Aldehydes and ketones

Aldehydes and ketones are organic compounds containing the carbonyl group >C=O (carbonyl compounds). In an aldehyde the carbonyl carbon is bonded to at least one hydrogen atom (the –CHO group), so aldehydes are easily oxidised and give the "silver mirror" test. In a ketone the carbonyl group sits between two hydrocarbon groups, so ketones resist mild oxidising agents. Methanal (its aqueous solution is formalin) is used for disinfection and to make resins, ethanal to make ethanoic acid, and propanone (acetone) is the most common solvent. This topic bridges alcohols and carboxylic acids: oxidising an alcohol gives an aldehyde, oxidising an aldehyde gives an acid.

§1. Structure and naming

The double bond of the carbonyl group is one σ-bond plus one π-bond. Oxygen is more electronegative than carbon, so the bond is strongly polarised: Cᵟ⁺=Oᵟ⁻. The positive carbon is open to attack by nucleophiles (species with a lone pair), so carbonyl compounds typically undergo nucleophilic addition (whereas alkenes undergo electrophilic addition).

Naming: under IUPAC, aldehydes take the ending -al and ketones -one.

• Methanal (formaldehyde) HCHO, ethanal (acetaldehyde) CH₃CHO, propanal C₂H₅CHO, butanal C₃H₇CHO.
• Propanone (acetone, dimethyl ketone) CH₃COCH₃, butanone (methyl ethyl ketone) CH₃COC₂H₅.

In aldehydes the carbonyl carbon is always C-1, so its position is not stated; ketones need a number: pentan-2-one, pentan-3-one.

§2. Isomerism and physical properties

Aldehydes and ketones are functional-group isomers: propanal and propanone share the formula C₃H₆O. They are also isomeric with unsaturated alcohols (for example CH₂=CH–CH₂OH) and cycloalkanols. Chain isomerism of aldehydes starts at C₄; position isomerism of the carbonyl group in ketones starts at C₅.

Physical properties: methanal is a pungent gas, and its roughly 40 % aqueous solution is called formalin; ethanal is a very volatile liquid (boiling point 21 °C); propanone is a liquid with a characteristic smell that mixes with water in all proportions. There is no hydrogen bonding between carbonyl molecules, so they boil lower than alcohols of similar mass, but, being polar, higher than alkanes. The lower members dissolve well in water because they form hydrogen bonds with water molecules.

Worked example

How many carbonyl compounds have the formula C₄H₈O?

1. Aldehydes: butanal CH₃CH₂CH₂CHO and 2-methylpropanal (CH₃)₂CHCHO.
2. Ketone: butanone CH₃COCH₂CH₃.
3. In total, 3 carbonyl compounds.

§3. Reduction and oxidation

Reduction (hydrogenation, Ni or Pt, heat): an aldehyde gives a primary alcohol, a ketone a secondary alcohol.

Oxidation is very easy for aldehydes: thanks to the hydrogen on the carbonyl carbon, –CHO becomes –COOH. These reactions are the tests for aldehydes:

In both, 1 mol of aldehyde groups corresponds to 2 mol Ag or 1 mol Cu₂O. Ketones give neither test; only strong oxidising agents attack them, breaking the carbon chain into a mixture of acids.

Another nucleophilic addition is the addition of hydrogen cyanide: CH₃CHO + HCN → CH₃CH(OH)CN (a hydroxynitrile). The nucleophile CN⁻ attacks the positive carbonyl carbon and hydrogen adds to the oxygen. This lengthens the carbon chain by one atom.

Combustion: saturated carbonyl compounds give n(CO₂) = n(H₂O): 2CH₃CHO + 5O₂ → 4CO₂ + 4H₂O.

Worked example

4.4 g of a saturated aldehyde gave 21.6 g of silver in the silver mirror test. Identify the aldehyde.

1. n(Ag) = 21.6/108 = 0.2 mol.
2. Assuming it is not methanal: n(aldehyde) = 0.2/2 = 0.1 mol, M = 4.4/0.1 = 44 → CH₃CHO.
3. Check: for methanal, n = 0.05 mol and M = 88, which does not match HCHO (30). Answer: ethanal.

§4. The special case of methanal, and condensation polymerisation

In methanal the carbonyl carbon carries two hydrogens. The first oxidation gives methanoic acid, which still contains an aldehyde group and is oxidised all the way to CO₂:

Methanal undergoes condensation polymerisation with phenol to give phenol–formaldehyde resins (Bakelite), with water eliminated. Formalin is used to preserve biological specimens, tan leather and disinfect, but methanal is toxic. Methanal and ammonia give methenamine (urotropine). Ethanal is the main raw material for ethanoic acid.

§5. Preparation

A primary alcohol gives an aldehyde, a secondary alcohol a ketone; tertiary alcohols are not oxidised under these conditions. In the laboratory the aldehyde is distilled off as it forms, otherwise it would be oxidised further to the acid.

§6. An official test item

A mixture of methanal and H₂ with an average molar mass of 12 g/mol was passed over a catalyst and methanol formed. The new mixture’s density relative to hydrogen is 8.4. Find the mole fraction of H₂ in it.

Worked example

1. 1 mol of mixture: 30x + 2(1 − x) = 12 → x = 5/14 mol HCHO, H₂ = 9/14 mol.
2. HCHO + H₂ → CH₃OH: for every y mol reacting, the total amount falls by y while the mass (12 g) is conserved.
3. New average molar mass 2·8.4 = 16.8: 12/(1 − y) = 16.8 → 1 − y = 10/14 → y = 4/14.
4. Remaining H₂: 9/14 − 4/14 = 5/14; fraction (5/14)/(10/14) = 0.5, i.e. 50 %.

Check yourself

1. What mass of silver forms when 3 g of methanal reacts with excess ammoniacal silver oxide?

2. Which alcohol forms when propanone is reduced?

3. Which reagent distinguishes an aldehyde from a ketone?

4. What mass of ethanol is needed to make 11 g of ethanal (100 % yield)?

5. Which ketone is a functional-group isomer of propanal?

6. What is formalin?

7. Heating 1.8 g of a saturated aldehyde with Cu(OH)₂ gave 3.6 g of Cu₂O. Identify the aldehyde.

8. What volume of H₂ (STP) is needed to reduce 0.5 mol of propanal completely?

Common mistakes

• Using 2 mol of Ag per mole for methanal too: methanal gives 4 mol of Ag.
• Thinking propanone gives a silver mirror.
• Writing a secondary alcohol as the reduction product of an aldehyde: aldehydes give primary, ketones secondary alcohols.
• Writing ethanol as the Kucherov product: the product is ethanal.
• Assuming carbonyl compounds undergo electrophilic addition like alkenes: they undergo nucleophilic addition.