Structure and general characteristics of carbohydrates
Carbohydrates are bifunctional compounds containing simultaneously several hydroxyl groups and one carbonyl group (aldehyde or keto group). It is the combination of two types of functional groups that determines the entire diversity of chemical properties of this class.
σ-bonds predominate in the molecular structure, and the hybridization of carbon atoms is sp³, with the exception of a single carbon atom in the carbonyl group, which is in sp²-hybridization (one π-bond). Carbohydrates are characterized by spatial isomerism; substances differing in the position of hydroxyl groups at asymmetric atoms are called diastereomers, and mirror forms are called enantiomers.
Physical properties are typical: white solids, readily soluble in water. Mono- and disaccharides have a sweet taste, whereas polysaccharides (starch, cellulose) are not sweet.
Cyclic forms and Haworth projections
In aqueous solution, glucose exists predominantly in a cyclic form. The cycle is formed due to intramolecular attack by the hydroxyl group of the fifth carbon atom on the aldehyde group of the first carbon atom, resulting in the formation of a hemiacetal. Glucose forms a six-membered pyranose ring (5 carbon atoms and 1 oxygen atom), while fructose forms a five-membered furanose ring.
In the Haworth projection, the ring is depicted as a flat hexagon: hydroxyl groups located on the right in the linear formula are directed downwards, while those on the left are directed upwards. For the Unified State Exam, glucose can be written with the molecular formula C₆H₁₂O₆, especially in task 32 when writing fermentation reactions.
Classification of carbohydrates
- By the number of carbon atoms: pentoses (ribose, deoxyribose) and hexoses (glucose, fructose, galactose). Deoxyribose differs from ribose by the absence of an OH group at the second carbon atom.
- By the type of carbonyl group: aldoses (contain an aldehyde group — glucose, galactose, ribose) and ketoses (contain a keto group — fructose). This classification determines chemical behavior.
- By the number of monomer units: monosaccharides, oligosaccharides (including disaccharides), and polysaccharides.
Chemical properties of monosaccharides
Properties due to hydroxyl groups (common to both aldoses and ketoses):
- Reaction with active metals (Na, K) with the release of hydrogen.
- Esterification with acids (nitric acid is most popular) to form esters.
- Qualitative reaction with Cu(OH)₂ without heating — formation of a bright blue solution (a sign of a polyhydric alcohol).
Properties of aldoses (due to the aldehyde group):
- Addition reactions: with H₂, HCN, NaHSO₃.
- Oxidation reactions: silver mirror (with ammoniacal Ag₂O solution), Cu(OH)₂ upon heating with the formation of a red-orange Cu₂O precipitate, oxidation with KMnO₄, K₂Cr₂O₇, and bromine water to the corresponding acids.
- Reduction reaction with hydrogen — formation of the hexahydric alcohol sorbitol.
Features of ketoses (fructose): all alcohol properties and addition reactions are preserved, but oxidation reactions are completely absent. Fructose does not give the silver mirror reaction and does not react with Cu(OH)₂ upon heating.
Disaccharides and their properties
The most important disaccharides are maltose, lactose, and sucrose. All of them contain a glucose residue. The ability of a disaccharide to undergo oxidation reactions is determined by the nature of the second (terminal) monosaccharide residue, which must retain a free hemiacetal (potentially aldehyde) group.
- Maltose (glucose + glucose) is a reducing disaccharide; it gives the silver mirror reaction and the reaction with Cu(OH)₂ upon heating.
- Lactose (galactose + glucose) is also a reducing disaccharide. Galactose differs from glucose in the spatial position of a single OH group.
- Sucrose (glucose + fructose) is a non-reducing disaccharide: fructose is a ketose, therefore the silver mirror reaction and oxidation with Cu(OH)₂, bromine water, and KMnO₄ do not occur.
Fermentation reactions
Fermentation reactions are catalyzed by enzymes and are the only carbohydrate reactions regularly encountered in task 32 of the Unified State Exam.
- Alcoholic fermentation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. Used in ethanol production.
- Lactic acid fermentation: C₆H₁₂O₆ → 2CH₃–CH(OH)–COOH. Lactic acid (2-hydroxypropanoic acid) is formed. It occurs in human muscles and during milk souring.
- Butyric acid fermentation: C₆H₁₂O₆ → C₃H₇COOH + 2CO₂ + 2H₂. Butyric (butanoic) acid is formed.
Properties of polysaccharides
- Starch gives a characteristic blue-violet coloration with iodine — this is a qualitative reaction used, for example, to detect starch in potatoes.
- Cellulose undergoes esterification reactions: with acetic acid it forms cellulose acetate, and with nitric acid it forms nitrocellulose (used in the production of explosives and films).
- Upon treatment with concentrated H₂SO₄, sucrose chars: sulfuric acid removes water, carbon is formed, and CO₂ and SO₂ are released.
- All di-, oligo-, and polysaccharides are capable of hydrolysis (most often in an acidic medium) to form the corresponding monosaccharides.
Key points
- Carbohydrates are bifunctional compounds with hydroxyl and carbonyl groups.
- Aldoses (glucose) exhibit all oxidation reactions; ketoses (fructose) do not.
- Qualitative reactions for polyhydric alcohols: Cu(OH)₂ without heating (blue solution); for the aldehyde group: silver mirror and Cu(OH)₂ with heating.
- Reducing disaccharides are maltose and lactose; the non-reducing one is sucrose.
- Three types of fermentation: alcoholic, lactic acid, and butyric acid — are catalyzed by enzymes.
- The qualitative reaction for starch is blue-violet coloration with iodine.
- Concentrated H₂SO₄ causes the charring of sucrose.
- All complex carbohydrates undergo hydrolysis to monosaccharides.