Physical Properties of Diastereomers

What is Diastereomers?

– We have defined stereoisomers as isomers whose atoms are bonded together in the same order but differ in how the atoms are directed in space.

– We have also considered enantiomers (mirror-image isomers) in detail.

– All other stereoisomers are classified as diastereomers, which are defined as stereoisomers that are not mirror images.

– Most diastereomers are either geometric isomers or compounds containing two or more chirality centers.

Cis-trans Isomerism on Double Bonds

– We have already seen one class of diastereomers, the cis-trans isomers, or geometric isomers.

– For example, there are two isomers of but-2-ene:

– These stereoisomers are not mirror images of each other, so they are not enantiomers. They are diastereomers.

 Cis-trans Isomerism on Rings

– Cis-trans isomerism is also possible when there is a ring present.

– Cis- and trans-1,2-dimethylcyclopentane are geometric isomers, and they are also diastereomerrs.

– The trans diastereomer has an enantiomer, but the cis diastereomer has an internal mirror plane of symmetry, so it is achiral.

What is Diastereomers?

Physical Properties of Diastereomers

– We have seen that enantiomers have identical physical properties except for the direction in which they rotate polarized light.

– Diastereomers, on the other hand, generally have different physical properties.

– For example, consider the diastereomerrs of but-2-ene (shown next).

– The symmetry of trans-but-2-ene causes the dipole moments of the bonds to cancel.

– The dipole moments in cis-but-2-ene do not cancel but add together to create a molecular dipole moment.

– The dipole–dipole attractions of cis-but-2-ene give it a higher boiling point than trans but-2-ene

Physical Properties of Diastereomers

– Diastereomers that are not geometric isomers also have different physical properties.

– The two diastereomers of 2,3-dibromosuccinic acid have melting points that differ by nearly 100 °C!

– Most of the common sugars are diastereomerrs of glucose.

– All these diastereomerrs have different physical properties.

– For example, glucose and galactose are diastereomeric sugars that differ only in the stereochemistry of one asymmetric carbon atom, C4.

Physical Properties of Diastereomers

– Because diastereomerrs have different physical properties, we can separate them by ordinary means such as distillation, recrystallization, and chromatography.

– As we will see in the next section, the separation of enantiomers is a more difficult process.

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