Question

In: Chemistry

1. If the boiling point of (R,R)-1,2-diaminocyclohexane is 110°C, what do you think the boiling point...

1. If the boiling point of (R,R)-1,2-diaminocyclohexane is 110°C, what do you think the boiling point of (S,S)-1,2-diaminocyclohexane is?

2. Based on measured roations, how can one predict how well one separated 1,2-diaminocyclohexane enantiomers? Please explain the answer and how results could be improved (how you can better separate the two enantiomers)

thankyou

Solutions

Expert Solution

Enantiomers are identical in most of the tests like melting point, boiling point, solubility, infrared spectroscopy, NMR spectroscopy, etc. So the boling point is same for (R,R)-1,2-diaminocyclohexane and (S,S)-1,2-diaminocyclohexane and is 110 deg C.

However, they behave differently when it comes to the rotation of plane-polarized light or when placed in a chiral environment.

The optical rotation is different for both enatiomers. One enantiomer rotate the plane polarized light to right that is dextrorotatory (here (R,R)-1,2-diaminocyclohexane) and the (S,S)-1,2-diaminocyclohexane rotate to left.(levorotatory)

Dextrorotary compounds have a positive specific rotation, while levorotary compounds have negative. Two enantiomers have equal and opposite specific rotations.

Thus if the optical rotation of the individual enantiomer is matching with the reported optical rotation value , then the enantiomer is pure. If not matching further crystallisation will give pure enantiomer or purify till it matches with the reported value.

A common way of separation of enantiomers uses the conversion into diastereomers, that are not mirror images of each other.

The resolution of the diastereomers is based on the different solubilities of the diastereomeric (R,R) tartrate salts that are obtained by reaction with (L)-(+)-tartaric acid. The (1R,2R)-amine-(2R,3R)-acid salt crystallizes from hot water, while the (1S, 2S)-amine-(2R,3R)-acid salt stays in solution. After recrystallization from water the obtained product is usually pure enough (>99 % according to literature.


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