What are carbohydrates?
Carbohydrates are made of carbon, hydrogen and oxygen. Many of them fit the formula Cx(H2O)y, which is why they were once called "hydrates of carbon". But that name is not exact: rhamnose (C6H12O5) is a carbohydrate that does not fit, and acetic acid (C2H4O2) fits but is not a carbohydrate.
A better definition: carbohydrates are polyhydroxy aldehydes or polyhydroxy ketones, or compounds that give these on hydrolysis. "Polyhydroxy" means many –OH groups. "Hydrolysis" means breaking a bond using water.
Classification of carbohydrates
By number of units (what hydrolysis gives)
- Monosaccharides: one unit, cannot be broken into simpler sugars. Glucose, fructose, ribose, galactose.
- Oligosaccharides: 2 to 10 units. Disaccharides (2 units) are the most common: sucrose, maltose, lactose.
- Polysaccharides: very many units. Starch, cellulose, glycogen. Not sweet and mostly not soluble in water.
By taste
Sweet ones that dissolve in water (mono and disaccharides) are called sugars. Polysaccharides are non-sugars.
Reducing and non-reducing sugars
A reducing sugar has a free aldehyde or keto group (or a ring that can open to give one). It reduces Tollens' reagent (silver mirror) and Fehling's solution (red precipitate). All monosaccharides are reducing. Maltose and lactose are reducing. Sucrose is non-reducing because both of its reducing ends are used up in the glycosidic bond.
Aldoses and ketoses
A monosaccharide with –CHO is an aldose; one with C=O inside the chain is a ketose. Add the carbon count: glucose is an aldohexose (6 C), fructose a ketohexose, ribose an aldopentose (5 C).
Glucose: how we get it and its open-chain structure
Preparation
- From sucrose: boil sucrose with dilute HCl or H2SO4 in alcohol. It splits into glucose and fructose.
- From starch (factory method): boil starch with dilute H2SO4 at about 393 K under 2–3 atm pressure. (C6H10O5)n + nH2O → nC6H12O6.
Proof of the open chain (clues from reactions)
- Formula C6H12O6.
- Heating with HI for a long time gives n-hexane: the 6 carbons are in one straight chain.
- With hydroxylamine it forms an oxime, and with HCN a cyanohydrin: there is a C=O group.
- Mild oxidation with bromine water gives gluconic acid (6 C): the C=O is an aldehyde (–CHO).
- Acetylation with acetic anhydride gives a pentaacetate: there are 5 –OH groups, on different carbons.
- Nitric acid oxidises both ends to give saccharic acid (a dicarboxylic acid): one end is –CHO and the other is a primary –CH2OH.
So glucose is CHO–(CHOH)4–CH2OH. It is D-(+)-glucose: "D" means the –OH on the lowest chiral carbon (C5) is on the right, like in D-glyceraldehyde; "(+)" means it turns plane-polarised light to the right. D and + are two separate facts.
Glucose: the ring structure, anomers and Haworth form
The open chain cannot explain everything:
- Glucose does not give Schiff's test and does not form the hydrogen sulphite (NaHSO3) addition product, though aldehydes should.
- Its pentaacetate does not react with hydroxylamine, so there is no free –CHO in it.
- Glucose exists as two crystal forms, α and β.
The answer: the –OH on C5 adds to the –CHO on C1 and closes a six-membered ring (five C and one O), called a pyranose ring, as it looks like pyran. C1 becomes a new chiral centre. If its new –OH is on the same side as the ring (down in Haworth form) we get α-D-glucose; the other way gives β-D-glucose. Such pairs, which differ only at C1, are called anomers, and C1 is the anomeric carbon. In water, the two forms slowly change into each other through the open chain (mutarotation).
The Haworth drawing shows the ring as a flat hexagon with groups pointing up or down.
Fructose
Fructose, C6H12O6, is the sweetest natural sugar, found in fruit and honey. It has a keto group at C2, so it is a ketohexose. It is laevorotatory: D-(–)-fructose. The –OH on C5 joins C2 to make a five-membered ring (four C and one O) called a furanose ring. It also has α and β anomers.
Disaccharides: sucrose, maltose, lactose
Two monosaccharides join with loss of water. The link, C–O–C, is the glycosidic linkage. Acid or an enzyme can break it again.
| Disaccharide | Made of | Link | Reducing? | Found in |
|---|---|---|---|---|
| Sucrose | α-D-glucose + β-D-fructose | C1 (glucose) – C2 (fructose) | No | Cane sugar, beet |
| Maltose | α-D-glucose + α-D-glucose | C1 – C4 | Yes | Malt, starch digestion |
| Lactose | β-D-galactose + β-D-glucose | C1 – C4 | Yes | Milk |
Invert sugar
Sucrose turns light to the right (+66.5°). When hydrolysed, it gives equal amounts of glucose (+52.5°) and fructose (–92.4°). Fructose turns light more strongly, so the mixture turns it to the left. The sign has flipped, so the product is called invert sugar, and the change is inversion.
Polysaccharides: starch, cellulose, glycogen
- Starch: the food store of plants (rice, wheat, potato). It is a mix of amylose (about 15–20%, a long unbranched chain of α-D-glucose with C1–C4 links, water-soluble, coils like a spring) and amylopectin (about 80–85%, branched: C1–C4 links in the chain and C1–C6 links at branch points, water-insoluble). Starch turns blue-black with iodine.
