find the observed rotation

Question

Methionine (65.0 mg) was dissolved in 15.0 mL of chloroform and placed in a 1.00 cm polarimeter cell. This solution produced an observed rotation (using the sodium D line at 20∘) of +1.07∘. What is the specific rotation of methionine?

I am getting 2469 but this is incorrect as the correct ansswer should be given in degrees to 3 sig. figs.

[α]20∘C = ????

D

Answer 1

The specific rotation, [a], is calculated from the observed rotation, a, adjusting for the concentration (in gr/ml) and the cell path length (in dm). Thus the specific rotation is independent of the concentration and the cell used. The specific rotation does depend on temperature and wavelength.

The units for [a] are deg-ml / gr-dm.

Thus

[a] = a observed / c x l

[a] = a observed / (0.065 gr/ 15 ml) x 0.1 dm = + 1.07 / 0.000433 = + 2469

This answer is suspect because the known [a] for Methionine is + 23.7 when dissolved in 5M aq HCl.

There are several reasons to suspect the data are incorrect. First, it would be wasteful to make up 15 ml of solution and only use that necessary to fill a 1 cm pathlength cell. Second, most polarimeter cells are 10 cm in length (1 dm). A 1.00 cm path length cell is most unusual. Third, chloroform is not a solvent in which amino acids are readily soluble. It is strange that the optical rotation measurement would be obtained on a chloroform solution.

The a observed would have to be + 0.0107 (perhaps consistent with the low solubility) or the cell would have to be 10 dm long (again highly unusual) for the [a] to be correct.

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