Questions: Carbohydrate Structure and Classification

Question 1

α-glucose and β-glucose are best described as:

• A. Epimers, differing at carbon 2
• B. Enantiomers, mirror images of each other
• C. Anomers, differing only at the anomeric carbon (C1)
• D. Constitutional isomers with different carbon skeletons

Answer: (C)

Anomers are a specific type of diastereomer that differ only at the anomeric carbon — the carbon derived from the carbonyl when the open chain cyclizes. In glucose, this is C1. In α-glucose the C1 hydroxyl is axial (trans to the CH₂OH group); in β-glucose it is equatorial (cis). Epimers differ at a single non-anomeric stereocenter; enantiomers are non-superimposable mirror images.

Question 2

In aqueous solution, glucose primarily exists in its open-chain aldehyde form.

• T. True
• F. False

Answer: False

This is a common misconception. In aqueous solution, glucose exists predominantly (>99%) in cyclic forms — mainly as the six-membered pyranose ring. The open-chain aldehyde form is present only in trace amounts at equilibrium. The ring forms because the C5 hydroxyl attacks the C1 aldehyde intramolecularly, forming a hemiacetal. The interconversion of α and β anomers via the open-chain form is called mutarotation.

Question 3

What structural feature distinguishes an aldose from a ketose? Give one example of each.

Model answer: An aldose has its carbonyl (C=O) at the terminal carbon of the chain, making it an aldehyde. A ketose has its carbonyl at an internal carbon, making it a ketone. Glucose is an aldose; fructose is a ketose.

The position of the carbonyl group determines functional group identity and reactivity. Aldoses (like glucose and galactose) have the carbonyl at C1, giving them aldehyde chemistry including reducing properties. Ketoses (like fructose) have the carbonyl at C2, making them ketones. Both form cyclic hemiacetal/hemiketal structures in solution, but the ring sizes and anomeric positions differ: aldoses preferentially form five- or six-membered rings at C1, while ketoses (with carbonyl at C2) often form five-membered furanose rings.