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Which of the following statements about anomers are correct?

1. Anomers differ in their stereochemistry only at the anomeric carbon (C-1).
2. alpha-D-glucopyranose and beta-D-glucopyranose are a pair of anomers.
3. Both anomeric forms of D-glucopyranose can be individually crystallised and purified.
4. When pure alpha-D-glucopyranose is dissolved in water, its specific optical rotation gradually changes over time.

Correct answer: Anomers differ in their stereochemistry only at the anomeric carbon (C-1).

Solution

Anomers are cyclic sugar isomers that differ in configuration only at C-1 (the anomeric carbon). Alpha and beta forms of D-glucopyranose are the classic example. Both can be crystallised as pure forms, and when either pure anomer is dissolved in water it undergoes mutarotation — a gradual change in optical rotation until equilibrium is reached. All four statements are therefore correct.

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