Aldehydes and Ketones

Carbonyl compounds are among the most important functional groups in organic chemistry. They contain the C=O (carbonyl) group and are divided into two main families: aldehydes and ketones. Understanding their chemistry is essential for Edexcel A-Level and underpins much of the organic synthesis you will encounter.

The Carbonyl Group

The carbonyl group consists of a carbon atom double-bonded to an oxygen atom (C=O). The key difference between aldehydes and ketones lies in what is attached to the carbonyl carbon:

In aldehydes, the carbonyl carbon is bonded to at least one hydrogen atom. The carbonyl group is always at the end of the carbon chain. The general formula is RCHO.
In ketones, the carbonyl carbon is bonded to two carbon-containing groups (alkyl or aryl). The carbonyl group is within the carbon chain. The general formula is RCOR'.

The C=O bond is polar because oxygen is more electronegative than carbon. This means the carbon atom carries a partial positive charge (δ+) and the oxygen carries a partial negative charge (δ−). This polarity is the reason carbonyl compounds undergo nucleophilic addition reactions.

Naming Aldehydes and Ketones

Aldehydes are named using the suffix -al. The longest chain must include the carbonyl carbon, and numbering always starts from the carbonyl end:

Methanal (HCHO) — one carbon
Ethanal (CH₃CHO) — two carbons
Propanal (CH₃CH₂CHO) — three carbons

Ketones are named using the suffix -one, with a number indicating the position of the carbonyl group:

Propanone (CH₃COCH₃) — the simplest ketone
Butanone (CH₃COCH₂CH₃) — four carbons
Pentan-2-one and pentan-3-one — positional isomers

Physical Properties

Carbonyl compounds cannot form hydrogen bonds with themselves (unlike alcohols), because they lack an O–H or N–H bond. However, they can form hydrogen bonds with water through the lone pairs on the oxygen atom. This means:

Small aldehydes and ketones are soluble in water
Boiling points are higher than alkanes of similar molecular mass (due to permanent dipole–dipole interactions) but lower than alcohols (which have stronger hydrogen bonding)
Solubility in water decreases as the hydrocarbon chain length increases

Nucleophilic Addition Reactions

The δ+ carbon of the carbonyl group is susceptible to attack by nucleophiles — species with a lone pair of electrons that they can donate. The general mechanism is nucleophilic addition:

The nucleophile attacks the δ+ carbonyl carbon, forming a new bond
The C=O pi bond breaks and both electrons move onto the oxygen, creating an alkoxide intermediate (with a negative charge on oxygen)
The alkoxide is protonated (gains H⁺) to form the final product

Reaction with HCN (Hydrogen Cyanide)

When an aldehyde or ketone reacts with HCN (in the presence of a trace of base such as KCN), the cyanide ion (CN⁻) acts as the nucleophile:

CN⁻ attacks the δ+ carbonyl carbon
The pi bond breaks and electrons move onto oxygen, forming an alkoxide
The alkoxide is protonated by HCN to form a hydroxynitrile (cyanohydrin)

This reaction is important because it extends the carbon chain by one carbon, which is useful in synthesis. The nitrile group can subsequently be hydrolysed to a carboxylic acid or reduced to an amine.

Safety note: HCN is extremely toxic, so in practice the reaction is carried out using KCN in acidified solution rather than HCN gas directly.

Reduction with NaBH₄ (Sodium Borohydride)

Sodium borohydride is a mild reducing agent that reduces carbonyl compounds to alcohols:

Aldehydes are reduced to primary alcohols
Ketones are reduced to secondary alcohols

The hydride ion (H⁻) from NaBH₄ acts as the nucleophile, attacking the δ+ carbonyl carbon. The mechanism is nucleophilic addition. NaBH₄ is dissolved in water or aqueous ethanol as the solvent.

Note that NaBH₄ is selective — it reduces C=O but does not reduce C=C double bonds. This selectivity makes it a valuable reagent in organic synthesis.

Oxidation of Aldehydes

Aldehydes can be oxidised to carboxylic acids. This is a key difference from ketones, which resist oxidation under normal conditions. The common oxidising agent is acidified potassium dichromate (K₂Cr₂O₇/H₂SO₄):

RCHO → RCOOH

During this reaction, the orange dichromate solution turns green (Cr³⁺ ions). Ketones show no colour change because they cannot be further oxidised without breaking a C–C bond.

Distinguishing Aldehydes from Ketones

Because aldehydes are more easily oxidised than ketones, two classic tests exploit this difference:

Tollens' Reagent (Silver Mirror Test)

Tollens' reagent contains silver(I) ions in aqueous ammonia, [Ag(NH₃)₂]⁺. When warmed with an aldehyde:

The aldehyde is oxidised to a carboxylate ion
Ag⁺ ions are reduced to metallic silver, which deposits as a silver mirror on the inside of the test tube
Ketones give no reaction with Tollens' reagent

Fehling's Solution (Copper Test)

Fehling's solution contains Cu²⁺ ions complexed with tartrate, giving a deep blue colour. When warmed with an aldehyde:

The aldehyde is oxidised to a carboxylate ion
Cu²⁺ ions are reduced to Cu⁺, forming a brick-red precipitate of copper(I) oxide (Cu₂O)
Ketones give no reaction with Fehling's solution

Both tests are positive for aldehydes and negative for ketones, providing a reliable way to distinguish between the two.

Summary of Key Reactions

Reaction	Aldehyde Product	Ketone Product
Reduction (NaBH₄)	Primary alcohol	Secondary alcohol
Oxidation (K₂Cr₂O₇)	Carboxylic acid	No reaction
HCN addition	Hydroxynitrile	Hydroxynitrile
Tollens' reagent	Silver mirror	No reaction
Fehling's solution	Brick-red precipitate	No reaction

Understanding carbonyl chemistry is fundamental to the rest of organic chemistry at A-Level. The nucleophilic addition mechanism recurs throughout the specification, and the ability to distinguish aldehydes from ketones using chemical tests is a common exam question.