If you're interested in chemistry, you might have heard of oxidation before. It's when something loses electrons. But in organic chemistry, there are two other ways to define oxidation: removing hydrogen or adding oxygen. When alcohols go through oxidation, they lose hydrogen and gain oxygen on the carbon next to the -OH group. This process is called oxidation of alcohols.
The structure of the alcohol affects the outcome of their oxidation. Regardless, all types of alcohol oxidation involve the same reagents.
See if you can identify these isomers of butanol (molecular formula: C4H10O). Are they primary, secondary, or tertiary alcohols?
'B' is a primary alcohol, 'A' is a secondary alcohol, and 'C' is a tertiary alcohol. 'A' is secondary because it has two R groups, while 'B' is primary because it has only one R group. 'C's three R groups make it tertiary. It is extremely important to understand this to see why primary, secondary, and tertiary alcohols have different outcomes upon oxidation. (Do refer to Alcohols should you struggle with this.)
There are two reagents used in alcohol oxidation - potassium dichromate (VI) and dilute sulfuric acid. Below is a table showing the molecular formulae and functions of these reagents.
The reaction mixture containing the two reagents is also known as acidified potassium dichromate (VI).
How do we oxidise primary alcohols?
Primary alcohols can undergo partial and full oxidation.
Primary alcohols contain only one R group attached to the C-OH carbon. Methanol, shown below, is an exception - it is a primary alcohol without any R groups.
Let’s now simplify the butan-1-ol structure further by representing the highlighted part with an R, and compare it with another molecule on the right.
Do you notice that the molecule on the right has one less hydrogen attached to the carbon? Also, instead of an OH group, the molecule on the right now has a double-bonded oxygen? To get from our primary alcohol to the molecule on the right, we use partial oxidation. This results in a product known as an aldehyde. The oxidation process also releases a water molecule.
The general chemical equation for the process is given below:
RCH2OH + Cr2O7 or [O] --------> RCHO + H2O
Remember to include the structural formula in your answers whenever you are asked to write chemical equations. Do also remember to expand on the R group! Also, [O] represents an oxidising agent.
The aldehyde can further oxidise into a carboxylic acid. In this reaction, oxygen is added to the H bonded to the C=O carbon to form an -OH group as circled below. The new molecule now contains the C=O and OH functional groups, which combine to form the carboxylic acid functional group.
In brief, partial oxidation of primary alcohols results in aldehydes, whereas full oxidation results in carboxylic acids.
To make aldehydes through partial oxidation, we use a technique called distillation with addition. We heat up a little bit of potassium dichromate and slowly add the alcohol to it. The aldehyde produced evaporates immediately and is condensed using a condenser. This is important because we don't want the aldehyde to keep oxidising into a carboxylic acid. We collect the aldehyde in the receiver.
For complete oxidation, we use a reflux apparatus. It has a reaction flask and a condenser on top. We heat it up under reflux to contain the vapours in the flask. This ensures that all the product is fully oxidised. Without reflux, the alcohol would turn into an aldehyde and evaporate off immediately. Reflux traps the aldehyde so it can oxidise further into a carboxylic acid. Remember to draw out the apparatus and practice for exams!
Secondary alcohols contain two R groups attached to the C-OH carbon. Butan-2-ol is the example given below, where the R groups are highlighted in orange.
Let’s now simplify the above structure further by representing the circled parts with R and R’, and compare it with another molecule on the right:
Do you notice that the molecule on the right has no hydrogens attached to the carbon? Also, instead of an OH group, the molecule on the right now has a double-bonded oxygen? Oxidation of a secondary alcohol results in the product on the right, known as a ketone. The oxidation process also releases a water molecule.
Note that ketones and aldehydes are isomers of each other. Aldehydes contain the carbonyl (C=O) group at the end of the carbon chain, as highlighted below. On the other hand, ketones have the carbonyl (C=O) group in the middle of the carbon chain. Furthermore, both functional groups have different names. Aldehydes use the suffix ‘-al’ (eg: propanal) whereas ketones use the suffix ‘-one’ (eg: propanone). This is shown in the example below.
The general chemical equation for the oxidation process is stated below:
RCH(OH)R’ + Cr2O7 or [O] --------> RCOR’ + H2O
Remember to include the structural formulae in your answers whenever you are asked to write chemical equations. Do also remember to expand on the R group!
Unlike aldehydes, which can be oxidised again into carboxylic acids, ketones cannot be oxidised further. This is because there are no carbon-hydrogen bonds on the carbonyl carbon left in ketones for oxidation to take place. Propanone is shown below.
As secondary alcohols will only be fully oxidised, the technique used would be heating under reflux.
The products of oxidation are contained in the vessel as the vapour condenses and drips back into the reaction flask. This prolongs the oxidation process, ensuring all the product is fully oxidised. The oxidation of secondary alcohols does not require immediate distillation.
Tertiary alcohols contain three R groups attached to the C-OH carbon. The example below is a molecule of 2-methylpropan-2-ol with the R groups circled in blue.
Let’s now simplify this molecule by replacing the circled R groups with R, R’ and R’’.
Since there are three R groups, there are no C-H bonds in tertiary alcohols. Thus, the lack of C-H bonds meant that tertiary alcohols can’t undergo oxidation. This is similar to why the ketones from the oxidation of secondary alcohols cannot be oxidised further. The enthalpy of the remaining C-C bonds is too high for oxidation to take place - breaking the C-C bonds for oxidation requires too much energy.
Even when 2-methylpropan-2-ol is heated with acidified potassium dichromate, the alcohol will remain unchanged.
How do we identify alcohols?
An alcohol can have a primary, secondary, or tertiary structure. To classify alcohols, we use a two-step process.
This step is based on the principle that tertiary alcohols cannot be oxidised.
When heated with orange acidified potassium dichromate, a solution containing primary or secondary alcohols turns green, whereas a solution containing tertiary alcohols remains orange. The colour change is due to the dichromate ion getting reduced into Cr3+ as it acts as an oxidising agent.
If we know from step 1 that an unknown alcohol is not a tertiary alcohol, we can carry out step 2. As we learnt, ketones cannot be oxidised further whereas aldehydes can. So, we use another colour change to identify any aldehydes present in our solution. If there is a colour change, there must be an aldehyde present, hence our alcohol must have been a primary alcohol.
There are two approaches to this step. One is to heat under Fehling’s solution. The solution containing the primary alcohol would turn from blue to brick red.
Another approach is to heat the solution under Tollen’s reagent. A silver mirror is formed in the solution containing the primary alcohol.
(The detailed mechanism behind the colour changes seen in both Fehling’s solutions and Tollen’s reagent is explored at A2 level.)
Here's a table to help you understand the differences in the oxidation of primary, secondary, and tertiary alcohols:
What is the oxidation of alcohol?
The oxidation of alcohol is a type of reaction where alcohol loses hydrogens or gains oxygens in the presence of an oxidising agent such as acidified potassium dichromate.
Are ketones formed by the oxidation of tertiary alcohols?
Ketones are formed by the oxidation of secondary alcohols. Tertiary alcohols cannot be oxidised.
Does oxidation of secondary alcohols need reflux?
Yes, reflux is needed to oxidise secondary alcohols to the final product, ketone.
What happens during oxidation of alcohols?
Only primary and secondary alcohols can be oxidised. Both alcohols will lose a proton, though only primary alcohols can gain extra oxygen to form carboxylic acids.
