Question

A Fischer projection of a monosaccharide is shown below:
A Fischer projection of a monosaccharide is shown below: CHO H + -OH CH2OH Classify this monosaccharide (e.g., aldotriose) Does it have the D or L configuration?

Answer 1

Short answer:

Given molecule 1 (one on the top) contains 3 three carbon atoms and a CHO (aldehyde) fuctional group and hence is classified as an aldotriose

The second molecule (one on the bottom) contains 5 carbon atoms and a C=O (ketone) functional group and is classified as a ketopentose.

Here, the first sugar is a D sugar and the second one is a L sugar.

Carbohydrates are biomolecules that contain hydrogen, oxygen and carbon which are composed of small repeating units. Their major function is storage of energy. The chemical structure of a carbohydrate (sugar) may be shown in a fischer projection or as a haworth projection formula. in a fischer projection, the carbon atoms in the open chain form of the sugar are connecte by vertical lines and hydrogen and oxygen atoms are attached to the carbon atoms by horizontal lines.

Sugars may be hydrolysed to break them down into simpler sugars, known as monosaccharides which further cannot be broken down to simpler sugars. For example, Sucrose may be hydrolysed to yield glucose and fructose wherein glucose and fructose are the corresponding monosaccharides.

Monosaccharides or simple sugars are classifed on the basis of:

(a) number of carbon atoms in the sugar as triose (3 carbon atoms), tetrose (4 carbon atoms), pentose (5 carbon atoms), hexose (6 carbon atoms) etc.

(b) the nature of the functional group present in them as aldoses (containing aldehyde) or ketoses (containing ketone)

Here, given molecule 1 contains 3 three carbon atoms and a CHO (aldehyde) fuctional group and hence is classified as an aldotriose

The second molecule contains 5 carbon atoms and a C=O (ketone) functional group and is classified as a ketopentose.

D and L notation

Monosaccharides with 3 or more carbon atoms may be considered as a derivative of glyceraldehyde. Glyceraldheyde is an optically active molecule with a chiral centre. It has two forms: D (dextrotatory form of glyceraldehyde) and L (levorotatory form of glyceraldehyde) enantiomers depending solely on the direction by which it rotates a plane polarized light passing through them (dextro- to the right, levo- to the left).

Sugars are classified as D or L by comparing its fischer projection structure to the D and L forms of glyceraldehyde. In the fischer projection formula of a monosaccharide, if the penultimate carbon atom has H atom on the left and the OH group on the right, it is assigned as a D sugar. Whereas, if the penultimate carbon atom has H atom on the right and OH on the left, it is assigned as a L sugar.

Here, the first sugar is a D sugar and the second one is a L sugar.