  Dec 29 '17 at 12:49
  • We can rotate whatever rotates freely (or nearly so) in the wild. Whether or not it changes some conformations is irrelevant. Also, this particular molecule would never become achiral, rotate it all you want. – Ivan Neretin Dec 29 '17 at 12:51
  • I meant about the sigma bond – Avyansh Katiyar Dec 29 '17 at 12:53
  • Yes, I got that. I meant the same. – Ivan Neretin Dec 29 '17 at 12:54
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    Wait, this is (2R,3S) thing, and hence a meso form. I take the last part back. You were right, it is nominally chiral and will become achiral if rotated 60°. Then again, actual molecules rotate around the sigma bond all the time, and you can't separate the "0° isomer" from the "60° isomer". – Ivan Neretin Dec 29 '17 at 13:00
  • So chirality is for a specific conformation and not a molecule as a whole? – Avyansh Katiyar Dec 29 '17 at 13:07
  • Chirality is there when you can separate and isolate the enantiomeric forms. In this case you can't. – Ivan Neretin Dec 29 '17 at 13:19
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    Wait. Wait. Wait. We need to be much more clear about what we're talking about. The front carbon is a chiral center regardless of rotation. In speaking of the chirality of a molecule, we generally do not limit ourselves to any one conformation, but rather all accessible ones. Most of the comments and answers assume that you're speaking about the molecule, but it's not 100% clear to me that that's what the book is also referring to. – Zhe Dec 29 '17 at 15:52
  • @Zhe it was that question (I edited the question) – Avyansh Katiyar Dec 29 '17 at 16:00
  • Your first structure (labeled II) is equal to structure I in the problem set. Is this an error or a result of copying the structure from somewhere else? – Terje D. Dec 29 '17 at 16:07
  • @Terje my bad that's a copying error – Avyansh Katiyar Dec 29 '17 at 16:10
  • @Zhe No, let’s really be clear. There is not really such a thing as a chiral carbon (even though the term keeps popping up in chemical literature). Molecules and centres can be chiral, only molecules can be achiral and carbon atoms (or other atoms) are typically asymmetric. So both carbons are asymmetric but the molecule is also achiral. – Jan Dec 30 '17 at 16:25
  • If I vote to close this as a duplicate, it will become unclear. I’m not going to vtc in this case. – Jan Dec 30 '17 at 16:27

1 Answers1

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It simply depends on what is the subject. The book asks for the displayed conformation, which we agree is chiral.

The question would have been more tricky to answer if it were about optical activity of a sample.

In this case we know the conformer can not be isolated at room conditions but this is not a general rule.

And one can always cool down, at least in principle.

Alchimista
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