Glucose and fructose are ______.

• A. anomers
• B. enantiomers
• C. diastereomers
• D. isomers

Answer: (D)

The idea being tested is that molecules can have the same formula but different structures. Glucose and fructose both have the same molecular formula, C6H12O6, but the way their atoms are connected differs. Glucose is an aldose hexose, with the carbonyl group at the end (an aldehyde) and a certain carbon skeleton, while fructose is a ketose hexose, with the carbonyl on a different carbon and a different skeleton. This difference in connectivity makes them structural (constitutional) isomers.

They are not enantiomers or diastereomers because those terms describe relationships between stereoisomers that differ in spatial arrangement around chiral centers, not simply a different connectivity. They are also not anomers, since that term refers to alpha/beta forms of the same cyclic sugar, whereas glucose and fructose are distinct molecules with different connectivity.