Disaccharides and polysaccharides: preparation and properties
Disaccharides have the formula C₁₂H₂₂O₁₁ and give two monosaccharides on hydrolysis. Sucrose (beet or cane sugar) consists of glucose and fructose residues: C₁₂H₂₂O₁₁ + H₂O → C₆H₁₂O₆ (glucose) + C₆H₁₂O₆ (fructose); it has no free aldehyde group, so it does not give the “silver mirror” before hydrolysis but does after boiling with acid. Maltose (malt sugar) gives two glucose molecules on hydrolysis. Polysaccharides are natural polymers (C₆H₁₀O₅)ₙ built of very many monosaccharide units. Starch consists of α-glucose residues, is a white powder insoluble in cold water, swells in hot water into a paste, gives a blue colour with iodine (the qualitative test) and hydrolyzes to glucose: (C₆H₁₀O₅)ₙ + nH₂O → nC₆H₁₂O₆. Cellulose is a fibrous polymer of β-glucose residues; it is the main material of plant cell walls, cotton fibre, paper and cloth; it gives no blue colour with iodine and humans cannot digest it (dietary fibre). Starch and cellulose have the same formula and differ in how glucose residues are linked. Cotton, a traditional crop of Uzbekistan, is mainly cellulose.
Starch and iodine: in class the teacher puts a drop of iodine solution on a cut potato; you record the colour change. Iodine solution must not be swallowed and stains clothes — not to be used on your own at home.