The dehydration of sucrose

Sucrose is a disaccharide with the formula C₁₂H₂₂O₁₁. On hydrolysis, it yields the two monosaccharides, glucose (aldohexose) and fructose (ketohexose), and on dehydration produces a complex carbonaceous solid residue.  

The reaction between sucrose and concentrated H₂SO₄

With this reaction, there is a time delay of almost one minute before the reaction proceeds. The acid starts to go yellow as the dehydration begins. The rate of dehydration then accelerates as the acid heats up because the reaction is exothermic. As the sugar molecules are stripped of water, the heat generated turns the water into steam which then expands the remaining carbon into a porous, smoking, black column. This expands out of the reaction vessel, producing a choking acrid vapour and the smell of burned sugar. At this stage I normally remind my students that sulfuric acid is highly corrosive and will burn skin so they must avoid contact with it. 

Kit 

• 50-60g Granulated sugar, (sucrose), C₁₂H₂O₁₁
• 25-30cm³ Concentrated sulfuric acid, H₂SO₄
• 100cm³ Beaker 
• Heat-proof mat 

Procedure 

Put the sugar into the beaker and stand on a heat-proof mat. With care pour the acid onto the top of the sugar and then stand back. Since this demonstration produces sulfur dioxide as a waste product, it should be performed in a fume cupboard. 

Special tips

Precise amounts of chemical are unnecessary but do not use any other form of sucrose other than normal (household) granulated sugar. 

Teaching goals

Pure sulfuric acid is an oily liquid. Odourless and colourless, the pure acid freezes at 10.5°C to produce a white crystalline solid consisting of a three dimensional hydrogen-bonded network which persists in the liquid and aqueous state, making such solutions viscous. The pure acid decomposes slightly on standing and warming to evolve sulfur trioxide and water. Thus the acid reaches its maximum boiling point mixture of 330°C and its maximum concentration of 98.33 per cent.  

Concentrated sulfuric acid has a molarity of 18M and has a strong affinity for water, making it very useful for drying gases which it does not react with, eg  SO₂, Cl₂, N₂, and O₂. As well as being a good reagent to dehydrate carbohydrates, concentrated H₂SO₄ also dehydrates crystalline hydrates, organic alcohols and acids. In the reaction with sucrose, the acid dehydrates the carbohydrate to carbon and then oxidises the carbon: 

C₁₂H₂₂O₁₁^Dehydrate→ 12C(s) + 11H₂O(l) ^Oxidise→ CO₂, SO₂

The acid is a powerful oxidising agent as shown by its reactions with solid halides:  

NaI + H₂SO₄→ HI + NaHSO₄

H₂SO₄ + 8HI → 4I₂ + H₂S + 4H₂O 

These reactions can be used to good effect when teaching oxidation number and balancing equations. It is worth noting that a solid potassium bromide and concentrated sulfuric acid mixture is used to convert organic alcohols to halogenoalkanes. 

CH₃CH₂OH + HBr → CH₃CH₂Br + H₂O   

The KBr-H₂SO₄ mixture generates hydrogen bromide in situ.