2.9.2Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols
Reactions of Alcohols — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 52 of 53 Unit quiz
Part 1 of 5Combustion
Alcohols release energy when burned in plenty of oxygen.
Complete combustion
- An alcohol burns in excess oxygen to form carbon dioxide and water only.
Ethanol equation
- C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l) represents the complete combustion of ethanol.
Limited oxygen
- In a restricted supply of oxygen, carbon monoxide and soot may form instead, which makes the flame smoky.
Energy trend
- The energy released per mole increases along the series because longer chains contain more C–H bonds to burn.
Part 2 of 5Converting Alcohols to Halogenoalkanes
The –OH group is replaced by a halogen atom; each halogen needs different reagents.
Converting Alcohols to Halogenoalkanes
Chlorination
- Phosphorus(V) chloride reacts vigorously at room temperature: C₃H₇OH + PCl₅ → C₃H₇Cl + POCl₃ + HCl.
Chlorine test
- Steamy white fumes of HCl with PCl₅ show that an unknown liquid contains an –OH group.
Bromination
- Warm the alcohol with KBr and 50% concentrated sulfuric acid, which generates HBr: 2KBr + H₂SO₄ → K₂SO₄ + 2HBr.
Iodination
- Warm the alcohol under reflux with red phosphorus and iodine, which form PI₃: 3C₂H₅OH + PI₃ → 3C₂H₅I + H₃PO₃.
| Product | Reagent | Conditions |
|---|---|---|
| Chloroalkane | PCl₅ | Room temperature, vigorous |
| Chloroalkane (tertiary only) | Concentrated HCl | Shake at room temperature |
| Bromoalkane | KBr + 50% H₂SO₄ | Warm |
| Iodoalkane | Red P + I₂ | Reflux |
Part 3 of 5Dehydration to Alkenes
Dehydration is an elimination reaction in which water is removed.
1
Heat the alcohol with concentrated phosphoric acid, which acts as an acid catalyst for the reaction.
2
The –OH group and a hydrogen atom on the neighbouring carbon are removed from the molecule.
3
A C=C double bond forms between the two carbon atoms that lost the groups.
4
The equation for ethanol is C₂H₅OH → CH₂=CH₂ + H₂O, and the catalyst is not shown in the equation.
5
The ethene gas can be collected over water because it is only sparingly soluble in water.
Part 4 of 5Why Conditions Matter
Acid strength
- Using 50% sulfuric acid avoids oxidising bromide ions to bromine, which concentrated acid would do.
Tertiary only
- Only tertiary alcohols react with concentrated hydrochloric acid at room temperature to give chloroalkanes.
Reflux
- Reflux allows prolonged heating of the red phosphorus and iodine mixture without losing the volatile iodoalkane.
Test value
- Reaction with PCl₅ is a qualitative test for –OH, but water and carboxylic acids also give steamy fumes.
Part 5 of 5Exam Checklist & Tips
2.9.2 – Key Points for the Exam
- Dehydration Removal of a water molecule from a compound, here forming an alkene from an alcohol.
- Halogenation Replacement of a hydrogen or –OH group by a halogen atom.
- Complete combustion Burning in excess oxygen to give only carbon dioxide and water.
- Phosphorus(V) chloride The solid PCl₅, used to test for –OH and to make chloroalkanes at room temperature.
Common Mistakes
- Writing H₃PO₄ in the dehydration equation as a reactant, when it acts as a catalyst and is not used up.
- Using concentrated sulfuric acid with KBr, which oxidises bromide ions instead of making HBr cleanly.
- Forgetting that POCl₃ and HCl are both products of the PCl₅ reaction.
- Balancing combustion equations without counting the oxygen already present in the alcohol molecule.
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