Topic 10 · page 16 of 27

Burning alcohols and losing water

Alcohols burn to carbon dioxide and water in plenty of air, and to carbon monoxide or soot when air is short. With concentrated phosphoric acid and heat an alcohol loses water and becomes an alkene, which is an elimination.

12 min read10.1710.17(i)10.17(iii)

Read first: Naming and classifying alcohols

Combustion 10.1710.17(i)

Alcohols burn in oxygen from the air. Burning is oxidation: the carbon and hydrogen are oxidised to carbon dioxide and water, and heat is released. Ethanol is used as a fuel, alone and as an additive to petrol.

Plenty of air: complete combustion. The products are carbon dioxide and water:

CX2HX5OH(l)+3 OX2(g)→2 COX2(g)+3 HX2O(l) \ce{C2H5OH(l) + 3O2(g) -> 2CO2(g) + 3H2O(l)}

Limited air: incomplete combustion. The products include carbon monoxide, which is toxic, and carbon (soot) if the air is more limited still:

CX2HX5OH(l)+2 OX2(g)→2 CO(g)+3 HX2O(l) \ce{C2H5OH(l) + 2O2(g) -> 2CO(g) + 3H2O(l)}
CX2HX5OH(l)+OX2(g)→2 C(s)+3 HX2O(l) \ce{C2H5OH(l) + O2(g) -> 2C(s) + 3H2O(l)}

MethodBalancing a combustion equation

  1. Write the formulae: the alcohol and O₂ on the left; CO₂ and H₂O on the right.
  2. Balance carbon first: one CO₂ for each carbon in the alcohol.
  3. Balance hydrogen next: one H₂O for every two hydrogens.
  4. Balance oxygen last. Count all the oxygen on the right, subtract the oxygen already in the alcohol, and halve the rest to get the O₂.
  5. If the numbers are fractions, double everything.

Worked exampleBalancing an equation, and a mass of CO₂

Balance the equation for the complete combustion of propan-1-ol. Then find the mass of CO₂ formed when 2.30 g of ethanol burns completely.

  1. Propan-1-ol is C₃H₇OH. Carbon: 3CO₂. Hydrogen: 8 H in total (7 + 1) gives 4H₂O.
  2. Oxygen on the right: 6 + 4 = 10 atoms. The alcohol supplies 1, so O₂ supplies 9 atoms, which is 4.5 O₂. Double everything:
  3. 2 CX3HX7OH+9 OX2→6 COX2+8 HX2O\ce{2C3H7OH + 9O2 -> 6CO2 + 8H2O}. Check oxygen: 2 + 18 = 20 on the left, 12 + 8 = 20 on the right.
  4. For ethanol, MrM_\text{r} = 2(12.0) + 6(1.0) + 16.0 = 46.0. n = 2.30 ÷ 46.0 = 0.0500 mol.
  5. The equation gives 2 mol CO₂ for each mol of ethanol, so n(CO₂) = 0.100 mol.
  6. MrM_\text{r}(CO₂) = 12.0 + 2(16.0) = 44.0, so mass = 0.100 × 44.0 = 4.40 g.

Dehydration: losing water 10.1710.17(iii)

Heat an alcohol with concentrated phosphoric acid. The –OH group and a hydrogen on the next carbon leave together as water, and a C=C forms. An alkene is made.

CHX3CH(OH)CHX3→CHX3CH=CHX2+HX2O \ce{CH3CH(OH)CH3 -> CH3CH=CH2 + H2O}
Propan-2-ol reacting with concentrated phosphoric acid and heat to give propene and water. In the propan-2-ol the OH on carbon 2 and an H on carbon 1 are drawn in orange. In the propene, a double bond appears between carbons 1 and 2, and a molecule of water is released.
Elimination of water. The OH (orange) and an H from the next carbon (orange) leave as water. A C=C forms.

The reaction is elimination: a small molecule, water, is removed and a C=C forms. It is the reverse of the hydration of an alkene in Topic 5.

Reagent concentrated phosphoric acid, H₃PO₄
Condition heat
Product an alkene and water
Type elimination (dehydration)
Test for the gas shake with bromine water: orange to colourless shows C=C

Concentrated phosphoric acid acts as an acid catalyst. It is used instead of concentrated sulfuric acid, which is also a strong oxidising agent and chars the organic compound.

The mechanism is not required for this reaction.

Check yourself

  1. Write the equation for the complete combustion of methanol.
  2. Write the equation for the complete combustion of butan-1-ol, C₄H₉OH.
  3. Write an equation for the incomplete combustion of propan-1-ol to carbon monoxide and water.
  4. Name the reagent and condition that change ethanol into ethene, and write the equation.
  5. Describe a test to show that the gas given off is an alkene.
  6. Explain why concentrated sulfuric acid is not used for this reaction.

Answers cover these until you have tried the questions

  1. 2 CHX3OH+3 OX2→2 COX2+4 HX2O\ce{2CH3OH + 3O2 -> 2CO2 + 4H2O}. Check oxygen: 2 + 6 = 8 on the left; 4 + 4 = 8 on the right.
  2. CX4HX9OH+6 OX2→4 COX2+5 HX2O\ce{C4H9OH + 6O2 -> 4CO2 + 5H2O}. Check oxygen: 1 + 12 = 13 on the left; 8 + 5 = 13 on the right.
  3. CX3HX7OH+3 OX2→3 CO+4 HX2O\ce{C3H7OH + 3O2 -> 3CO + 4H2O}. Check oxygen: 1 + 6 = 7 on the left; 3 + 4 = 7 on the right. (Or 2 CX3HX7OH+6 OX2→6 CO+8 HX2O\ce{2C3H7OH + 6O2 -> 6CO + 8H2O}.)
  4. Concentrated phosphoric acid and heat: CHX3CHX2OH→CHX2=CHX2+HX2O\ce{CH3CH2OH -> CH2=CH2 + H2O}.
  5. Shake the gas with bromine water. It goes from orange to colourless if the gas is an alkene.
  6. Concentrated sulfuric acid is a strong oxidising agent and also chars the organic compound, so a mixture of products forms. Phosphoric acid does not oxidise.