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3.3.8 Aldehydes and Ketones (Nucleophilic addition reactions: (carbonyl…
3.3.8 Aldehydes and Ketones
Aldehydes
functional group: RCHO
suffix -al
no numbers as the aldehyde group is always at the end of the chain
readily oxidised to carboxylic acids by acidified potassium dichromate, K₂Cr₂O₇/H⁺ (and by Fehling's and Tollens)
Ketones
functional group: RCOR'
suffix -one
need numbering; not propanone or butanone as the carbonyl group can only be in one position
cannot be oxidised easily as a C-C bond must be broken
Chemical test:
Fehling's solution: warm solution with Fehling's solution. If brick red precipitate of copper(I) oxide is produced, it is an aldehyde
ketones give no reaction; Fehling's solution remains blue
Tollens reagent: warm solution with Tollens reagent. If a silver mirror forms on the inside of the test tube as the Ag⁺ is reduced to metallic silver, it is an aldehyde
ketones give no reaction
Reduction of aldehydes and ketones:
aldehydes are reduced to primary alcohols
ketones are reduced to secondary alcohols
reagent: NaBH₄ (aq) [H]
nucleophile: H⁻
nucleophilic addition reactions
Nucleophilic addition reactions:
carbonyl compounds with KCN followed by dilute acid, to produce hydroxynitriles
aldehydes and unsymmetrical ketones form mixtures of enantiomers when reacting with KCN followed by dilute acid
CN⁻ ions are highly toxic
important in organic synthesis as it increases the length of the carbon chain by one
racemic mixture of two enantiomers produced as the :CN⁻ may attack from above or below the flat C=O group
Physical properties:
carbonyl group is strongly polar Cδ+=Oδ-; permanent dipole-dipole forces between molecules
boiling point of aldehydes/ketones is higher than alkanes of similar Mr due to dipole-dipole forces, but not as high as alcohols as they can form hydrogen bonds
shorter chain aldehydes and ketones are fully soluble in water as hydrogen bonds can form between oxygen of carbonyl group water; increase carbon chain = less soluble
Sammer Sheikh