Chemistry · Organic chemistry · Grade 10

Carbohydrates: glucose and starch

Why does bread turn blue when iodine is dropped on it, while cotton doesn't?

The situation

A pinch of grape sugar, a lump of sugar, a piece of flatbread and a tuft of cotton — all of these are carbohydrates: using sunlight, a plant makes glucose from carbon dioxide and water, and from glucose it builds sugar, starch and cotton fibre. Drop iodine on bread and it turns blue; drop it on cotton and nothing happens. Glucose forms a silver mirror, but table sugar does not. These substances have similar formulas — so why are they so different, and how can we tell them apart?

Topic

Carbohydrates: from glucose to cellulose. Cotton, bread and sugar: glucose from photosynthesis, sucrose, starch and cellulose; starch turns blue with iodine

Key ideas

Glucose: ring ⇌ chain
Hydrolysis of sucrose
Starch + iodine

Formulas

6 CO_{2} + 6 H_{2} O \overset{\overset{}{sunlight, chlorophyll}}{\to} C_{6} H_{12} O_{6} + 6 O_{2}

Photosynthesis: forming 1 mol of glucose needs at least ≈ 2800 kJ of energy — it comes from sunlight (the same amount is released when glucose burns).

C_{6} H_{12} O_{6} + 2 Cu (OH)_{2} \overset{\overset{}{t}}{\to} C_{6} H_{12} O_{7} + Cu_{2} O ↓ + 2 H_{2} O

When heated, the aldehyde group of glucose reduces $Cu (OH)_{2}$ to red $Cu_{2} O$ (practical 3.21).

C_{12} H_{22} O_{11} + H_{2} O \overset{\overset{}{H^{+}, t}}{\to} C_{6} H_{12} O_{6} + C_{6} H_{12} O_{6}

Hydrolysis of sucrose: glucose and fructose (invert sugar) are formed.

(C_{6} H_{10} O_{5})_{n} + n H_{2} O \overset{\overset{}{H^{+}, t}}{\to} n C_{6} H_{12} O_{6}

Hydrolysis of starch (and cellulose): the final product is glucose; dextrins and maltose appear on the way.

\ln \frac{α - α_{\infty}}{α_{0} - α_{\infty}} = - k t

University: the inversion of sucrose is first order: the angle of rotation $α$ is measured against time in a polarimeter, and $k$ is found from it.

Theory

Carbohydrates and monosaccharides (3.18)

Carbohydrates are polyhydroxy aldehydes and polyhydroxy ketones, whose composition is often $C_{n} (H_{2} O)_{m}$ , together with the substances formed from them. They are made in green plants by photosynthesis: $6 CO_{2} + 6 H_{2} O \overset{}{\to} C_{6} H_{12} O_{6} + 6 O_{2}$ (≈ +2800 kJ/mol). Monosaccharides do not hydrolyse: glucose (grape sugar) is an aldohexose, fructose (fruit sugar, the sweetest natural sugar) is a ketohexose, and both are $C_{6} H_{12} O_{6}$ — isomers. In solution glucose exists mostly in the ring form: the –OH on C5 adds to the aldehyde group and forms a six-membered pyranose ring; the new –OH (C1) can sit in two ways relative to the plane of the ring — the α (axial) and β (equatorial) anomers; they convert into each other through the open form (at equilibrium there is more β). Properties: as a polyhydric alcohol it turns $Cu (OH)_{2}$ into a blue solution in the cold; as an aldehyde it gives a silver mirror and, on heating, red $Cu_{2} O$ ; it is reduced by hydrogen to sorbitol. Fermentation: alcoholic ( $C_{6} H_{12} O_{6} \overset{}{\to} 2 C_{2} H_{5} OH + 2 CO_{2}$ , yeast) and lactic ( $C_{6} H_{12} O_{6} \overset{}{\to} 2 CH_{3} CH (OH) COOH$ — yoghurt, pickled cabbage). In the body glucose is oxidised and releases energy: $C_{6} H_{12} O_{6} + 6 O_{2} \overset{}{\to} 6 CO_{2} + 6 H_{2} O$ .

Disaccharides: sucrose and maltose (3.19)

Disaccharides consist of two monosaccharide residues, $C_{12} H_{22} O_{11}$ . Sucrose (sugar from sugar beet and sugar cane) is α-glucose and β-fructose joined through both anomeric carbons (C1 and C2): a free aldehyde group cannot form, so it is a non-reducing disaccharide: it gives no silver mirror and, on heating, no red precipitate with $Cu (OH)_{2}$ (but in the cold, as a polyhydric alcohol, it gives a blue solution). It is hydrolysed by acid or by the enzyme invertase: the mixture of glucose + fructose is invert sugar (found in honey). Maltose (malt sugar, formed when starch breaks down) is two glucose units joined by an α-1,4 bond; C1 of the second residue is free, so it is a reducing disaccharide. Lactose (milk sugar) — galactose + glucose — is also reducing. Sucrose melts and decomposes at ≈ 186 °C (caramel); at 20 °C about 2 g dissolves in 1 mL of water.

