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*Revision Materials and Past Papers* 2.1.1 Atomic structure and isotopes 2.1.2 Compounds, formulae and equations 2.1.3 Amount of substance 2.1.4 Acids 2.1.5 Redox 2.2.1 Electron structure 2.2.2 Bonding and structure 3.1.1 Periodicity 3.1.2 Group 2 3.1.3 The halogens 3.1.4 Qualitative analysis 3.2.1 Enthalpy 3.2.2 Reaction Rates 3.2.3 Chemical equilibrium 4.1 Basic concepts and hydrocarbons 4.1.2 Alkanes 4.1.3 Alkenes 4.2.1 Alcohols 4.2.2 Haloalkanes 4.2.3 Organic synthesis 4.2.4 Analytical techniques 5.1.1 How fast? 5.1.2 How far? 5.1.3 Acids, bases and buffers 5.2.1 Lattice enthalpy 5.2.2 Enthalpy and entropy 5.2.3 Redox and electrode potentials 5.3.1 Transition elements 5.3.2 Qualitative analysis 6.1.1 Aromatic compounds 6.1.2 Carbonyl compounds 6.1.3 Carboxylic acids and esters 6.2.1 Amines 6.2.2 Amino acids, amides and chirality 6.2.3 Polyesters and polyamides 6.2.4 Carbon–carbon bond formation 6.2.5 Organic synthesis 6.3.1 Chromatography and qualitative analysis 6.3.2 Spectroscopy Required Practicals

4.2.1 Alcohols

Properties of alcohols Reactions of alcohols

Reactions of Alcohols

Specification Reference 4.2.1 (b)–(e)

Quick Notes

  • Alcohols undergo complete combustion in oxygen to form CO2 and H2O. Reaction is exothermic and used for fuel.
    • Alcohols can be oxidised with acidified dichromate (K2Cr2O7/H2SO4):
      • Primary alcohols: to aldehyde (distillation), to carboxylic acid (reflux)
      • Secondary alcohols: to ketone (heat)
      • Tertiary alcohols: do not oxidise
  • We use [O] to represent the oxidising agent in equations.
  • Alcohols undergo elimination (dehydration) to form alkenes.
    • Conditions: concentrated H3PO4 or H2SO4, heat
  • Alcohols react with halide ions in the presence of acid (e.g. NaBr/H2SO4) to form halogenoalkanes.

Full Notes

Combustion of Alcohols

Alcohols burn completely in oxygen to form carbon dioxide and water, releasing heat.

General equation: CnH2n+1OH + (3n/2)O2 → nCO2 + (n+1)H2O

This reaction is exothermic, making alcohols useful as fuels.

Example (ethanol): C2H5OH + 3O2 → 2CO2 + 3H2O

Oxidation of Alcohols

Primary and secondary alcohols can be oxidised using an oxidising agent, represented by [O] in reactions.

The oxidising agent is usually acidified potassium dichromate(VI), K2Cr2O7/H2SO4. It changes from orange to green when reduced.

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For organic chemistry, oxidation often means a carbon gains a bond to a more electronegative element (usually oxygen). We use an oxidising agent to supply oxygen and show it as [O] in equations. Common oxidising agents include acidified K2Cr2O7 and acidified KMnO4.

Primary alcohols (1°)

Primary alcohols oxidise first to aldehydes, then to carboxylic acids with heat and acidified dichromate.

OCR (A) A-Level Chemistry overview of primary alcohol oxidation pathway to aldehyde and carboxylic acid.

1. To aldehydes (distillation) — remove product as it forms to prevent further oxidation.

OCR (A) A-Level Chemistry apparatus for distillation to obtain aldehyde from primary alcohol oxidation.

Distillation is used because aldehydes have low boiling points and leave the reaction mixture as vapour.

For Example:CH3CH2OH + [O] → CH3CHO + H2O   (Ethanol → Ethanal)

2. To carboxylic acids (reflux) — heat under reflux to allow further oxidation.

OCR (A) A-Level Chemistry apparatus for reflux to oxidise aldehyde to carboxylic acid.

Reflux continually condenses vapours back into the flask so the aldehyde can be further oxidised.

For Example:CH3CHO + [O] → CH3COOH   (Ethanal → Ethanoic acid)

Overall from ethanol: CH3CH2OH + 2[O] → CH3COOH + H2O

Secondary alcohols (2°)

Secondary alcohols oxidise to ketones when heated with acidified potassium dichromate(VI).

OCR (A) A-Level Chemistry oxidation of a secondary alcohol to a ketone using acidified dichromate.

Observation: Orange Cr2O72− turns green (Cr3+ formed).

For Example:CH3CH(OH)CH3 + [O] → CH3COCH3 + H2O   (Propan-2-ol → Propanone)

Tertiary alcohols (3°)

Tertiary alcohols do not undergo oxidation under normal conditions because there is no hydrogen on the carbon bearing the OH group.

Elimination to Form Alkenes

Alcohols can lose water (elimination) to form alkenes — a dehydration reaction.

OCR (A) A-Level Chemistry dehydration of alcohols to alkenes under acidic conditions and heat.

General reaction: CnH2n+1OH → CnH2n + H2O

For Example:CH3CH2OH → CH2=CH2 + H2O

Note that different alkenes may form depending on the position of the OH group in the alcohol.

Substitution to Form Haloalkanes

Alcohols undergo nucleophilic substitution with halide ions in the presence of acid to form haloalkanes.

OCR (A) A-Level Chemistry preparation of bromoalkanes from alcohols using NaBr and H2SO4.

This is a key method of preparing haloalkanes from alcohols.

Summary