Organic Chemistry MCAT Practice Exam

In carbohydrate nomenclature using Fischer projection, OH on the right indicates which configuration?

1. L (-)

2. D (+)

3. Alpha configuration

4. Beta configuration

The correct answer is: D (+)

In carbohydrate nomenclature, the Fischer projection is a two-dimensional representation of a molecule where the most oxidized carbon is placed at the top. The configuration of a carbohydrate can be determined by the placement of the hydroxyl groups (OH) on the chiral carbon furthest from the carbonyl group. When the OH group on this chiral carbon is located on the right side in the Fischer projection, it indicates that the carbohydrate is in the D configuration. Specifically, this refers to the relationship of the structure to D-glyceraldehyde, the simplest sugar, which serves as a reference point. Thus, if the OH group is on the right, the sugar is classified as a D-sugar, which is characteristic of most naturally occurring sugars. In contrast, if the OH group were on the left, it would indicate an L configuration. The other choices, referring to alpha and beta configurations, pertain to anomeric carbon stereochemistry in cyclic forms of carbohydrates and are not directly related to the Fischer projection identification regarding the D or L nomenclature.