Question

There are three containers that the labels are lost. We know that they contain (a) racemic a-phenylethylamine, (b) pure (-) a-phenylethylamine and (c) a mixture with 90% optical purity of the dextrorotatory amine. If you can only use a polarimeter, how could you differentiate these three containers? The specific rotation of (+)-a-phenylethylamine at 22 °C is +40.3. (No calculation is necessary)

Answer 1

1. Enantiomers having opposite sign and the value is same.
2. (-)-a-phenylethylamine is having -40.3 rotation.
3. Racemic mixture (one enantiomer 50%+other enatiomer 50%) having rotation 0.

So. (b) Rotation of (-)-a-phenylethyl amine is -40.3 rotation.

(a) Racemic phenylethylamine is having 0 rotation.

(c) a mixture with 90% optical purity of the dextrorotatory amine will show + optical rotation. Dexrorotatory amine is major. a mixture with 90% optical purity rotation is + sign.