Carbohydrates

The essentials

Carbohydrates are optically active polyhydroxy aldehydes or ketones, or compounds that give these on hydrolysis. Monosaccharides (glucose, fructose, ribose) cannot be hydrolysed further. Oligosaccharides give 2–10 monosaccharide units on hydrolysis; disaccharides such as sucrose and maltose give two. Polysaccharides (starch, cellulose, glycogen) give a large number. Sugars that reduce Fehling's solution and Tollens' reagent are reducing sugars; all monosaccharides are reducing sugars.

Key points

• Glucose (C₆H₁₂O₆) is an aldohexose. Evidence for its open-chain structure: with HI it gives n-hexane (straight chain of six carbons); with hydroxylamine an oxime and with HCN a cyanohydrin (C=O present); bromine water oxidises it to gluconic acid (–CHO present); acetic anhydride gives a pentaacetate (five –OH groups); nitric acid gives saccharic acid (a primary –OH present).
• Glucose does not give Schiff's test or react with NaHSO₃, and its pentaacetate does not react with hydroxylamine. So it exists mainly as a six-membered ring (pyranose) formed when the C-5 –OH adds to the –CHO group. The two forms differing only at C-1, α and β, are called anomers.
• Fructose is a ketohexose; it forms a five-membered (furanose) ring. It is laevorotatory: D-(–)-fructose.
• D and L: a sugar is D if the –OH on the lowest asymmetric carbon is on the right (like D-glyceraldehyde) and L if on the left. D/L is not the same as (+)/(–) rotation.
• Sucrose: α-D-glucose (C-1) linked to β-D-fructose (C-2); non-reducing. Its hydrolysis gives an equimolar laevorotatory mixture called invert sugar.
• Maltose: two α-D-glucose units, C-1 to C-4; reducing. Lactose (milk sugar): β-D-galactose and β-D-glucose, C-1 to C-4; reducing.
• Starch: amylose (about 15–20%, unbranched, water-soluble) and amylopectin (about 80–85%, branched with C-1–C-6 links). Cellulose: unbranched chain of β-D-glucose units. Glycogen: animal starch, highly branched, stored in liver, muscles and brain.

Common mistakes

• Assuming a D-sugar must be dextrorotatory; fructose is D but laevorotatory.