Aldehydes and Ketones: Properties and Reactions

Aldehydes and ketones are two families of organic compounds that both contain the carbonyl group, \( C=O \), a carbon atom double bonded to an oxygen atom. The difference between them lies entirely in what is attached to that carbon.

In an aldehyde, the carbonyl carbon is bonded to at least one hydrogen atom, giving the general formula \( RCHO \). In a ketone, the carbonyl carbon is bonded to two carbon-containing groups, giving the general formula \( RCOR' \). This small structural difference has a big effect on how each compound reacts, which is exactly why examiners like to ask you to tell them apart.

Aldehydes are named with the suffix -al, for example ethanal, \( CH_3CHO \), and propanal, \( CH_3CH_2CHO \). Because the carbonyl carbon is always carbon 1, no locant number is needed.

Ketones are named with the suffix -one, for example propanone, \( CH_3COCH_3 \), and butan-2-one, \( CH_3COCH_2CH_3 \). Here a number is often required to show where the carbonyl group sits in the chain, since it can occupy more than one position.

The carbon to oxygen double bond is polar because oxygen is more electronegative than carbon, leaving the carbon slightly positive, \( C^{\delta+} \), and the oxygen slightly negative, \( O^{\delta-} \). This permanent dipole allows dipole to dipole attractions between molecules, so aldehydes and ketones have higher boiling points than alkanes of similar relative molecular mass.

They cannot form hydrogen bonds with each other, however, because there is no hydrogen attached directly to the oxygen. This means their boiling points are still lower than those of comparable alcohols or carboxylic acids. Small aldehydes and ketones, such as ethanal and propanone, are miscible with water because the oxygen atom can hydrogen bond with water molecules, but solubility drops off quickly as the hydrocarbon chain gets longer.

The \( \delta+ \) carbon of the carbonyl group is open to attack by nucleophiles, so aldehydes and ketones typically react by nucleophilic addition across the double bond. A nucleophile attacks the carbon, the pi bond electrons move onto the oxygen, and a negatively charged intermediate forms before being protonated.

Two nucleophilic addition reactions come up repeatedly at this level:

• Reduction: sodium tetrahydridoborate, \( NaBH_4 \), supplies a hydride ion, \( H^- \), as the nucleophile. Aldehydes are reduced to primary alcohols and ketones are reduced to secondary alcohols.
• Addition of hydrogen cyanide: the cyanide ion, \( CN^- \), attacks the carbonyl carbon to form a hydroxynitrile, adding one carbon atom to the chain. This is a useful way of extending a carbon skeleton, alongside methods using Grignard reagents.

Because the carbonyl carbon becomes bonded to four different groups after addition of \( HCN \) to an unsymmetrical aldehyde or ketone, the product is often a chiral molecule formed as a racemic mixture. You can explore why this happens, and what a racemic mixture means, on the chirality and optical isomers page.

Oxidation is where aldehydes and ketones part company. Aldehydes are readily oxidised to carboxylic acids because the hydrogen attached to the carbonyl carbon can be removed. Ketones have no such hydrogen on the carbonyl carbon, so they resist oxidation under the same mild conditions. This single difference is the basis of every classic identification test in this topic.

Tollens' reagent (the silver mirror test)

Tollens' reagent contains the diamminesilver(I) ion, \( [Ag(NH_3)_2]^+ \), which acts as a mild oxidising agent. When warmed with an aldehyde, the silver ions are reduced to metallic silver, which coats the inside of the test tube as a bright silver mirror, while the aldehyde is oxidised to a carboxylic acid. Ketones give no reaction because they cannot be oxidised this way.

Fehling's solution

Fehling's solution contains blue copper(II) ions complexed in an alkaline solution. Warming it with an aldehyde reduces the copper(II) to copper(I) oxide, seen as a brick red precipitate, while the aldehyde is again oxidised to a carboxylic acid. A ketone leaves the solution blue with no precipitate forming.

Brady's reagent, a solution of 2,4-dinitrophenylhydrazine in methanol and sulfuric acid, reacts with both aldehydes and ketones. It cannot tell them apart, but it confirms that a carbonyl group is present at all, since alcohols, carboxylic acids and esters give no reaction with it.

The nitrogen atom of the hydrazine acts as the nucleophile, attacking the carbonyl carbon, and the product is an orange or yellow crystalline solid. The melting point of this solid can be compared against data tables to identify the exact aldehyde or ketone used, which is why the test is sometimes used alongside melting point analysis for a full identification.

The iodoform test identifies a specific structural feature: a methyl ketone, \( CH_3COR \), or ethanal itself. Warming the compound with iodine and sodium hydroxide produces a pale yellow precipitate of triiodomethane, \( CHI_3 \), with a distinctive antiseptic smell. Aldehydes other than ethanal, and ketones that are not methyl ketones, give a negative result.

The carboxylic acid formed from oxidising an aldehyde behaves very differently from the parent carbonyl compound; see carboxylic acids, acyl chlorides and esters for how that group reacts further.

A student is given two unlabelled bottles, one containing propanal and one containing propanone. Describe a test that would distinguish them.

Add a small sample of each to separate portions of Fehling's solution and warm gently in a water bath. Propanal, an aldehyde, will be oxidised, turning the solution from blue to a brick red precipitate of copper(I) oxide. Propanone, a ketone, has no hydrogen on its carbonyl carbon to remove, so it cannot be oxidised and the solution stays blue. The same distinction could equally be made with Tollens' reagent, looking for a silver mirror instead of a colour change.

Related lessons

• Uk A Level Chemistry