Oxidation Reactions of Alcohols

Alcohols

What is oxidation of an alcohol in terms of hydrogen?

Oxidation involves the loss of hydrogen from the alcohol molecule.

Which alcohols can be oxidised?

Primary and secondary alcohols.

Why can primary and secondary alcohols be oxidised?

They have a hydrogen atom attached to the carbon bonded to the –OH group.

Why are tertiary alcohols not oxidised in this way?

The carbon bonded to the –OH group has no hydrogen atom attached to it.

What functional group is formed when a primary or secondary alcohol is oxidised?

A carbonyl group, C=O.

What is a carbonyl group?

A C=O functional group.

What does a secondary alcohol form when oxidised?

A ketone.

What is the general formula of a ketone?

RCOR.

What is a ketone?

An organic compound containing a carbonyl group bonded to two carbon-containing groups.

Can a ketone normally be oxidised further under these conditions?

No.

What does a primary alcohol initially form when oxidised?

An aldehyde.

What is the general formula of an aldehyde?

RCHO.

What is an aldehyde?

An organic compound containing a terminal –CHO group, formed by partial oxidation of a primary alcohol.

What can happen if an aldehyde is oxidised further?

It forms a carboxylic acid.

What is the general formula of a carboxylic acid?

RCOOH.

What is a carboxylic acid?

An organic compound containing the –COOH functional group.

What is the oxidation sequence for a primary alcohol?

Primary alcohol → aldehyde → carboxylic acid.

What is the oxidation sequence for a secondary alcohol?

Secondary alcohol → ketone.

What happens to a tertiary alcohol with acidified potassium dichromate(VI)?

No oxidation occurs under these conditions.

What reagent is commonly used to oxidise alcohols?

Acidified potassium dichromate(VI).

What chemicals make up acidified potassium dichromate(VI)?

Potassium dichromate(VI) and dilute sulfuric acid.

How is an oxidising agent represented in simplified organic equations?

[O].

What does [O] represent?

An oxygen atom supplied by the oxidising agent.

What colour change occurs when acidified potassium dichromate(VI) oxidises an alcohol?

Orange to green.

What does the orange-to-green colour change indicate?

The dichromate(VI) ions have been reduced while the organic compound has been oxidised.

What is the equation for oxidation of propan-1-ol to propanal?

CH₃CH₂CH₂OH + [O] → CH₃CH₂CHO + H₂O.

What type of alcohol is propan-1-ol?

A primary alcohol.

What type of compound is propanal?

An aldehyde.

What is the equation for oxidation of propanal to propanoic acid?

CH₃CH₂CHO + [O] → CH₃CH₂COOH.

What is the overall oxidation of propan-1-ol to propanoic acid?

CH₃CH₂CH₂OH + 2[O] → CH₃CH₂COOH + H₂O.

What is the equation for oxidation of propan-2-ol to propanone?

CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O.

What type of alcohol is propan-2-ol?

A secondary alcohol.

What type of compound is propanone?

A ketone.

How do you oxidise a primary alcohol to an aldehyde without further oxidation?

Use distillation with addition and distil off the aldehyde as it forms.

Why must an aldehyde be removed as it forms during partial oxidation?

Aldehydes are easily oxidised further to carboxylic acids.

What practical technique is used to obtain an aldehyde from a primary alcohol?

Distillation with addition.

What is distillation with addition?

Heating a reaction mixture while adding another liquid and distilling off the product as it forms.

What is heated initially during distillation with addition?

The oxidising agent.

How is the alcohol added during distillation with addition?

It is slowly added to the heated oxidising agent.

Why does the aldehyde distil off as soon as it forms?

It has a lower boiling temperature than the alcohol from which it is made.

What happens to the aldehyde after it distils?

It condenses and is collected in a receiver.

How do you oxidise a primary alcohol completely to a carboxylic acid?

Heat it under reflux with excess acidified potassium dichromate(VI).

What practical technique is used for complete oxidation of a primary alcohol?

Heating under reflux.

What is heating under reflux?

Heating a reaction mixture with a vertical condenser so vapours condense and return to the flask.

Why is heating under reflux used for complete oxidation?

It prevents volatile reactants and products from escaping, allowing oxidation to continue.

What happens to vapours during reflux?

They rise into the condenser, cool, condense and return to the reaction flask.

How do you oxidise a secondary alcohol to a ketone?

Heat it under reflux with acidified potassium dichromate(VI).

What is the key difference between making an aldehyde and making a carboxylic acid from a primary alcohol?

For an aldehyde, distil it off as it forms; for a carboxylic acid, heat under reflux to allow complete oxidation.

Why should you remember the difference between distillation and reflux in alcohol oxidation?

The technique determines whether oxidation of a primary alcohol stops at the aldehyde or continues to the carboxylic acid.

What products can butan-1-ol form by oxidation?

Butanal and, with further oxidation, butanoic acid.

What products can pentan-2-ol form by oxidation?

Pentan-2-one.

How can you predict whether an alcohol forms an aldehyde or ketone?

Identify whether the alcohol is primary or secondary: primary forms an aldehyde, secondary forms a ketone.