The Fischer projection of D-fructose differs from D-glucose primarily in the arrangement of hydroxyl groups and the position of the carbonyl group. In D-glucose, the carbonyl group is located at the first carbon atom (C1) as an aldehyde, making it an aldose. The hydroxyl groups are arranged along the carbon backbone, with specific stereochemistry at C2, C3, C4, and C5, which ultimately defines it as D-glucose.
On the other hand, D-fructose is a ketose, meaning its carbonyl group is found at the second carbon atom (C2), leading to a different configuration. In the Fischer projection, this results in a distinct arrangement of the hydroxyl groups compared to D-glucose. For example, at C3, the orientation of the hydroxyl group in fructose is different from that in glucose. The overall structure of D-fructose showcases a ketone functional group instead of an aldehyde, which is a key feature that sets it apart.
In summary, while both sugars share the same molecular formula (C6H12O6), their structural formulas as depicted in Fischer projections highlight their differences: the position of the carbonyl group and the configuration of hydroxyl groups determine the unique characteristics of D-fructose compared to D-glucose.