Reactions of Alcohols

This lesson covers: 

  1. Combustion of alcohols
  2. Oxidation of alcohols
  3. Structures of aldehydes, ketones and carboxylic acids
  4. Substitution of alcohols to produce halogenoalkanes
  5. Dehydration of alcohols to produce alkenes

Complete combustion of alcohols

Alcohols combust completely when burned in an excess of oxygen, breaking all C-C and C-H bonds. This results in the production of carbon dioxide and water, along with the release of heat energy.


For example, the complete combustion of ethanol is represented by the equation:

C2H5OH(l) + 3O2(g) ➔ 2CO2(g) + 3H2O(g)

Oxidation of alcohols using potassium dichromate(VI) solution

Oxidation of alcohols can be performed using a potassium dichromate(VI) solution (K2Cr2O7), acidified with dilute sulfuric acid. The solution changes colour from orange to green as the reaction proceeds due to the reduction of dichromate(VI) ions (Cr2O72-) to chromium(III) ions (Cr3+).


The extent of oxidation depends on the alcohol's structure:

  • Primary alcohols are oxidised to aldehydes and then to carboxylic acids.
  • Secondary alcohols are oxidised to ketones only.
  • Tertiary alcohols do not oxidise under these conditions.

Controlling oxidation of primary alcohols

The oxidation of primary alcohols can proceed in two stages, initially forming an aldehyde and subsequently a carboxylic acid:

Diagram showing the oxidation of primary alcohols to aldehydes and carboxylic acids.

Here, [O] represents an oxidising agent.


To isolate the aldehyde, gently heat the alcohol in a distillation apparatus with a limited amount of potassium dichromate(VI) and distil the aldehyde as it forms to prevent further oxidation. To obtain the carboxylic acid, heat the alcohol with an excess of dichromate(VI) under reflux conditions.

Oxidising secondary alcohols

Secondary alcohols, such as propan-2-ol, can be converted into ketones by refluxing with acidified dichromate(VI).


This process does not allow for further oxidation of the ketone:

Diagram showing the oxidation of a secondary alcohol to a ketone with the use of an oxidising agent.

Structure of aldehydes, ketones and carboxylic acids

Aldehydes and ketones are characterised by the presence of a carbonyl functional group (C=O), but differ in their structure:

  • Aldehydes have a hydrogen atom and an alkyl group attached to the carbonyl carbon.
  • Ketones have two alkyl groups attached to the carbonyl carbon.


Carboxylic acids feature a carboxyl functional group (COOH) attached to an alkyl group.

Diagram showing the structure of aldehydes, ketones, and carboxylic acids with carbonyl and carboxyl groups.

Converting alcohols to halogenoalkanes

Alcohols can undergo a substitution reaction to become halogenoalkanes when reacted with sodium halides, such as NaBr, at room temperature in the presence of concentrated sulfuric acid as a catalyst. The reaction involves the generation of a hydrogen halide in situ, which then reacts with the alcohol.


The general equation for this reaction is:

RCH2OH + NaX + H2SO4 ➔ RCH2X + NaHSO4 + H2O


For example, propan-2-ol can be converted to 2-bromopropane using NaBr and concentrated H2SO4:

(CH3)2CHOH + NaBr + H2SO4 ➔ (CH3)2CHBr + NaHSO4 + H2O

Dehydrating alcohols to form alkenes

Dehydration of alcohols, an elimination reaction facilitated by a concentrated acid catalyst such as concentrated H2SO4, results in the formation of alkenes through the elimination of water:

alcohol ➔ alkene + water


For instance, dehydrating ethanol with concentrated sulfuric acid produces ethene:

CH3CH2OH ➔ CH2=CH2 + H2O