On an exam, a student was asked to draw the Fischer projection of L-glucose, but he had only memorized the structure of D-glucose. He wrote the structure of D-glucose and switched the hydroxyl group on C5 from the right to the left. Was his answer correct? If not, what is the name of the aldose that he drew?

Verified step by step guidance

1. Understand the difference between D- and L- configurations: The D- and L- designations in sugars refer to the configuration of the chiral carbon furthest from the carbonyl group (C=O). In D-glucose, the hydroxyl group (-OH) on the last chiral carbon (C5) is on the right in the Fischer projection, while in L-glucose, it is on the left.
2. Analyze the student's approach: The student switched the hydroxyl group on C5 from the right to the left, which is consistent with the L-configuration. However, the configuration of all other chiral carbons (C2, C3, and C4) must also be inverted to correctly represent L-glucose.
3. Check the structure drawn: If the student only switched the hydroxyl group on C5 and left the configurations of C2, C3, and C4 unchanged, the resulting structure is not L-glucose. Instead, it represents a different sugar with a specific name.
4. Identify the sugar: The sugar drawn by the student is the enantiomer of D-glucose, which is called D-mannose. This occurs because switching the hydroxyl group on C5 alone does not invert the entire molecule to the L-configuration.
5. Conclude the answer: The student's answer was incorrect. To correctly draw L-glucose, the configurations of all chiral carbons (C2, C3, C4, and C5) must be inverted relative to D-glucose.

Key Concepts

Fischer Projection

The Fischer projection is a two-dimensional representation of a molecule, particularly useful for depicting the stereochemistry of carbohydrates. In this format, the vertical lines represent bonds that project behind the plane of the page, while horizontal lines represent bonds that project out towards the viewer. This method is essential for visualizing the spatial arrangement of atoms in sugars, allowing for easy identification of stereoisomers.

D- and L- Configuration

The D- and L- designations refer to the configuration of the chiral carbon furthest from the aldehyde or ketone group in a sugar molecule. In the case of glucose, D-glucose has the hydroxyl group on the right at this carbon, while L-glucose has it on the left. This distinction is crucial for understanding the stereochemistry of sugars and their biological roles, as D- and L- forms can have different properties and functions.

Aldose

An aldose is a type of sugar (monosaccharide) that contains an aldehyde group (-CHO) at one end of the molecule. Glucose is a common example of an aldose. When the student incorrectly modified D-glucose, he inadvertently created a different aldose, specifically D-mannose, by switching the hydroxyl group on C5. Understanding the classification of sugars as aldoses or ketoses is fundamental in carbohydrate chemistry.