Combustion and oxidation of alcohols

 When 50cm3 of ethanol is mixed with 50cm3 of water in
a 100cm3 measuring cylinder, the total volume is only
98cm3. Can you explain why there is 2cm3 of liquid
 When water and ethanol are mixed, hydrogen bonds
form between the molecules. This draws the molecules
closer together than in pure ethanol or pure water,
reducing the volume
Learning objectives
• Describe the combustion of alcohols.
• Describe the oxidation of primary and secondary
• Outline the resistance to oxidation of tertiary
Combustion of alcohols
 Demonstration – combustion of ethanol
 What are the products of the combustion of ethanol in a
plentiful supply of oxygen?
 Carbon dioxide and water
 Write and balance the equation
 C2H5OH + 3O2  2CO2 + 3H2O
Oxidation of alcohols
 Using a selection of alcohols, we are going to test to see
what happens when we oxidise alcohols
1. Put 10 drops of acidified potassium dichromate into
the bottom of each test tube (one for each alcohol you
are testing)
2. Add two drops of alcohol to the test tube (label with
the alcohol name)
3. Observe the test tubes over the next 15 minutes and
note down any observations you see
Oxidation of alcohols
Oxidation of alcohols
 Primary and secondary alcohols can be oxidised using an
oxidising agent e.g. A solution containing acidified
dichromate ions (H+/Cr2O72-)
 This is made using sulfuric acid and potassium
 During the reaction, the acidified potassium dichromate
changes from orange to green
Primary alcohols
 Gentle heating
 Oxidises to produce an ALDEHYDE (functional group -CHO)
 The product must be distilled off straight away otherwise...
Primary alcohols
 Stronger heating
 Complete oxidation forming a CARBOXYLIC ACID (-COOH)
 When making a carboxylic acid, it is heated under REFLUX
before distilling off the product
 Reflux is the continual boiling and condensing of a reaction
mixture to ensure that the reaction takes place without the
flask boiling dry
Secondary alcohols
 Oxidised to produce ketones (C-CO-C)
 Cannot be oxidised further
Tertiary alcohols
 Resistant to oxidation
 The oxidising agent should remain orange in colour
Exam questions
 Can you draw structural isomers of C4H9OH that are
 What products are formed when each isomer is heated
in an excess of acidified potassium dichromate?
PRIMARY - aldehyde, then carboxylic acid
SECONDARY - ketone
TERTIARY - no reaction
 Page 153 – complete the questions
 Homework
 Exam questions on the wiki