Structure of Monosaccharides

Commonly called simple sugars. (e.g. glucose and fructose).
They are either aldehydes or ketones with one carbonyl group and at least two hydroxyl groups (-OH).
The empirical formula is CH2O
The common formula for all monosaccharides is (CH2O)n, where n can be any integer from 3 to 7.
Monosaccharides are grouped based on the value of n:
  • Trioses: n = 3
  • Tetrose: n = 4
  • Pentose: n = 5
  • Hexose: n = 6
  • Heptose: n = 7

The most common and best known hexose is glucose, its chemical formula is: C6 H12O2.
Figure1.1 Carbonyl or Carboxyl group
There are two possible structures of the monosaccharides; straight chain and ring structure.

-Straight chain structure-
Even in the straight chain structure there are two kinds. D and L nomenclature.
'D' and 'L' here refers to the position of -OH group attached to the second last carbon. 'D' is used when the -OH group is on the 'right' hand side, and 'L' is used when -OH group is on the 'left' hand side.
Now take glucose as an example.
Figure 1.2: D-glucose and L-glucose
As you can see in D-glucose the -OH group attached to the second last carbon is on the right hand side and in L-glucose the -OH group attached to the second last carbon is on the lef hand side.

-Ring Structure-
There is evidence that glucose and fructose molecules can exist as ring structures when in solution. This happens because the shape and flexibility of the molecule brings the aldehyde or ketone carbonyl group close to one of the hydroxyl groups, and they react to form a cyclic structure.
Haworth projections show the structures of carbohydrates in a way that makes comparisons easier.
Cyclic structure for glucose ~