Polysaccharides: starch and cellulose (3.20, 3.22)

Starch and cellulose are both $(C_{6} H_{10} O_{5})_{n}$ , but their structures differ. Starch is made of α-glucose residues: amylose (≈ 20–25 %, α-1,4 linked, coiled into a helix) and amylopectin (branched, with α-1,6 branches). Iodine molecules enter the amylose helix as a chain — a dark blue colour (the qualitative test for starch); on heating the colour disappears and on cooling it returns. Acid or enzymes break starch down step by step into dextrins, maltose and glucose. Glycogen is the storage polysaccharide of animals. Cellulose is made of β-glucose residues: neighbouring residues are alternately “flipped”, the chain is straight, and the chains pack into strong fibres through hydrogen bonds — it is insoluble in water and does not turn blue with iodine. Cotton fibre is over 90 % cellulose, wood ≈ 40–50 %. Paper, nitrocellulose (smokeless powder), acetate and viscose fibres (artificial fibres) are made from cellulose, and glucose and ethanol are made by hydrolysis. The Fergana Furan Compounds Chemical Plant (opened in 1946) produced cotton cellulose, furfural and industrial ethyl alcohol.

Practical 3.21: qualitative reactions of carbohydrates

1) **With $Cu (OH)_{2}$ :** add a few drops of $CuSO_{4}$ to 2 mL of NaOH solution — a blue precipitate; add the carbohydrate solution. Glucose, fructose, sucrose and maltose (like polyhydric alcohols) turn the precipitate into a dark blue solution. Then heat: glucose, fructose and maltose give yellow and then red $Cu_{2} O$ ; sucrose gives no red precipitate (if boiled for a long time, $Cu (OH)_{2}$ turns into black $CuO$ ). 2) Silver mirror: ammoniacal silver solution + carbohydrate, water bath at 60–70 °C: reducing sugars give a mirror. 3) Iodine: starch — dark blue; cellulose (cotton) and sugars give no colour (the yellow of iodine). 4) Hydrolysis of sucrose: after boiling for a few minutes with 1–2 drops of $H_{2} SO_{4}$ and neutralising with NaOH, the $Cu (OH)_{2}$ test is positive. Safety: wear goggles when working with alkali and acid; do not point a boiling test tube at yourself or others; pour away the ammoniacal silver solution after the experiment. Tasting is forbidden in the laboratory.

University: sugar laboratory

Reducing sugars (with a free hemiacetal –OH: glucose, fructose, maltose, lactose, cellobiose) reduce Fehling's reagent (an alkaline tartrate complex of $Cu^{2 +}$ → red $Cu_{2} O$ ) and Tollens' reagent (silver mirror); sucrose and raffinose do not. Inversion: sucrose rotates the plane of polarised light to the right ( ${[α]}_{D}^{20}$ = +66.5°), glucose +52.7°, fructose −92.4° — after hydrolysis the mixture rotates to the left (hence “invert” sugar). Acid hydrolysis is first order: $α (t) = α_{\infty} + (α_{0} - α_{\infty}) e^{- k t}$ , $k$ increases with acid concentration and temperature; the half-life is $t_{1 / 2} = \ln 2 / k$ . Starch hydrolysis is followed with the iodine test: blue → violet (amylodextrins) → reddish-brown (erythrodextrins) → colourless (maltose, glucose). If dried fruit (raisins, prunes, dried apricots) is extracted with water, the solution is rich in glucose and fructose — the $Cu (OH)_{2}$ test is positive; they can be separated by paper or thin-layer chromatography. Monosaccharides are divided into the D- and L-series (by the last asymmetric C atom); glucose and fructose in nature are D-series. The interconversion of the open and ring forms is ring–chain tautomerism. Derivatives: deoxy sugars (2-deoxyribose in DNA), amino sugars (glucosamine; its N-acetyl derivative is a unit of chitin), ascorbic acid (vitamin C). More polysaccharides: glycogen (animal starch), chitin (the shell of insects), inulin (a polymer of fructose), pectin (fruit jelly), hyaluronic acid and heparin (connective tissue; an anticoagulant drug). Hydrolysis is fast in acidic medium; glycosidic bonds are much more stable in alkaline medium.

Limits of the model

The rates of the reactions in the test tubes are simplified: the silver mirror is noticeable from ≈ 35 °C, $Cu_{2} O$ from ≈ 65 °C, hydrolysis from ≈ 60 °C; time is sped up. The fading of the starch–iodine colour on heating is taken to be linear at ≈ 60–80 °C. The specific rotations in the polarimeter are the values at 20 °C, and $k$ (25 °C, 1 mol/L HCl) = 0.012 min⁻¹ is an approximate model value (it rises by a factor of ≈ 3.5 for every 10 °C); time: 1 s = 1 min. The glucose and fructose rings are shown in a true chair/envelope shape, with bond lengths ≈ 1.43–1.53 Å; the disaccharide linkages are approximate; the amylose and cellulose models are schematic (H atoms are not shown, the rings are flat). The fermentation animation shows only the overall result of more than 10 enzymatic steps.

Did you know?

Cotton fibre is the purest cellulose in nature: over 90 % of it is cellulose (in wood this share is ≈ 40–50 %).
The starch–iodine colour fades on heating and returns on cooling: the blue is given by polyiodide chains sitting inside the amylose helix; when hot, the helix loosens and the iodine escapes.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

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