- Cellulose: the most common carbohydrate in plants, the main part of the cell wall. A straight chain of β-D-glucose with C1–C4 links. Chains lie side by side and hold each other with H-bonds, so it is strong. Humans lack the enzyme to break β-links, so we cannot digest it; cows can, with help from gut microbes.
- Glycogen: the animal store of glucose, in liver, muscle and brain. Built like amylopectin but even more branched. Sometimes called animal starch. Yeast and fungi also have it.
Why carbohydrates matter
- Fuel: glucose is burnt in cells to release energy.
- Storage: starch in plants, glycogen in animals.
- Structure: cellulose in plant walls; wood, cotton and paper.
- Building blocks: ribose and deoxyribose are part of RNA and DNA.
- Industry: honey (invert sugar), textiles, paper, lacquers, brewing.
Try it at home
- Put a drop of tincture iodine (from a first-aid box) on a slice of raw potato, a piece of bread and a bit of sugar solution.
- Predict first: which turns blue-black?
- Check: potato and bread go blue-black (starch). Sugar water stays brown (no starch).
- Now chew a small piece of bread for 2 minutes. It starts to taste sweet: saliva is turning starch into maltose. In the 3D, step 5, set the type to amylose and watch the coil that holds iodine.
Key formulas and definitions
- Glucose, fructose: C₆H₁₂O₆ · Sucrose, maltose, lactose: C₁₂H₂₂O₁₁ · Starch, cellulose: (C₆H₁₀O₅)ₙ
- Monosaccharide + monosaccharide → disaccharide + H₂O
- C₁₂H₂₂O₁₁ + H₂O → C₆H₁₂O₆ (glucose) + C₆H₁₂O₆ (fructose)
- Glucose + Br₂ water → gluconic acid (proves –CHO) · Glucose + HNO₃ → saccharic acid
- Glucose + HI (heat) → n-hexane (proves straight 6-C chain)
- n units in a chain → (n − 1) glycosidic bonds and (n − 1) H₂O removed
- Molar mass: glucose 180 g/mol, sucrose 342 g/mol, glucose residue C₆H₁₀O₅ = 162 g/mol
Worked examples
1. Classify: glucose, sucrose, starch, ribose, lactose, cellulose.
Step 1: One unit → monosaccharides: glucose, ribose. Step 2: Two units → disaccharides: sucrose, lactose. Step 3: Many units → polysaccharides: starch, cellulose.
2. How many water molecules are removed when 5 glucose units join in a chain? What is the molar mass of the product?
Step 1: 5 units in a line need 4 links, and each link removes 1 H₂O. So 4 H₂O. Step 2: Mass = 5 × 180 − 4 × 18 = 900 − 72 = 828 g/mol. Answer: 4 water molecules, 828 g/mol.
3. A starch sample has molar mass 162 000 g/mol. About how many glucose units does it have?
Step 1: Each glucose unit inside the chain is C₆H₁₀O₅ = 162 g/mol (glucose minus one water). Step 2: n ≈ 162 000 ÷ 162 = 1000. Answer: about 1000 glucose units.
4. How many grams of glucose are formed when 34.2 g of sucrose is fully hydrolysed?
Step 1: Moles of sucrose = 34.2 ÷ 342 = 0.1 mol. Step 2: 1 sucrose gives 1 glucose + 1 fructose, so 0.1 mol glucose. Step 3: Mass = 0.1 × 180 = 18 g. Answer: 18 g glucose (and 18 g fructose).
5. Glucose pentaacetate is formed with acetic anhydride. What does this tell us, and what is the mass gained per mole?
Step 1: Pentaacetate means 5 acetyl groups, so 5 –OH groups. Step 2: Each –OH → –OCOCH₃ swaps H (1) for COCH₃ (43): gain 42 per group. Step 3: 5 × 42 = 210 g per mole. Answer: 5 –OH groups; 180 → 390 g/mol.
6. Why is sucrose non-reducing while maltose is reducing?
Step 1: A sugar reduces Tollens' or Fehling's only if one anomeric carbon is free (its ring can open to –CHO). Step 2: In sucrose, C1 of glucose is joined to C2 of fructose: both anomeric carbons are locked. Step 3: In maltose, C1 of one glucose joins C4 of the other; C1 of the second glucose is free. Answer: sucrose has no free anomeric carbon; maltose has one.
7. Sucrose (+66.5°) is hydrolysed. Glucose is +52.5° and fructose is −92.4°. Find the sign of the mixture.
Step 1: Equal moles of both form. Average rotation ≈ (52.5 + (−92.4)) ÷ 2 ≈ −20°. Step 2: The value is negative, so the mixture is laevorotatory. Answer: it changes from + to −, which is why it is called invert sugar.
Common mistakes
- Saying all compounds of form Cx(H₂O)y are carbohydrates. Acetic acid fits the formula but is not a carbohydrate.
- Mixing D/L with +/−. D is about the position of –OH on C5; + is about the direction light turns. They are not linked.
- Calling sucrose a reducing sugar because it is sweet. It is non-reducing: both anomeric carbons are tied up.
- Thinking starch and cellulose differ in units. Both are glucose; starch has α-links, cellulose has β-links.