Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: metalgearrex21 on March 26, 2019, 04:31:49 PM
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I did an experiment in the lab recently to measure optical rotation of enantiomers. I messed up on one of the samples and received very little product following crystallisation, 70mg to be precise.
For my report I have to include this with the specific rotation of these results.
So for the three samples I dissolved 70mg of each in 12ml of ethanol for polarimetry analysis (concentration is low I know).
So to calculate specific rotation would this be the right method:
concentration: 0.070g/12ml = 0.00583 g/ml
observed rotation = 0.53
path length = 2dm
specific rotation = 0.53/2*0.00583 = 45.5 degrees?
Is this the correct method for doing the calculation? or is it just a messed up result because the concentration is so low??
Any help would be great cause I am struggling to get an answer!
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You probably did the calculation with the given calculation procedure, so it should be correct. To judge whether your measurement is reasonably correct can only be decided when the rotation of the pure enantiomer is known.
Remember about significant digits.
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Yeah it looks OK. The concentration is good as well, I quite often mesaured samples with as low concentration as 1,2 mg/mL (0,012g/mL) and it went well.
There are multiple ways how to confirm your measurement, for example, you can do the same sample in tube that is 10cm. Or you can dilute the sample and measure it again. If your meassurement is correct, then you should get the same value in the end no matter the sample concentratio and path lenght.
You can also compare with literature values as suggested by AWK (if they are known). FUrthermore, if you measured both eantiomers, then you should get the same value with opposite signs
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From literature it says rotation of enantiomers of ibuprofen is -,+ 54.5 degrees in methanol. I know that rotation can different by the solvent your using and temperature etc. It's just I used the formula below but then I saw the following formula somewhere else and thought I might be using the wrong one.
The formula I think I have to use: https://imgur.com/a/nkhR3D4
Why are there two different formulas for the specific rotation?
I just want to make sure I have it correct on my report.
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The other one you showed is historically used because the concentration was not in g/ml but 1g/100ml which you then need to multiply by 100 but should give you the same result (you can try it and see)
THe fact that you see only 45 degress might have multiple causes: a) your sample is not enantiomericaly pure b) experimental error (different concentration, temperature..)
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Ok, so I did it out using the other formula and got the same answer :)
100*0.53/281.166 = 45.45 degrees
Is using that formula just putting my answer into grams per 100ml then?
Because I seen this example then and thought this might be the method how I get it into g/100ml
https://imgur.com/a/M5JRUis
But when I use this conc. I get 0.45 degrees as my answer.
100ml*0.070g/12ml = 0.583g/100ml
Then I plug this concentration back into my formula and get 0.53/2*0.583 = 0.45?
Thanks for the help so far and sorry for all the questions!
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Yep thats the way how to do it.
Also, congrats, your sample is quite enantiomericaly pure ;)