Write your answer in the space provided or on a separate sheet of paper. The spe

Question

Write your answer in the space provided or on a separate sheet of paper. The specific rotation of pure (-)-2-butanol is -13.5 degree. what % of a mixture of the two enantiomeric forms is the (+)-form if the specific rotation of this mixture is +7.0 degree ? (-)-Lactic acid has a specific rotation of -3.8 degree. What is the specific rotation of solution containing 7.5 g of (-)-lactic acid and 2.5 g of (+)-lactic acid? Calculate the e.e. of a mixture containing 8.0 g of (-)-glyceraldohyde and 2.0 of (+)-glyceraldehyde. Can a molecule be chiral if it contains no asymmetric carbons? Explain brief. Is it thcorcticaily possible to separate the pair of compounds below by distillation? Explain briefly.

Explanation / Answer

55. optical purity (%) = (specific rotation of mixture/specific rotation of pure sample) x 100

we have (-)-2-butanol rotation = -13.5 degree

So, (+)-2-butanol will have = +13.5 degree

mixture = +7.0 degree so, (+) is in excess

Feed values,

optical purity (%) = (7/13.5) x 100

= 51.852 % excess of (+) enantiomer

Remaining = 100 - 51.852 = 48.148 %

which has equal (+) and (-) forms

So overall we have = 51.852 + (48.148/2) = 75.926 % of (+) form in the mixture.

59. The two compounds shown here are distereomeric to each other. Diastereomers have more then one stereocentres in them and have different physical properties. Here we have 2 stereocentres in the molecule. So their physical properties would be also different. Thus, it is possible to separate the two by distillation as they would have different boiling points.

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