Carbonyl Compounds

This lesson covers: 

1. The carbonyl group in aldehydes and ketones
2. Synthesising aldehydes and carboxylic acids from primary alcohols
3. Synthesising ketones from secondary alcohols
4. Oxidation of aldehydes to carboxylic acids
5. Reduction of aldehydes and ketones to alcohols
6. Hydroxynitrile formation

Aldehydes and ketones contain a carbonyl group

Aldehydes and ketones are both carbonyl compounds, meaning they contain a carbon-oxygen double bond called the carbonyl group.

The key difference lies in the position of this functional group:

• Aldehydes have their carbonyl group at the end of a carbon chain. Their names end in -al.
• Some common examples of aldehydes are:

• Ketones have their carbonyl group within the carbon chain. Their names end in -one and often have a number indicating the carbon with the C=O group.
• Some common examples of ketones are:

Oxidising primary alcohols to aldehydes and carboxylic acids

Primary alcohols can be oxidised to aldehydes and further to carboxylic acids using acidified potassium dichromate(VI) (K₂Cr₂O₇) as an oxidising agent.

The oxidation reactions are:

Where:

• [O] represents the oxidising agent.
• R is an alkyl group.

The orange dichromate(VI) ion (Cr₂O₇^2-) is reduced to the green chromium(III) ion (Cr³⁺) during these oxidation reactions.

Controlling alcohol oxidation to get aldehydes or acids

You can control how far primary alcohols are oxidised by using different reaction conditions:

• Gently heating the alcohol with acidified K₂Cr₂O₇ in a distillation set-up stops at the aldehyde product.
• Refluxing the alcohol (or aldehyde) with acidified K₂Cr₂O₇ continues the oxidation to the carboxylic acid.

Oxidising secondary alcohols to ketones

Ketones are produced when secondary alcohols are oxidised with acidified K₂Cr₂O₇.

The oxidation reaction is:

• [O] represents the oxidising agent.
• R and R’ represent alkyl groups.

Ketones cannot be oxidised further by acidified K₂Cr₂O₇.

Aldehydes are easily oxidised to carboxylic acids

Aldehydes can be oxidised by oxidising agents such as acidified potassium dichromate(VI) to form carboxylic acids. The colour change observed during this oxidation is from orange to green, as the Cr(VI) in the dichromate ion is reduced to Cr(III). However, ketones do not undergo oxidation under the same conditions, so colour change is seen.

For example, methanal is oxidised to methanoic acid:

HCHO + [O] ➔ HCOOH

By contrast, propanone does not react with oxidising agents:

CH₃COCH₃ + [O] ➔ no reaction

This difference in reactivity occurs because the hydrogen atom attached to the carbonyl carbon in an aldehyde is weakly acidic, so it is easily displaced. By contrast, ketones lack that weakly acidic hydrogen, so oxidation does not occur.

Reduction of aldehydes and ketones to alcohols

Aldehydes and ketones can be reduced to primary and secondary alcohols respectively using reducing agents like sodium borohydride (NaBH₄) dissolved in a mixture of water and methanol. 

Carbonyl compounds undergo nucleophilic addition reactions because the carbonyl carbon is electrophilic due to the polarisation of the C=O double bond, which makes it susceptible to attack by nucleophiles.

The reduction reactions are:

Where:

• [H] represents the reducing agent.
• R and R’ represent alkyl groups.

Mechanism of carbonyl reduction

The reduction of carbonyl compounds, such as ethanal, to alcohols follows a nucleophilic addition mechanism:

1. The reducing agent provides hydride ions (H^-) which act as nucleophiles.
2. The hydride ion attacks the partially positive carbonyl carbon, forming a new C-H bond.
3. Protons from the solvent, such as water, are added to the oxygen, forming the hydroxyl group (-OH).

Aldehydes and ketones form hydroxynitriles

Aldehydes and ketones can react with hydrogen cyanide (HCN) in nucleophilic addition reactions to produce hydroxynitriles which conains both the cyano (-CN) and hydroxy (-OH) substituents. This reaction is useful because it increases the length of the carbon chain by 1 carbon atom.

The reactions are:

Where

• R and R’ represent alkyl groups.

Hydrogen cyanide is toxic so it is often generated in situ by mixing sodium cyanide or potassium cyanide with sulfuric acid.

Mechanism of hydroxynitrile formation

The formation of hydroxynitriles from carbonyl compounds, such as ethanal, follows a nucleophilic addition mechanism:

1. CN^- attacks the partially positive carbonyl carbon, transferring the electrons to oxygen.
2. Protonation of oxygen by water (or acid) to give the -OH group.