Aldehydes and ketonesEdexcel A-Level Chemistry: Revision notes

Aldehydes and ketones both contain the carbonyl group, C=O. In an aldehyde the carbonyl carbon is at the end of the chain, bonded to at least one hydrogen: –CHO, named with the suffix -al (propanal, CH₃CH₂CHO). In a ketone the carbonyl carbon is inside the chain, bonded to two carbon atoms: R–CO–R′, named with -one and a position number where needed (pentan-2-one, CH₃COCH₂CH₂CH₃).

Oxygen is more electronegative than carbon, so the C=O bond is polar: the carbon is δ+ and the oxygen δ−. The δ+ carbon is the site attacked by nucleophiles. Propanal and propanone are functional group isomers (both C₃H₆O).

Aldehydes and ketones have no hydrogen atom bonded to oxygen, so their molecules cannot form hydrogen bonds with each other. Their molecules attract through permanent dipole–dipole forces (from the polar C=O) and London forces. Their boiling temperatures are therefore higher than those of alkanes of similar relative molecular mass but lower than those of alcohols, which hydrogen bond.

The oxygen has lone pairs that can accept a hydrogen bond from water. Short-chain carbonyl compounds such as ethanal and propanone therefore mix with water. Solubility falls as the hydrocarbon chain lengthens, because the non-polar chain cannot hydrogen bond with water.

Aldehydes are easily oxidised to carboxylic acids; ketones are not. This gives three tests:

• Fehling's (or Benedict's) solution: warm with an aldehyde and the blue copper(II) solution gives a brick-red precipitate of copper(I) oxide. Ketones: stays blue.
• Tollens' reagent (ammoniacal silver nitrate, containing [Ag(NH₃)₂]⁺): warm with an aldehyde and a silver mirror forms as Ag⁺ is reduced to Ag. Ketones: no change.
• Acidified potassium dichromate(VI): warm with an aldehyde and the solution goes from orange to green as Cr₂O₇²⁻ is reduced to Cr³⁺. Ketones: stays orange.

Using [O] for the oxidising agent: RCHO + [O] → RCOOH, for example CH₃CHO + [O] → CH₃COOH.

Lithium tetrahydridoaluminate, LiAlH₄, in dry ether is a reducing agent that supplies hydride ions, H⁻, acting as a nucleophile. Using [H] for the reducing agent:

• aldehyde → primary alcohol: CH₃CH₂CHO + 2[H] → CH₃CH₂CH₂OH (propan-1-ol)
• ketone → secondary alcohol: CH₃COCH₃ + 2[H] → CH₃CH(OH)CH₃ (propan-2-ol)

The ether must be dry because LiAlH₄ reacts violently with water. Dilute acid is added afterwards to release the alcohol.

Carbonyl compounds react with hydrogen cyanide, HCN, with a little KCN, to form hydroxynitriles. KCN provides the cyanide ion CN⁻, because HCN alone is a weak acid and gives too few CN⁻ ions.

Mechanism:

1. The lone pair on the carbon of CN⁻ attacks the δ+ carbonyl carbon (curly arrow from the lone pair to carbon).
2. The C=O π bond breaks (curly arrow to oxygen), giving an intermediate with a negative oxygen.
3. O⁻ removes H⁺ from HCN (or water) to give the hydroxynitrile; CN⁻ is regenerated.

Example: CH₃CHO + HCN → CH₃CH(OH)CN, 2-hydroxypropanenitrile. This is nucleophilic addition. The reaction also adds a carbon atom to the chain.

Optical activity: the carbonyl group is planar, so CN⁻ attacks from either side equally. For ethanal the product has a chiral centre, so equal amounts of two enantiomers form: a racemic mixture, which does not rotate plane-polarised light. Propanone gives (CH₃)₂C(OH)CN, which has no chiral centre.

2,4-dinitrophenylhydrazine (2,4-DNPH) reacts with the C=O group of both aldehydes and ketones to give an orange precipitate. It tests for a carbonyl group only; it does not distinguish aldehydes from ketones. To identify the compound, filter and recrystallise the precipitate, measure its melting temperature and compare it with data-book values for known derivatives.

Iodine in the presence of alkali (I₂ with NaOH(aq)) gives a pale yellow precipitate of triiodomethane, CHI₃, with compounds containing the CH₃CO– group (ethanal and methyl ketones such as propanone and butanone). Propanal and pentan-3-one do not react.