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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asked Dec 29, 2017 at 12:43
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2$\begingroup$ Related: Is (2R,3S)-butane-2,3-diol chiral? $\endgroup$– user7951Dec 29, 2017 at 12:49
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$\begingroup$ 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. $\endgroup$– Ivan NeretinDec 29, 2017 at 12:51
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$\begingroup$ I meant about the sigma bond $\endgroup$– Avyansh KatiyarDec 29, 2017 at 12:53
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1$\begingroup$ 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". $\endgroup$– Ivan NeretinDec 29, 2017 at 13:00
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1$\begingroup$ 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. $\endgroup$– ZheDec 29, 2017 at 15:52
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1 Answer
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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.
answered Dec 29, 2017 at 14:05