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 no 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. A proton from the solvent, such as water, is added to the oxygen to form 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 contains 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 (i.e. within the reaction mixture) by mixing sodium cyanide or potassium cyanide with dilute sulfuric acid. This avoids handling or storing large quantities of gaseous or liquid hydrogen cyanide.

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.

Aldehydes and unsymmetrical ketones form racemic mixtures

Aldehydes and unsymmetrical ketones contain a planar carbonyl group (C=O) which allows attacking reagents to approach from either side with equal likelihood. This leads to the formation of racemic mixtures containing both enantiomers in equal amounts.

For example, when ethanal reacts with acidified potassium cyanide:

1. The planar C=O double bond allows the CN- nucleophile to attack from either side of the plane.
2. Attacking from above or below the plane produces two different enantiomers.
3. Since both directions of attack are equally likely, the reaction yields equal amounts of both enantiomers of 2-hydroxypropanenitrile, resulting in a racemic mixture.

In contrast, symmetrical ketones do not form racemic mixtures when attacked by nucleophiles. Due to their symmetry, attack from either side of the carbonyl group produces the same product rather than two different enantiomers. For instance, when propanone reacts with acidified potassium cyanide, the product is the non-chiral molecule 2-hydroxy-2-methylpropanenitrile.