Question

Problem 27, Chemistry 0225 - 2019 You take a bottle from your -200 °C freezer and discover that the label has been partially eaten by low-temperature rats. Enough of it is legible, however, so that you know that it contains either: (R, R)-1,2-dimethylcyclohexane or (R, S)-1,2-dimethylcyclohexane. To resolve (sorry!) the problem, you allow the bottle to warm to room temperature and measure the optical rotation of the contents, hoping to match it to the known rotation of one of the two possibilities. Surprisingly, you discover that there is no optical rotation whatsoever. Which stereoisomer did you have? Explain carefully.

Answer 1

Mirror plane ме Me Me (1R,2R)-1,2-dimethylcyclohexane Optically active (1R,25),1,2-dimethylcyclohexane Optically in active as it is meso compound which has a internal plane of symmetry. That is why in spite of having two chiral centers it shows no optical rotation Since no optical rotation is shown by the unknown sample it must be (1R,25)-1,2-dimethylcyclohexane