Topic 17.2 Carbonyl compounds

• A carbon atom joined by a double bond to an oxygen atom. • -C=O • Carbonyl compounds can be classed as either aldehydes or ketones.

• A hydrogen atom is bonded to the carbonyl group. • General formula: RCHO • =O at the end of carbon chain • Aledehydes are oxidised further to produce carboxylic acids.

• There is only hydrocarbon groups joined to the carbonyl group. • Cannot be further oxidised as they do not have hydrogen atom attached to their carbonyl.

• Contain -C=O group. • Polar bond. • Electron density is greater near the δ- O atom.

• Distinctive smells. • Boiling temperature are between those of alkanes (low) and alcohol (high). • Boiling points increase with chain length. • No hydrogen bonding but permanent dipole dipole attraction.

• Smaller aldehydes and ketones are soluble in water - they can form hydrogen bonds with water. • Solubility decreases as chain length increases- hydrocarbon part of the molecules become more significant.

Oxidised: Primary alcohol —> aldehyde —> carboxylic acid

Oxidised: Secondary alcohol —> ketone —> X

Tertiary alcohol —> X

Aldehyde: brick red, precipitate solid Ketone: stays blue, no visible change

Aldehyde: silver mirror, black precipitate Ketone: stays colourless, no visible change

Aldehydes: decolourises Ketones: stays purple, no visible change

Aldehyde: colour change, orange to green Ketone: stays orange, no visible change

Propanal: green Propanone: orange (no change)

Propanal: light blue Propanone: dark blue

Propanal: white precipitate Propanone: colourless

Aldehydes and ketones can be reduced to alcohols using lithium tetrahydridoaluminate (LiAlH4) dissolved in dry ether. -Reducing agent represented by [H]

-Aldehydes can be oxidised to form carboxylic acids -Ketones are not easily oxidised (assume they aren’t) -Oxidising agents shown as [O]

• Triidomethane reaction. • Carbonyl compound added to an alkaline solution of iodine. • Mixture warmed then cooled. • Yellow precipitate forms - this is a positive result. • Test for CH3CO group (found in ethanal and all methyl ketones).

-Aldehydes and ketones react with HCN in an alkaline solution of KCN -These are addition reactions where a hydrogen atom joins to the oxygen in the carbonyl group and the cyanide group attaches to the carbon of the carbonyl group

OH group- hydroxy CN group- nitrile The products are known as hydroxynitriles

Step 1: nucleophilic attack by a cyanide ion on the δ+ carbon atom of the carbonyl group.

Step 2: the intermediate reacts with a hydrogen cyanide molecule.

• As C=O is planar there is an equal chance of the CN- ion attacking from each side of the plane. • Even though the product contains a chiral centre, the product is not optically active as there are equal amounts of each enantiomer - a racemic mixture.