Carbonyl Compounds

1. O p orbitals overlap δ+ δ- O C C Ketones and aldehydes contain the carbonyl functional group, C=O. Carbonyl Compounds Formation of the C=O carbonyl π bond π bond formed by sideways overlap of p orbitals above and below the plane of the molecule. The bonding electrons are drawn towards the more electronegative oxygen atom. Electrons drawn towards the more electronegative oxygen atom, creating a dipole in the C=O bond.

2. Physical Properties of Carbonyl Compounds Aldehydes have a lower boiling point than the comparable alcohol or carboxylic acid. They have dipole dipole forces, which are weaker than the hydrogen bonds present in alcohols and carboxylic acids. Uses of aldehydes (methanal): manufacturing plastic coating such as formica; preserving and embalming; feedstock in pharmaceuticals, perfumes and flavouring agents. Uses of ketones (propanone): solvents – like nail varnish removed, in paints and varnishes. Miscibility with Water: The polarity of carbonyl compounds is sufficient to enable the lower members of the homologous series to be completely miscible with water. Water will form hydrogen bonds to the carbonyl group. Aldehydes and ketones with more than 4 carbon atoms become increasingly immiscible in water because the hydrocarbon chain is hydrophobic – it hinders the formation of hydrogen bonds between the lone pair of electrons on the oxygen atom of carbonyl group and water molecules. . .

3. Reduction reaction of carbonyl compounds Reduction reactions of carbonyl compounds form alcohols. This reaction occurs by a nucleophilic addition mechanism + 2[H] Propanal + 2[H]  Propan-1-ol NaBH4 (sodium borohydride) in the presence of water • The lone pair of electrons from the :H- nucleophile attack the electron deficient carbon atom of the carbonyl functional group forming a dative covalent bond. • At the same time, the π bond in the C=O bond breaks to produce a negatively charged intermediate. Both π electrons are on the oxygen atom • The intermediate ion rapidly reacts with a hydrogen atom of a water molecule • The organic product formed is an alcohol.

4. Characteristic Tests for Carbonyl Compounds • Identifying a carbonyl compound • Add 2,4-dinitrophenylhydrazine (2,4-DNPH) • Bright orange-yellow crystals will form if the compound in an aldehyde or ketone • Filter the precipitate and purify by recrystallisation • Measure the melting point of dry crystals • Compare the known melting point from data bases to identify the actual carbonyl compound • 2. Aldehyde or Ketone? • Add warm Tollen’s Reagent • Only aldehydes will give a positive test result: if an aldehyde is present, a ‘silver mirror’ will form Tollen’s Reagent is ammoniacal silver nitrate. It is a mild oxidising agent containing the complex ion [Ag(NH3)2]+ . If an aldehyde is present, it will be oxidised to a carboxylic acid, and the Ag+ in the Tollen’s reagent will be reduced to Ag. This is what forms the ‘silver mirror’. Ag+ (aq) + e- Ag (s)