Carbohydrates include polyhydroxy aldehydes, polyhydroxy ketones and substances producing these units on hydrolysis. Glucose has molecular formula C6H12O6 and exists mainly in cyclic forms in water, in equilibrium with a small open-chain fraction. The open-chain aldehyde form helps explain its reducing behaviour. A simple carbon–hydrogen–oxygen ratio alone is not a sufficient definition of a carbohydrate.
Condensation joins sugar units through glycosidic links while releasing water. Joining two glucose molecules into a disaccharide removes one H2O, giving C12H22O11. A long glucose-based polysaccharide is commonly represented by repeating unit (C6H10O5)n; exact finite chains include end groups. Hydrolysis reverses link formation. Starch and glycogen store energy, while cellulose provides structural support; differing link arrangements give different properties despite related monomers.
Example 1
Derive the formula for a disaccharide formed from two C6H12O6 molecules.
2C6H12O6−H2O=C12H22O11. A single glycosidic link forms with loss of one water molecule.
Example 2
How many water molecules are released to make a single unbranched chain of five glucose units?
Five monomers require four connecting links, so four H2O molecules are released. In general a finite chain of n units has n−1 links.
Watch out:Many fit a related ratio, but the class is defined by structure and hydrolysis products; deoxyribose is a counterexample to that simple formula.
Carbohydrate structures and condensation practice | Year 12 Chemistry | Ascent Arcade