14.2 : Reactions of Alcohols

Question 1

What are the reactions alcohols can undergo ?

Answer 1

Combustion
Oxidation
Dehydration
Substitution

Question 2

Describe combustion of alcohols

Answer 2

They burn completely in plentiful supply of oxygen to produce carbon dioxide and water.

This is an exothermic reaction, releasing a large quantity of energy in the form of heat. As the number of carbon atoms in the alcohol chain increases the quantity of heat released per mole also increase.

Question 3

How can alcohols be distilled ? Include catalyst, result, method and any colour changes.

Answer 3

If the alcohol is primary.

Dilute sulfuric acid catalyst.

Result = aldehyde

Method :

Gently heat alcohol with acidified potassium dichromate.
Distill aldehyde out of the reaction mixture as it forms ( prevents any further reaction with oxidising agent .

Dichromate ions change colour from orange to green.

Question 4

How can primary alcohols be refluxed ? Include catalyst, result and method

Answer 4

Catalyst = dilute sulfuric acid

Result : carboxylic acid

Method :

Strongly heat alcohol under reflux, with excess acidic potassium dichromate. Heating under reflux ensures that any aldehyde formed in the reaction undergoes oxidation to the carboxylic acid.

Question 5

How can secondary alcohols be distilled ? Include catalyst, result, method and any colour changes

Answer 5

Result : ketones

Catalyst : dilute sulfuric acid

Method :

Strongly heat alcohol under reflux, with oxidising mixture.

Dichromate ions : orange to green

Question 6

Describe oxidation of tertiary alcohols

Answer 6

They do not undergo oxidation reactions.

Acidified dichromate remains orange when added to alcohol.

Question 7

Define ‘dehydration reaction’

Answer 7

Any reaction in which a water molecule is removed from the starting material.

It is a type of elimination reaction.

Question 8

Describe dehydration of alcohols

Answer 8

Product : alkene and water

Catalyst = concentrated sulfuric acid or concentrated phosphoric acid

Method :

Alcohol is heated under reflux in the presence of an acid catalyst.

Question 9

Describe substitution of alcohols

Answer 9

Product : haoalkane

Method :

When the alcohol is heated under reflux with sulfuric acid and a sodium halide, the hydrogen bromide is formed.

This HBr then reacts with the alcohol to produce the haloalkane.