Carbohydrates. Monosaccharides: preparation and properties
Carbohydrates are widespread in nature; the first ones studied fitted the formula Cₙ(H₂O)ₘ, hence the name. By structure they are divided into monosaccharides (not hydrolyzed: glucose, fructose, ribose), disaccharides (give two monosaccharides on hydrolysis: sucrose, maltose) and polysaccharides (starch, cellulose). A monosaccharide name is the carbon count plus “-ose”: hexoses C₆H₁₂O₆, pentoses C₅H₁₀O₅. In open form glucose is CH₂OH–(CHOH)₄–CHO — an aldehyde alcohol (one –CHO and five –OH); in solution it is in equilibrium with six-membered ring α- and β-forms. Fructose is its isomer, a ketone alcohol (it has a >C=O group), and is sweeter; glucose decolourizes bromine water while fructose does not — one way to tell them apart. Glucose is a sweet, colourless crystalline solid, very soluble in water. Thanks to the aldehyde group it gives the “silver mirror” and Cu(OH)₂ (red Cu₂O on heating) tests; as a polyhydric alcohol it turns Cu(OH)₂ into a blue solution; hydrogen reduces it to sorbitol. It ferments under enzymes: alcoholic fermentation C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. In green plants glucose forms in photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (light, chlorophyll); in the body it is slowly oxidized to release energy: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O.
Find “carbohydrates”, “sugars” and “fibre” on food packages and list which are monosaccharides (glucose, fructose) and which disaccharides (sucrose). Do not burn or heat anything.