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Have you ever accidently left a bottle of wine open in a cupboard, dusty and forgotten about? When you return to it, the chances are that it will be sour and acidic. The alcohol within the wine has reacted with the oxygen in the air and turned into a completely different type of molecule altogether. This is an example of the oxidation of alcohols.
You might already know from Redox that oxidation has a few different definitions:
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:
That seems like oxidation to us!
Check out the deep dive at the end of the article forbreakdownown of the alpha carbon's oxidation state in different alcohols, and how it changes in oxidation reactions.
We oxidise alcohols by heating them with an oxidising agent. This is typically potassium(VI) dichromate (Na2Cr2O7). 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:
We'll recap the definitions of primary, secondary, and tertiary alcohols in the course of this article, but if you want a reminder, head over to Alcohols for a full explanation and multiple examples.
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.
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.
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.
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
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.
Functional groups lost on you? Don't worry - we've got an article for that! Check out Functional Groups to find out all you need to know about different organic groups, their formulae, and the families they're found in.
Write an equation for the partial oxidation of ethanol. Give the conditions for the reaction and name the organic product formed.
The partial oxidation of ethanol (CH3CH2OH) results in ethanal (CH3CHO) and water. It uses distillation and an excess of the alcohol.
CH3CH2OH + [O] → CH3CHO + H2O
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
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
Write an equation for the full oxidation of ethanol. Give the conditions for the reaction and name the organic product formed.
The full oxidation of ethanol (CH3CH2OH) produces ethanoic acid (CH3COOH) and water. It uses reflux and an excess of acidified potassium(VI) dichromate.
CH3CH2OH + 2[O] → CH3COOH + H2O
In brief, partial oxidation of primary alcohols results in aldehydes, whereas full oxidation results in carboxylic acids.
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.
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.
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
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.
Write an equation for the oxidation of propan-2-ol. Give the conditions for the reaction and name the organic product formed.
The oxidation of propan-2-ol (CH3CH(OH)CH3) produces propanone (CH3COCH3) and water. It uses acidified potassium dichromate and reflux.
CH3CH(OH)CH3 + [O] → CH3COCH3 + H2O
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.
Why can't we break, for example, a C-C bond in an oxidation reaction? Well, C-C bonds are very strong and stable and so breaking them requires a lot of energy. This simply isn't favourable for oxidation reactions.
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.
|Oxidation type||Alcohol structure||Reaction conditions||Organic product||Equation|
|Primary alcohol (partial oxidation)||RCH2OH||Distillation, excess alcohol||Aldehyde (RCHO)||RCH2OH + [O] → RCHO + H2O|
|Primary alcohol (full oxidation)||RCH2OH||Reflux, excess oxidising agent||Carboxylic acid (RCOOH)||RCH2OH + 2[O] → RCOOH + H2O|
|Secondary alcohol||RCH(OH)R||Reflux||Ketone (RCOR)||RCH(OH)R + [O] → RCOR + H2O|
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 of alcohols uses a mechanism similar to the E2 mechanism you see in Alcohol Elimination Reactions. It essentially involves converting the -OH hydroxyl group into a better leaving group, which is then eliminated from the molecule. However, this mechanism is extremely complicated and you aren't expected to know it for your exams. Simply focus on learning the reactants, products, and conditions for different alcohol oxidation reactions and you'll ace that test paper!
Remember how we said at the start of the article that oxidising alcohols increases the oxidation state of the alpha carbon? Let's see if that is true by calculating the oxidation state of the alpha carbon in alcohols, aldehydes, carboxylic acids, and ketones.
You might never have calculated the oxidation state of a specific carbon atom in an organic molecule before. Here's a simple process that should get you started.
We've worked out the oxidation state of the alpha carbon atoms in different alcohols and their oxidation products in the diagram below. Remember that an R group is a shorthand for an alkyl group, and so counts as a C-C bond - it has no effect on the alpha carbon's oxidation state.
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:
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.
|Species||Observation with potassium dichromate||Oberservation with Tollens' reagent||Observation with Fehling's solution|
|Aldehyde||Green solution turns orange||Colourless solution forms silver mirror deposit||Blue solution forms dark red precipitate|
|Ketone||Solution remains green (no visible reaction)||Solution remains colourless (no visible reaction)||Solution remains blue (no visible reaction)|
Note that we can only use Tollens' reagent and Fehling's solution to oxidise aldehydes into carboxylic acids. We can't use them to oxidise primary or secondary alcohols into aldehydes or ketones respectively. Directly oxidising alcohols requires a strong oxidising agent, and Tollen's reagent and Fehling's solution are both too weak. However, acidified potassium dichromate is up for the job!
The oxidation of alcohol is a type of reaction where an alcohol loses hydrogen and gains oxygen in the presence of an oxidising agent, such as acidified potassium(VI) dichromate.
Ketones are formed by the oxidation of secondary alcohols. Tertiary alcohols cannot be oxidised.
Yes - oxidising alkanes forms alcohols.
When alcohols are oxidised, they lose a hydrogen and form an extra C-O bond. However, only primary and secondary alcohols can be oxidised. Oxidising primary alcohols forms aldehydes and carboxylic acids, whilst oxidising secondary alcohols forms ketones. Both reactions also produce water.
True or false? Oxidation is the gain of electrons.
False - oxidation is the LOSS of electrons.
Outline two other definitions of oxidation specific to organic chemistry.
Name two species we mix the alcohol with in alcohol oxidation reactions and state their purpose in the reaction.
Name the technique for oxidising ethanol into ethanal and justify its use.
Distillation. The ethanal produced evaporates as soon as it is formed and so can't be oxidised further into ethanoic acid.
Name the technique used to oxidise propan-2-ol into propanone.
What are the two steps to identifying the class of alcohols?
1. Heating the unknown solution under acidified potassium dichromate (VI).
2. Heating the unknown solution under Fehling’s solution or Tollen’s reagent (if NOT tertiary).
Name the technique used for the full oxidation of ethanol into ethanoic acid. Justify its use.
Reflux. Reflux prevents any gaseous vapours from escaping the system. This traps the ethanal formed first in the reaction vessel and allows it to be oxidised further into ethanoic acid.
Give the colour change you'd expect to see when the following molecules react with acidifed potassium(VI) dichromate.
Give the colour change you'd expect to see when the following molecules react with Tollens' reagent.
Give the colour change you'd expect to see when the following molecules react with Fehling's solution.
Acidified potassium(VI) dichromate, Tollens' reagent, and Fehling's solution can all be used to distinguish between _____.
Aldehydes and ketones.
Pentan-1-ol is oxidised into:
Give the conditions for each reaction and name the type of oxidation.
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