Chapter 9: Aldehydes and Ketones

Aldehydes and ketones are carbonyl compounds containing the C=O functional group.

  • Aldehydes can be oxidized to form carboxylic acids by heating under reflux with potassium dichromate (oxidizing agent) and concentrated sulphuric acid
  • Aldehydes can be reduced to primary alcohols and ketones to secondary alcohols using NaBH4 as the reducing agent e.g. reduction of propanal CH3CH2CHO + 2[H] → CH3CH2CH2OH

  • Nucleophilic addition of cyanide ions can be used to extend the length of a carbon chain
  • Reactions of carbonyls with KCN and dilute acid, produce a hydroxy nitrile. Aldehydes and unsymmetrical ketones form two optical isomers, as there is an equal chance of either isomer being formed. This is a racemic mixture.

  • Cyanide-containing compounds can be toxic or irritants

Naming:

  • Aldehydes end in AL
  • Ketones end in ONE
  • Pick the longest chain of carbon atoms which includes the C=O
  • Substituent positions are based on the carbon with O attached

CH3CH2CH2CH2CH2CHO[Hexanal ]

CH3COCH2CH2CH2CH3[hexan-2-one]

Making Aldehydes and Ketones

Oxidization

Aldehydes

  • Oxidation of primary (1°) alcohols – the risk of oxidation to acids

E.g. CH3CH2OH (I) + [O] [Ethanol] → CH3CH0 (I) [ ethanal]+ H2O (I)

However, this can happen

CH3CHO (I)[Ethanal ] + [O] →CH3COOH (I) [ethanoic acid]

  • It is essential to distill off the aldehyde before it gets oxidized to the acid
  • The alcohol is dripped into a warm solution of acidified K2Cr2O7 (Cr2O72-/H+)
  • The orange Cr2O72- [dichromate (VI)] is reduced to the green Cr3­+
  • The aldehyde has a low boiling point – no hydro bonding – it distills off
  • If it didn’t distill if it would be oxidized to the equivalent carboxylic acid
  • To oxidize alcohol straight to the acid you would reflux the mixture

Ketones

  • Oxidation of secondary (2°) alcohols.

e.g.  CH3CHOHCH3 (I) + [O] → CH3COCH3 (I) + H2O (I)

            Propan-2-ol                          propanone

Identifying a Carbonyl Compound

Tollens’ Reagent

Reagent: Tollens’ reagent formed by mixing aqueous ammonia and silver nitrate. The active substance is the complex ion of [Ag (NH3 )2 ]+.

Conditions: heat gently

Reaction: aldehydes only are oxidized by Tollens’ reagent into a carboxylic acid. The silver (I) ions are reduced to silver atoms

Observation: with aldehydes, a silver mirror forms coating the inside of the test tube. Ketones result in no change.

CH3CHO + 2Ag+ + H2O → CH3COOH + 2Ag + 2H+

Fehling’s Solution

Reagent: Fehling’s solution containing blue Cu 2+ ions.

Conditions: heat gently

Reaction: aldehydes only are oxidized by Fehling’s Solution into a carboxylic acid. The copper (II) ions are reduced to copper (I) oxide.

Observation: Aldehydes: Blue Cu 2+ ions in solution change to a red precipitate of Cu2O. Ketones do not react.

 CH3CHO + 2Cu2+ + 2H2O → CH3COOH + Cu2O + 4H+

The triiodomethane (iodoform) reaction

Aldehydes and ketones that possess at least one hydrogen atom on the carbon atom adjacent to the carbonyl group to react with iodine in the presence of sodium hydroxide. The reaction results in the formation of yellow crystals of triiodomethane.

CH3CHO (I) [Ethanal]+ 3I2 (aq) + 4NaOH (aq)→ CHI3 (s) [iodoform]+ HCOONa (aq) + 3NaI (aq) + 3H2O (I)