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Chiral molecules are classified as dextrorotatory (+) or levorotatory (-) depending on whether they rotate plane-polarized light (ppl) to the right or left.

My question is, how is such a measurement made? What instrumentation is used and how would you determine that ppl has been rotated right or left?

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The instrument used is called a polarimeter. You basically shine monochromatic linear polarized light through a solution of known concentration and you measure the rotation of the light on the other end of a tube with defined length. You then normalize the rotation according to concentration and length of your tube to get the specific rotation which is then independent from concentration and length and only depends on temperature, solvent and wavelength of the light.

You can determine if it's rotated left or right by changing the concentration since the observed rotation is directly proportional to concentration. You also need to do this to determine if your rotation is more than a full 360°, since a measurement of 10° could also be 370° or 730° or 1090° or also −350°, −710°, and so on.

Let's look at this using a simple example and let us assume our tube has a length of 10 cm (which is the length used for the specific rotation, so we don't need to worry about this). Specific rotation is also normalized to a concentration of 1 g/100 mL.

To determine our specific rotation we could do 3 measurements at 1 g/100 mL, 2 g/100 mL and 0.5 g/100 mL.

  • 1 g/100 mL : +160°
  • 2 g/100 mL : −40°
  • 0.5 g/100 mL : −100°

Now what does this tell us? If the specific rotation would be +160° then double the concentration should be 320° and half should be +80°, so that's clearly not the case. If it would be −200° instead of +160°, then double the concentration would be −400° which would show up as −40° and half the concentration would be −100°. So our specific rotation in this case is −200°.

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  • $\begingroup$ Can you give an example of a rotation of more than $360^\circ$ at 1% concentration? $\endgroup$ – Raoul Kessels Jul 1 '18 at 21:43
  • $\begingroup$ @RaoulKessels according to 10.1002/cber.190704002126 D-Hexol bromocamphorsulphonate has a specific rotation of 2640. $\endgroup$ – DSVA Jul 1 '18 at 21:55
  • $\begingroup$ Your reference is wrong. I have checked the original article (published in 1907) and no optical resolution is done. I have researched a bit more and it was not until 1914 that Werner resolved the hexol. Then, it is true that it has an amazing rotation with a maximum at 560 nm. It might be good to state that rotations above $360\circ$ are uncommon, especially in organic chemistry where the optical activity is most abundant. $\endgroup$ – Raoul Kessels Jul 2 '18 at 14:13
  • $\begingroup$ @RaoulKessels ok sorry, didn't knew that. I found it referencedquite a few times so I assumed it's in there. And yes it might be uncommon but it's possible (especially if you go to higher concentrations) so one should look out for that. $\endgroup$ – DSVA Jul 2 '18 at 16:14
  • $\begingroup$ Helicenes would be an example of organic molecules with very high optical activity $\endgroup$ – Raoul Kessels Jul 3 '18 at 6:30

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