1. Definition and classification
Carbohydrates: optically active polyhydroxy aldehydes or ketones, or compounds giving them on hydrolysis. The formula Cₓ(H₂O)ᵧ is only a rough guide (rhamnose C₆H₁₂O₅ is a carbohydrate; ethanoic acid C₂H₄O₂ is not).
By hydrolysis: monosaccharides (glucose, fructose, ribose) → oligosaccharides (2–10 units: sucrose, maltose, lactose) → polysaccharides (starch, cellulose, glycogen). Aldoses (–CHO) and ketoses (C=O), named by carbon count (aldohexose, ketohexose, aldopentose). Mono- and disaccharides are sweet, soluble sugars; polysaccharides are non-sugars.
Reducing sugars reduce Tollens’ and Fehling’s (free aldehyde or α-hydroxy ketone, or a ring that opens to one): all monosaccharides, maltose, lactose. Non-reducing: sucrose, polysaccharides.
Importance: energy (glucose, starch, glycogen), structure (cellulose), materials (cotton, paper, ethanol), ribose and deoxyribose in nucleic acids.
| Class | Example | Reducing? |
|---|---|---|
| Monosaccharide | Glucose | Yes |
| Disaccharide | Sucrose | No |
| Disaccharide | Maltose | Yes |
| Polysaccharide | Starch | No |
Formulas
- A disaccharide gives two monosaccharides
Key terms
- Monosaccharide
A carbohydrate that cannot be hydrolysed to a simpler sugar.
- Reducing sugar
A sugar that reduces Tollens’ reagent and Fehling’s solution because it has a free (or potentially free) carbonyl group.
2. Structure of glucose
Open chain HOCH₂(CHOH)₄CHO (aldohexose; C2–C5 chiral; 2⁴ = 16 stereoisomers). Evidence:
| Reagent | Product | Shows |
|---|---|---|
| HI, prolonged heating | n-Hexane | Straight C₆ chain |
| NH₂OH / HCN | Oxime / cyanohydrin | C=O group |
| Br₂ water | Gluconic acid (C₆) | C=O is –CHO |
| (CH₃CO)₂O | Pentaacetate | Five –OH, on different carbons |
| HNO₃ | Saccharic acid HOOC(CHOH)₄COOH | Primary –CH₂OH |
| C₆H₅NHNH₂ (1 mol / 3 mol) | Phenylhydrazone / osazone | C=O; C1 and C2 react |
Cyclic (pyranose) structure: C5–OH adds to C1–CHO (hemiacetal). Explains: no Schiff’s test, no NaHSO₃ adduct, pentaacetate unreactive to NH₂OH, two crystalline forms, two methyl glucosides. C1 is the anomeric carbon: α (+111°, m.p. 419 K) and β (+19.2°, 423 K); mutarotation to +52.5° (about 36% α, 64% β).
D/L: –OH on the lowest chiral carbon (C5) right = D, left = L (relative to glyceraldehyde); independent of (+)/(−).
| Form | Rotation |
|---|---|
| α-D-Glucose | +111° |
| β-D-Glucose | +19.2° |
| Equilibrium | +52.5° |
Formulas
- Osazone formation
- Pentaacetate
Key terms
- Anomers
Cyclic stereoisomers of a sugar that differ only in configuration at the anomeric carbon (C1 of glucose).
- Mutarotation
The gradual change in specific rotation of a freshly prepared solution of one anomer to a constant equilibrium value.
3. Fructose and the bromine-water test
Fructose: ketohexose HOCH₂CO(CHOH)₃CH₂OH, C=O at C2; gives n-hexane with HI; cyanohydrin → hydrolysis → HI gives 2-methylhexanoic acid (glucose gives heptanoic acid). Ring: C5–OH + C2 → five-membered furanose (α and β anomers). D-(−), −92.4°.
Fructose reduces Tollens’ and Fehling’s (it isomerises in alkali), and gives the same osazone as glucose. Bromine water (mild, not alkaline) oxidises glucose (decolourised) but not fructose:
| Test | Glucose | Fructose |
|---|---|---|
| Bromine water | Decolourised | No change |
| Fehling’s | Positive | Positive |
Formulas
- Bromine water oxidises glucose
Key terms
- Furanose
A five-membered ring form of a sugar containing four carbons and one oxygen.
- Ketose
A monosaccharide containing a ketone group.
4. Disaccharides
Glycosidic linkage: C–O–C link formed between the anomeric –OH of one sugar and an –OH of another, with loss of water.
| Disaccharide | Units | Link | Reducing? |
|---|---|---|---|
| Sucrose | α-D-glucose + β-D-fructose | C1 (Glc) – C2 (Fru), α1→β2 | No |
| Maltose | 2 α-D-glucose | α-1,4 | Yes |
| Lactose | β-D-galactose + D-glucose | β-1,4 | Yes |
Sucrose (+66.5°) hydrolyses (H⁺ or invertase) to an equimolar glucose + fructose mixture of rotation about −20°: invert sugar. Maltose comes from starch (diastase) and lactose from milk.
| Sugar | Link |
|---|---|
| Sucrose | α1 → β2 |
| Maltose | α1 → 4 |
| Lactose | β1 → 4 |
Formulas
- Hydrolysis of maltose
Key terms
- Glycosidic linkage
The ether (C–O–C) link joining two monosaccharide units through an anomeric carbon.
- Invert sugar
The equimolar mixture of glucose and fructose formed by hydrolysis of sucrose; laevorotatory although sucrose is dextrorotatory.
5. Polysaccharides
Starch (plants): amylose 15–20% (unbranched α-1,4, water-soluble, blue with I₂) + amylopectin 80–85% (α-1,4 chains branched by α-1,6 about every 20–25 units, insoluble). Hydrolysis: starch → dextrins → maltose → glucose.
Cellulose (cell walls, cotton): unbranched β-1,4 chains, hydrogen-bonded into fibres; not digested by humans (no enzyme for β-links); digested by ruminants with gut microbes.
Glycogen (liver, muscle): α-1,4 with α-1,6 branches about every 8–12 units — more branched than amylopectin.
Mass relation: each chain unit C₆H₁₀O₅ (162) gives one glucose (180).
| Polysaccharide | Link | Role |
|---|---|---|
| Starch | α-1,4 (+ α-1,6) | Plant store |
| Cellulose | β-1,4 | Structure |
| Glycogen | α-1,4 + α-1,6 | Animal store |
Formulas
- Complete hydrolysis of starch
Key terms
- Amylopectin
The branched, water-insoluble component of starch with α-1,4 and α-1,6 links.
- Glycogen
The highly branched glucose polymer that stores energy in animals.
Exam tips
Common mistakes
- Using Fehling’s solution to distinguish glucose from fructose (both are positive; use bromine water).
- Calling α- and β-glucose enantiomers (they are anomers, differing at C1 only).
- Linking D with (+).
- Writing lactose as glucose + glucose, or giving cellulose α-links.
How marks are usually awarded
- Evidence for glucose structure (1 mark per reaction): reagent, product and conclusion.
- Disaccharide structure (2 marks): units, carbons joined, α/β, reducing or not.
- Polysaccharide comparisons (3 marks): one point each for unit, link/branching and function.