Oxidation of Alcohols

Principles of alcohol oxidation

You might already know from Redox that oxidation has a few different definitions:

1. The addition of oxygen.
2. The removal of hydrogen.
3. The loss of electrons, which leads to an increase in oxidation state.

How do these definitions apply to alcohol oxidation reactions? They all start to make sense when we look at the alcohol's alpha carbon. This is the carbon atom that is joined to the -OH hydroxyl group.

When alcohols are oxidised, the alpha carbon experiences the following changes:

1. It forms an additional bond to oxygen.
2. It loses a hydrogen atom.
3. It has an increase in oxidation state.

That seems like oxidation to us!

Oxidation of alcohols reaction

We oxidise alcohols by heating them with an oxidising agent. This is typically potassium(VI) dichromate (K₂Cr₂O₇). To speed up the reaction, we acidify the potassium dichromate using a concentrated sulphuric acid catalyst (H₂SO₄).

However, not all alcohol oxidation reactions are the same. The exact conditions and products differ depending on the alcohol's classification. Let's take a look:

• Primary alcohols (RCH₂OH) are partially oxidised to aldehydes (RCHO) using distillation. They can then be fully oxidised to carboxylic acids (RCOOH) using reflux.
• Secondary alcohols (RCH(OH)R) are oxidised to ketones (RCOR) using reflux. They can't be oxidised any further.
• Tertiary alcohols can't be oxidised at all.

Now that we know the basics of alcohol oxidation, we can explore the different reactions more closely. We'll start by learning about the oxidation of primary alcohols.

Oxidation of primary alcohols

Do you remember the difference between primary, secondary, and tertiary alcohols? It is to do with the number of R groups attached to their alpha carbon.

• In primary alcohols (shown with the symbol 1°), the alpha carbon is bonded to zero or one R group.
• In secondary alcohols (shown with the symbol 2°), the alpha carbon is bonded to two R groups.
• In tertiary alcohols (shown with the symbol 3°), the alpha carbon is bonded to three R groups.

Primary, secondary, and tertiary alcohols, shown with their alpha carbon, hydroxyl group, and R group(s) highlighted.StudySmarter Originals

Notice what this means for the alpha carbon of a primary alcohol in terms of hydrogen atoms. Primary alcohols have two hydrogen atoms attached to their alpha carbon (with the exception of methanol (CH₃OH), which has three). At the start of this article, we found out that each alcohol oxidation reaction requires the alpha carbon to lose one hydrogen atom. This means that primary alcohols can be oxidised twice - they undergo both partial and full oxidation.

Partial oxidation of primary alcohols

When we first oxidise primary alcohols, we partially oxidise them. Like all alcohol oxidation reactions, this uses potassium(VI) dichromate acidifed with concentrated sulphuric acid. However, to limit the oxidation to just partial oxidation, we use distillation and an excess of alcohol. We end up with an aldehyde (RCHO) and water (H₂O).

Here's the equation for the partial oxidation of primary alcohols. Note that we've represented the oxidising agent using [O], which is typical notation for oxidation reactions.

RCH₂OH + [O] → RCHO + H₂O

The partial oxidation of primary alcohols produces an aldehyde. The molecules' R groups and functional groups are highlighted.StudySmarter Originals

Compare the two structures above: the alcohol and the aldehyde. Overall, we swap the alcohol's -OH hydroxyl group for a C=O double bond (known as the carbonyl group) and we remove a hydrogen atom from the alpha carbon. The primary alcohol's alpha carbon gains an extra bond with oxygen and loses hydrogen. This leaves us with the characteristic -CHO aldehyde carbonyl group.

Full oxidation of primary alcohols

Primary alcohols can be oxidised twice, and looking at the structure of an aldehyde, it is easy to see how. Aldehydes still contain a hydrogen atom bonded to their alpha carbon and so they can undergo further oxidation. Once again, we use acidified potassium dichromate, but this time we use heat the mixture under reflux. The reaction produces a carboxylic acid (RCOOH). Note that no water is produced when we oxidise an aldehyde.

RCHO + [O] → RCOOH

The oxidation of aldehydes produces a carboxylic acid. The molecules' R groups and functional groups are highlighted.StudySmarter Originals

Now compare the aldehyde and the carboxylic acid. In this second oxidation reaction, we remove the aldehyde alpha carbon's remaining hydrogen atom and form an extra C-O bond in its place. The hydrogen then joins onto the other side of the C-O bond, forming an -OH hydroxyl group. Overall, the aldehyde's alpha carbon gains a bond with oxygen and loses a hydrogen atom. We're left with the -COOH carboxyl group of carboxylic acids.

It is entirely possible miss out the middle step and jump straight from a primary alcohol to a carboxylic acid by oxidising the alcohol twice in one go. Oxidising a primary alcohol in this way is known as full oxidation. We don't bother with distillation and instead simply heat the alcohol under reflux with an excess of acidified potassium(VI) dichromate. The overall reaction requires two moles of the oxidising agent for each mole of alcohol and results in a carboxylic acid and water.

