francis
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Optical purity of (-)-a-phenylethylamine
Hello,
We isolated (S)-(-)-a-phenylethylamine from the racemate using tartaric acid.
When measuring the optical purity, the notes said to use 0.1g per mL of ethanol.
However, I used 0.0127g/mL. (0.127g dissolved in 10mL ethanol).
When measured in the polarimeter, the observed rotation was -0.180 degrees
I now should calculate the specific rotation, based on the literature value (-30.5 degrees).
But the literature value used a concentration of 0.1g/mL.
I was going to calculate the specific rotation like this:
specific rotation = observed rotation / concentration * pathlength
For mine:
-0.180 / (0.0127g/mL * 0.5dm) = -28.3 degrees
Then to measure the optical purity:
optical purity = (calculated specific rotation / specific rotation of pure compound) * 100%
But since the specific rotation of the pure compound (the literature value) was determined at c=0.1g/mL, do I need to change the equation?
eg would
(-28.3 / -30.5) * 100% = 92.9% purity
be incorrect?
I was quite chuffed to see such a high optical purity - until I read that the literature value was taken at concentration of 0.1g/mL of ethanol.
Is my optical purity actually much lower?
Thanks
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DJF90
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The factor of concentration in the equation means that it can be a variable, and what you've dine is correct. I suggest that they use 0.1g/ml as it
makes the maths simpler (yes they're likely lazy!).
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francis
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Cheers, thanks very much
Well, yep the literature value is 0.1g/mL and so the lab manual said to use that amount...I stuffed it up though, for some reason thinking it was
0.1g/10mL therefore 0.01g/mL
Thanks again
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