Alcohols and their reactionsEdexcel International A Level Chemistry: Revision notes
Section 1
Naming and classifying alcohols
Alcohols contain the –OH functional group and are named with the suffix -ol and a number giving the position of the OH on the longest chain containing it, e.g. propan-2-ol, 2-methylbutan-2-ol. You must be able to draw structural, displayed and skeletal formulae.
Classify by the carbon bonded to OH:
- Primary: attached to one other carbon (or none), e.g. butan-1-ol
- Secondary: attached to two other carbons, e.g. butan-2-ol
- Tertiary: attached to three other carbons, e.g. 2-methylpropan-2-ol
Section 2
Combustion
Alcohols burn in oxygen to form carbon dioxide and water (complete combustion):
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
Butan-1-ol: C₄H₉OH + 6O₂ → 4CO₂ + 5H₂O. In a limited supply of oxygen, incomplete combustion gives carbon monoxide and/or carbon (soot).
To balance, count oxygen last: oxygen atoms on the right, minus the one in the alcohol, divided by 2.
For a mole calculation, work from moles of alcohol to moles of CO₂ using the balanced equation, then multiply by the molar mass (CO₂ = 44.0 g mol⁻¹).
Section 3
Converting alcohols into halogenoalkanes
The OH group is replaced by a halogen (substitution). Mechanisms are not required.
- PCl₅ (at room temperature): R–OH + PCl₅ → R–Cl + POCl₃ + HCl. Steamy fumes of HCl are a qualitative test for the –OH group.
- KBr with 50% concentrated H₂SO₄, heated: HBr forms in situ (KBr + H₂SO₄ → KHSO₄ + HBr), then R–OH + HBr → R–Br + H₂O.
- Red phosphorus and iodine: PI₃ forms in situ and converts R–OH to R–I.
Steamy fumes show that an –OH group is present, but they do not tell you whether the alcohol is primary, secondary or tertiary.
Section 4
Dehydration to alkenes
Heating an alcohol with concentrated phosphoric acid removes water in an elimination (dehydration) reaction, forming an alkene:
CH₃CH₂OH → CH₂=CH₂ + H₂O
The OH is lost together with a hydrogen from an adjacent carbon. Where there are different adjacent carbons, a mixture of alkenes can form: butan-2-ol gives but-1-ene and but-2-ene (E and Z isomers).
Must know
- Name alcohols; classify as primary, secondary or tertiary
- Complete combustion gives CO₂ and H₂O; incomplete gives CO and/or C
- PCl₅: chloroalkane + POCl₃ + HCl (steamy fumes, test for –OH)
- KBr + 50% H₂SO₄ gives bromoalkane; red P + I₂ gives iodoalkane
- Conc. H₃PO₄: elimination to an alkene (dehydration)
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Alcohols and their reactions
- A technician is given a colourless liquid labelled as propan-2-ol, a solvent used in cleaning products, and wants to confirm that it is an alcohol and then convert it into a halogenoalkane.The technician adds phosphorus(V) chloride to a sample of the liquid. State what is observed and write the equation for the reaction with propan-2-ol.2 marks
- Butan-1-ol, C₄H₉OH, is being investigated as a possible biofuel. Relative atomic masses: C = 12.0, H = 1.0, O = 16.0.Calculate the mass of carbon dioxide formed when 3.70 g of butan-1-ol burns completely in excess oxygen.2 marks
- A chemist makes two different compounds from ethanol, one by a halogenation reaction and one by an elimination reaction, in separate experiments.Ethanol is converted into bromoethane using potassium bromide and 50% concentrated sulfuric acid. State the conditions, show how the reactive reagent is formed, and write the equation for the reaction of ethanol with that reagent.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).