Question

Draw Fischer projections of L-glucose and L-fructose.

Short answer

L-glucose

L-fructose

Step-by-step solution

Fischer projections

Fischer projections are used to represent a three-dimensional molecule in a two-dimensional plane. It depicts the configuration of all the stereocenters of a molecule.

It is generally used for compounds having more than one stereocenter like sugars.

A Fischer projection contains horizontal and vertical lines. The horizontal lines give the functional groups, and the vertical line represents the carbon chain. The point of intersection of the vertical line with the horizontal line represents stereo carbon.

D and L configuration

The sugars are classified based on their stereochemistry of the farthest stereo carbon from the carbonyl group. It is assigned by comparing the compound with the D and L isomer of glyceraldehydes.

The D-form of glyceraldehydes has the hydroxyl group of the stereo carbon to the right, and the hydroxyl group in L-glyceraldehyde is on the left. Therefore, all the sugars with the bottom chiral carbon having a hydroxyl group on the right are D-sugars, and that on the left is L-sugars.

D- sugars and L- sugars are enantiomers.

Structure of given sugars

L-glucose-

The stereoisomer of glucose in which the hydroxyl group of the bottom chiral carbon is positioned to the right is L-glucose. Glucose is aldohexose. Therefore, it contains six carbon atoms and an aldehyde group.

The Fischer projection for L-glucose is:

L-fructose

The stereoisomer of fructose in which the hydroxyl group of the bottom chiral carbon is positioned to the right is L-fructose. Fructose is ketohexose. Therefore, it contains six carbon atoms and a ketone group.

The Fischer projection for L-fructose is:

L-fructose