RCH₂OH + 2[O] → RCOOH + H₂O

In brief, partial oxidation of primary alcohols results in aldehydes, whereas full oxidation results in carboxylic acids.

Partial and full oxidation method

You might have noticed the different conditions required for partial and full oxidation of primary alcohols. Whilst partial oxidation uses distillation and an excess of the alcohol, full oxidation requires reflux and an excess of the oxidising agent. We make these changes in order to control the extent of the oxidation reaction.

• In partial oxidation, we want to oxidise the alcohol just once and so manipulate the conditions to prevent further oxidation.
  • Using an excess of alcohol means that there isn't enough oxidising agent to oxidise the alcohol a second time.
  • Aldehydes have lower boiling points than alcohols. Using distillation means that the aldehyde evaporates as soon as it is formed, preventing any further reaction with the oxidising agent.

• In full oxidation, we want to oxidise the alcohol twice and so manipulate the conditions to ensure further oxidation.
  • Using an excess of the oxidising agent means that there is more than enough oxidising agent for a second oxidation reaction.
  • Reflux is a technique that causes any gaseous vapours to condense and fall back into the reaction vessel. Using reflux prevents the aldehyde formed in partial oxidation from evaporating and escaping the system. Instead, the aldehyde is trapped in the reaction vessel, where it can reach higher temperatures and react further with the oxidising agent.

The following diagram compares the typical set-up for the partial and full oxidation of primary alcohols. By simply changing the relative amounts of the reactants, alongside altering the reaction conditions, we can start with the same reactants and end up with two completely different products.

The set-up for partial (distillation; left) and full (reflux; right) oxidation of primary alcohols.Image credits: http://www.chembook.co.uk/

Oxidation of secondary alcohols

Secondary alcohols contain two R groups attached to the C-OH alpha carbon. This leaves them with just one hydrogen atom. As a result, secondary alcohols can only be oxidised once. We oxidise secondary alcohols by heating them under reflux with acidified potassium chromate, forming a ketone (RCOR) and water. Using an excess of the oxidising agent doesn't make a difference - ketones simply can't be oxidised any further!

Here's the equation for the reaction:

RCH(OH)R + [O] → RCOR + H₂O

The oxidation of secondary alcohols produces a ketone. Like before, the molecules' R groups and functional groups are highlighted.StudySmarter Originals

Compare the two structures: the alcohol and the ketone. We swap the alcohol's -OH hydroxyl group for a carbonyl group C=O double bond and we remove a hydrogen atom from the alpha carbon. Overall, the secondary alcohol's alpha carbon gains an extra bond with oxygen and loses hydrogen. This leaves us with the characteristic -CO- ketone carbonyl group.

Unlike aldehydes, which can be oxidised again into carboxylic acids, ketones cannot be oxidised further. This is because there are no C-H bonds left on the ketone's alpha carbon and so oxidation can't take place.

Oxidation of tertiary alcohols

Tertiary alcohols contain three R groups attached to the C-OH alpha carbon. If you refer back the diagram earlier in the article, you can see that this means that the alpha carbon isn't bonded to any hydrogen atoms - it has no C-H bonds. As a result, tertiary alcohols can't be oxidised. Heating a tertiary alcohol with acidified potassium(VI) dichromate has no effect.

Comparing oxidation of alcohols

To summarise all that we've learned in this article, we've created a table that pulls together the oxidation reactions of primary, secondary, and tertiary alcohols.

We've also made a useful diagram to help you visualise the products of alcohol oxidation reactions and their structures. The diagram highlights the molecules' R groups and their different functional groups.

The oxidation reactions of primary, secondary, and tertiary alcohols, and their organic products. The molecules' R groups and functional groups are highlighted.StudySmarter Originals

Oxidation of alcohols: Testing for aldehydes and ketones

There are a couple of useful applications for the oxidation of alcohols. We can use what we know about alcohol oxidation reactions to test for aldehydes and ketones.

Remember that aldehydes (produced by oxidising a primary alcohol) can be oxidised further, whilst ketones (produced by oxidising a secondary alcohol) can't. Many oxidising agents give a distinct colour change when they react which allows us to positively distinguish between these two families. You need to know about three oxidising agents in particular:

• We've already met potassium(VI) dichromate. It is naturally a vibrant orange colour, but when it reacts, it turns green.
• Another oxidising agent is Tollens' reagent. It is naturally colourless, but when it successfully oxidises a species it forms a silver mirror deposit.
• The third oxidising agent you could be tested on is Fehling's solution. If the blue solution forms a dark red precipitate, you know that an oxidation reaction has taken place.

The colour changes of various oxidising agents when they successfully oxidise another species.StudySmarter Originals

To distinguish between aldehydes and ketones, you simply warm them gently with one of the oxidising agents above. The following table summarises the different colour changes you'll expect to see.