[antimatter101]

I just finished the chapter on stereochemistry. I recall that the specific rotation of Penicillin V (Phenoxymethylpenicillin) is =233 degrees. But it exceeds 180 degrees! Shouldn't it be -127 degrees instead? You will know if you learnt angles.

[sjb]

I just finished the chapter on stereochemistry. I recall that the specific rotation of Penicillin V (Phenoxymethylpenicillin) is =233 degrees. But it exceeds 180 degrees! Shouldn't it be -127 degrees instead? You will know if you learnt angles.

Possibly, but this could be resolved by taking measurements at e.g. different concentrations. There is nothing to stop a specific rotation even being in the range of -359.99999...° ≤ θ ≤ 359.99999...°

[Dan]

Specific rotation [α] is not measured in degrees - it is often quoted in degrees, but this is a common mistake.

The calculation is:

[α] = 100α/cl

Where α = measured rotation (deg); c = concentration (g/100cm³); l = path length (dm)

So the units are:

(100 deg 100 cm³)/(g dm) = 10⁴ deg cm³ g^-1 dm^-

or 10³ deg cm² g^-1

it is the measured rotation α that is in degrees, and the polarimeter can not distinguish +233 from -127. However, the specific rotation of Penicillin V was probably* measured under the following (conventional) conditions:

c = 1.0 g/100cm³
l = 1.0 dm

in that case, we know [α] = +233 10³ deg cm² g^-1, and that:

233 = 100α/(1.0 x 1.0)

So in this case the measured rotation α = +2.33 deg.

There is little ambiguity here, because the other possibility is that the α = -357.67 deg, which is not practically realistic for a dilute solution.


*If you have the concentration and path length at which [α] was measured (should be given at the source), you can calculate the value of α (in derees) that was measured in the experiment.

[antimatter101]

Thanks, I was a bit dumb.