Q.Why are carbohydrates generally optically active?

Epimers are a special class of diastereomers that differ at just a single stereocentre out of several.

Definition

When two stereoisomers, each having more than one chiral (asymmetric) carbon, differ in configuration at exactly one of those chiral carbons — with the configuration at every other centre identical — they are called epimers. Being diastereomers, epimers have different physical properties (m.p., specific rotation, solubility), unlike enantiomers.

Examples

• D-glucose and D-mannose differ only in configuration at C-2; they are C-2 epimers. …

The presence of several chiral (asymmetric) carbon atoms in their structure is what makes carbohydrates optically active.

Explanation

Most carbohydrates are polyhydroxy aldehydes or ketones. In their open-chain structures, several carbon atoms (e.g. C2, C3, C4, C5 in glucose) are each bonded to four different groups — making them chiral (asymmetric) carbon atoms. A molecule containing one or more such chiral centres lacks a plane/centre of symmetry and can exist as non-superimposable stereoisomers; each such stereoisomer rotates the plane of plane-polarised light passing through it, either clockwise (dextrorotatory, +) or anticlockwise (laevorotatory, −).

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Erythrose is a four-carbon aldose (aldotetrose); its D-form is drawn in Fischer projection with the -CHO group at the top, both -OH groups on the right, and -CH2OH at the bottom.

Erythrose is an aldotetrose (4-carbon sugar with an aldehyde group). Its Fischer projection (vertical carbon chain, horizontal bonds pointing towards the viewer, vertical bonds pointing away) for the naturally occurring D-form is written top-to-bottom as:

C1: CHO

C2: H-C-OH (OH on the right)

C3: H-C-OH (OH on the right)

C4: CH2OH

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