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Author Topic: Practical Aspects of measuring optical rotation  (Read 5117 times)

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IsotopeBill

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Practical Aspects of measuring optical rotation
« on: August 27, 2012, 11:14:21 AM »

I'm interested in a monograph or sort of a how-to manuscript on measuring optical rotation.  For example, if you've recently synthesized a previously-unknown chiral compound, and want to measure it's optical rotation, what factors go into determining the solvent, cell size, wavelength, etc?  Or does it much matter provided everything is reported?  I haven't had much experience in this area, other than measuring the rotary power of known compounds for comparison to literature values.  My internet searches seem to be awash in undergraduate laboratory experiments, which have been informative but I'm looking for something more.

Thanks for any help.
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Alberto_Kravina

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Re: Practical Aspects of measuring optical rotation
« Reply #1 on: August 27, 2012, 12:03:55 PM »

I don't know how other people see it, but in my opinion optical rotation does not matter that much any more when it comes to analysis of small molecules. for the determination of the ee you need a SFC or HPLC trace with a racemate to really proof that you achieved enantiomer separation on your column. The things I look at in (e.g. http://onlinelibrary.wiley.com/doi/10.1002/anie.201204145/abstract, just to show off  ;D ) is the sign and look that the magnitude is in a plausible range. I also measured the same compound 48 hours later for my own curiosity, and the value was about 60% off. I also saw a total synthesis paper where the value they got (at same solvent, conc...) was 20 and the isolated natural product 200, if I reacall correctly.

however, I still think that any student should have made a bunch of orot measurements in his/her studies, just to know what it is about. CD spectra are more familiar with people studying racemisations, but maybe someone else knows better than me


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Dan

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Re: Practical Aspects of measuring optical rotation
« Reply #2 on: August 27, 2012, 09:10:31 PM »

For example, if you've recently synthesized a previously-unknown chiral compound, and want to measure it's optical rotation, what factors go into determining the solvent, cell size, wavelength, etc?  Or does it much matter provided everything is reported?

It doesn't matter provided everything is reported. However, it is convention to measure optical rotation at a concentration around 1 g/100 mL in chloroform at 20 or 25 °C using a sodium lamp (589 nm). I've only ever encountered cells with a path length of 1 dm.
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fledarmus

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Re: Practical Aspects of measuring optical rotation
« Reply #3 on: August 28, 2012, 04:03:26 AM »

For example, if you've recently synthesized a previously-unknown chiral compound, and want to measure it's optical rotation, what factors go into determining the solvent, cell size, wavelength, etc?  Or does it much matter provided everything is reported?

It doesn't matter provided everything is reported. However, it is convention to measure optical rotation at a concentration around 1 g/100 mL in chloroform at 20 or 25 °C using a sodium lamp (589 nm). I've only ever encountered cells with a path length of 1 dm.

"provided everything is reported" is absolutely critical. Any measurement of optical rotation should include solvent, temperature, and wavelength.

There are some experimental considerations though, that can be helpful. I'm not so sure about a "convention" of using chloroform as a solvent - it seems to me that almost all of the rotations I've seen were run in methanol, but that might have been just because of the places I worked. 

If you have a strongly colored or dark solution, it might be difficult to get a good reading using a 1 dm cell length at 1g/100 ml. Also, many compounds of interest aren't that highly soluble, and a compound that isn't completely dissolved will give incorrect measurements. For those cases, you can either reduce concentrations or use a shorter cell (they are available for many of the newer polarimeters). The cell path length is very carefully measured by the manufacturer and is usually etched into the side of the cell, to three or four significant digits.

Also, a polarimeter will only read from +180 to -180 degrees, for reasons which should be obvious and are left to the student to explain. It is good practice, although rarely done in any of the labs I've been in, to take optical rotation measurements at two different concentrations to make sure you don't have a very strongly rotating compound. And sometimes you will have a compound that has such a small rotation that it is very difficult to measure accurately. In these cases, if you cannot increase the concentration sufficiently to get an accurate measurement, you can sometimes get a stronger rotation by changing the wavelength of light. Most polarimeters now have at least two lamps, sodium and mercury, or a tunable laser so you have a selection of wavelengths to choose from. This can make comparisons to literature values somewhat troublesome - although you can calculate a standard rotations based on concentration and path length, I don't know of any calculation to standardize rotation based on wavelength.
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Dan

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Re: Practical Aspects of measuring optical rotation
« Reply #4 on: August 28, 2012, 04:31:51 AM »

I'm not so sure about a "convention" of using chloroform as a solvent - it seems to me that almost all of the rotations I've seen were run in methanol, but that might have been just because of the places I worked. 

I find the majority of small organic molecules in the lit to be run in chloroform, but it depends on what you're making and convention in your lab. I've always gone for chloroform unless the compound is not sufficiently soluble in it - in which case I tend to use acetone, methanol or water.
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IsotopeBill

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Re: Practical Aspects of measuring optical rotation
« Reply #5 on: August 29, 2012, 03:22:23 AM »

Thanks for everybody's help; it was quite informative.  I agree that there are better methods for measuring ee and the like, but we need to measure some optical rotations anyways, and I wanted some practical advice.  I think I've only done this once or twice since I've been out of school, and always measuring known compounds for comparison.

Thanks again.
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