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I know that a compound doesn't need to have a chiral carbon to be optically active. So how can I find out whether a compound drawn on paper is chiral without access to any experiments? e.g. 1,2,2-tribromopropane

The question is expected to be solved using only pen and paper

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  • But a sample of the compound in a polarimeter and try to measure optical activity. – MaxW Dec 20 '15 at 20:14
  • @MaxW The problem is the question like these show up in MCQ based college entrance exams with no excess to lab. – sudo_dudo Dec 20 '15 at 20:16
  • @Jan I am expected to answer such questions without access to lab. I want to know if there is no chiral carbon in a compound how would I know that it has a non-superimposable mirror image? – sudo_dudo Dec 20 '15 at 20:20
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    Draw it's mirror image, if the two images are not superimposable then the molecule is chiral. – ron Dec 20 '15 at 20:41
  • It still looks like homework - in such simple cases checking for chiral atom is ok. – Mithoron Dec 20 '15 at 21:04

1 Answers1

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A substance will be optically active if it is not identical to its mirror image. Check out this great answer to see why. So the dullest and brute-force-est way to find out whether a molecule is chiral is to draw it, to mirror it, and then to attempt to overlay image and mirror image using only rotation. (Remember to draw molecules three-dimensionally!)

But there are a few shortcuts. You can look at the molecule as a whole and check out what symmetry features it has — determine its point group. If a molecule has a centre of symmetry (i.e. inversion results in the identical molecule), a plane of symmetry or improper rotation (which is a rotation followed by mirroring along a plane orthogonal to the rotation axis) it is achiral. If you cannot find any of these symmetry elements, the molecule is chiral.

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