Disaccharides


Condensation of Sugars

Disaccharides are formed by the condensation reactions of two simple sugar molecules. Condensation is the loss of water in a chemical reaction. Two OH groups, one from each sugar molecule, come together to release water and form an oxygen bridge between. One of the OH groups is attached to the anomeric carbon (the carbon that has 2 oxygens bonded to it).

Here you see the formation of sucrose from the 6-membered form of glucose and the 5-membered form of fructose. Note that linear fructose has a ketone rather than an aldehyde group. Which carbon in glucose and in fructose would be the carbonyl carbon in the linear form?



Another example is the condensation of 2 molecules of glucose.




Examples of Disaccharides

Sucrose is the disaccharide of glucose and fructose. This is common table sugar and it comes from sugar cane and sugar beets. Maple syrup also contains sucrose.
Maltose is derived from the coupling of two molecules of glucose. It is produced when the enzyme amylase breaks down starch. Maltose is formed in germinating cereal grains and is important in the production of alcohol by fermentation.
This is a disaccharide of galactose and glucose. Lactose is also called milk sugar and it makes up between 2 and 8 % of milk.



Step 1: Protonation of Sugars

Most reactions involve the combination of an electrophile and a nucleophile. Remember that a strongly electrophilic carbon is formed by the protonation of a simple sugar.



The cation on carbon is stabilized by the adjacent oxygen atom. The empty p orbital on carbon can overlap with the filled p orbital on oxygen. The carbon is still electron-poor though and will react rapidly with nucleophiles.


Step 2: Addition of a Nucleophile

Most reactions can be viewed as the addition of a nucleophile to an electrophile. In acid-base reactions, the base is also a nucleophile and combines with the proton, an electrophile. When carbonyl compounds are reduced by borohydride reagents, a nucleophilic hydride is added to the nucleophilic carbonyl carbon.

In each of the reactions below, a curved arrow indicates the movement of an electron pair from the nucleophile to the electrophile. This is a very useful way to show the mechanism of the reaction. Never show an arrow from an electrophile to a nucleophile!



An available hydroxy oxygen atom on a simple sugar acts as a nucleophile and adds to the electrophilic carbon of the sugar atom. Following the addition, the bridging oxygen loses a proton.




Professor Patricia Shapley, University of Illinois, 2012