Aldehydes and Ketones: Carbonyl Chemistry

Aldehydes and ketones are carbonyl compounds that contain a C=O group: in an aldehyde the carbonyl carbon is bonded to at least one hydrogen (R–CHO), and in a ketone it is bonded to two carbon groups (R–CO–R′); their characteristic reaction is nucleophilic addition. In brief: propanal is oxidized by Tollens’ reagent to give a silver mirror, while its isomer propanone (acetone) is not; both are reduced to alcohols by NaBH₄.

The carbonyl group is one of the most important functional groups in organic chemistry and in biology: sugars, steroid hormones, flavors such as vanillin and many drugs contain it. This article explains the structure and naming of aldehydes and ketones, their physical properties, their addition, oxidation and reduction reactions, and the laboratory tests that distinguish them.

What is the structure of the carbonyl group?

The carbonyl carbon is sp² hybridized, with three sigma bonds in a plane at about 120° and a π bond to oxygen. The C=O bond is about 122 pm long and strongly polar, since oxygen is more electronegative: the carbon has a partial positive charge and the oxygen a partial negative charge. The carbon is therefore attacked by nucleophiles, and the oxygen by electrophiles such as H⁺. The dipole moment of acetone is 2.9 D.

How are they named?

• Aldehydes: the suffix -al; the carbonyl carbon is always C1 and no locant is needed. Examples: methanal (HCHO), ethanal (CH₃CHO), propanal.
• Ketones: the suffix -one with a locant for the carbonyl carbon. Examples: propanone (CH₃COCH₃), butanone (CH₃COCH₂CH₃), pentan-2-one and pentan-3-one.
• Common names: formaldehyde, acetaldehyde, acetone, benzaldehyde.

What are their physical properties?

Aldehydes and ketones are polar, and so their boiling points are higher than those of alkanes of similar mass, but lower than those of alcohols, since they cannot form hydrogen bonds with themselves. They can accept hydrogen bonds from water, so the lowest members (up to about four carbons) are soluble in water: acetone is miscible with water. They often have pleasant smells, particularly the higher ones.

What are the main reactions?

Nucleophilic addition

A nucleophile attacks the carbonyl carbon, the π bond breaks and the oxygen becomes an alkoxide, which is then protonated. Aldehydes are more reactive than ketones, since the carbon is less hindered and less stabilized by electron-donating groups. Examples:

• Hydrogen cyanide: RCHO + HCN → RCH(OH)CN (a cyanohydrin), which lengthens the carbon chain by one carbon.
• Reduction with hydride: NaBH₄ or LiAlH₄ give alcohols: aldehydes give primary alcohols and ketones give secondary alcohols. CH₃COCH₃ → CH₃CH(OH)CH₃.
• Grignard reagents: RMgX adds to give alcohols after hydrolysis (primary from methanal, secondary from other aldehydes, tertiary from ketones).
• Alcohols: with an acid catalyst, aldehydes form hemiacetals and acetals, which is the way in which sugars form rings.

Condensation with 2,4-dinitrophenylhydrazine

Both aldehydes and ketones react with 2,4-dinitrophenylhydrazine (Brady’s reagent) to give an orange or yellow precipitate. The melting point of the crystals was used to identify the carbonyl compound.

Oxidation

Aldehydes are easily oxidized to carboxylic acids, as the carbonyl carbon has an H atom: RCHO → RCOOH. Ketones resist oxidation by mild oxidizers, and only break up under vigorous conditions.

Enolization and the aldol reaction

Hydrogen atoms on the carbon next to the carbonyl group (α-hydrogens) are slightly acidic (pK_a about 19–20 for ketones), and a base can remove one to give an enolate, which then adds to another carbonyl group. This aldol reaction forms carbon–carbon bonds and is fundamental in synthesis and in biochemistry.

How do you distinguish aldehydes and ketones?

Worked examples

Example 1: identifying an unknown carbonyl compound

A compound C₃H₆O gives an orange precipitate with 2,4-DNPH and a silver mirror with Tollens’ reagent. The first test shows a carbonyl group, and the second shows that it is an aldehyde: propanal. If the Tollens’ test were negative and the iodoform test positive, it would be propanone.

Example 2: predicting the product of reduction

NaBH₄ reduces butanone (CH₃COCH₂CH₃) to butan-2-ol, a secondary alcohol, and propanal to propan-1-ol, a primary alcohol.

Example 3: silver from Tollens’ reagent

RCHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → RCOO⁻ + 2Ag + 4NH₃ + 2H₂O. How much silver is formed from 0.50 g of ethanal (44.05 g/mol)? n = 0.50 / 44.05 = 0.01135 mol, and n(Ag) = 2 × 0.01135 = 0.02270 mol, so m(Ag) = 0.02270 × 107.87 = 2.45 g.

Where are they used?

• Methanal (formaldehyde): a 37 % aqueous solution, formalin, preserves biological specimens; large amounts go to make resins and plastics.
• Propanone (acetone): a widely used solvent (nail polish remover) and a raw material for plastics such as PMMA. It is also a ketone body, produced in the body in fasting or diabetes.
• Flavors and fragrances: benzaldehyde (almonds), cinnamaldehyde (cinnamon), vanillin (vanilla) and many ketones in perfumes.
• Biochemistry: glucose is an aldehyde in its open-chain form, fructose is a ketone, and steroid hormones such as testosterone contain ketone groups.

Where does the name aldehyde come from?

Justus von Liebig coined the name aldehyde in 1835, as a contraction of the Latin alcohol dehydrogenatus, meaning “alcohol that has lost hydrogen”, which describes the way in which they are made by the oxidation of alcohols. The silver mirror test was introduced by Bernhard Tollens in 1882, and Hermann von Fehling’s solution for sugars in 1848.

Which mistakes are common?

• Mixing up the two classes: an aldehyde has an H on the carbonyl carbon, a ketone has two carbon groups.
• Expecting a ketone to give a silver mirror: only aldehydes do.
• Forgetting that the C=O carbon of an aldehyde is C1: propanal needs no locant.
• Thinking that ketones form hydrogen bonds with themselves: they do not.
• Confusing the reduction products: aldehydes give primary alcohols, ketones secondary ones.