Aldehydes and Ketones

Key Terminology
Term Definition
Aldehydes Organic compound with the group CHO, formed by oxidation of 1° alcohols
Ketones Organic compound with the group CO, formed by oxidation of 2° alcohols
Aldehydes and ketones are both carbonyl compounds, containing a C=O bond. Aldehydes can be easily
oxidised to carboxylic acids, whilst ketones cannot as a C-C bond must be broken. Strong oxidising
agents shorted the chain and produce O2 and H2O.
Aldehydes cause acidified potassium dichromate to turn from orange to green. Aldehydes cause blue
Fehling’s Solution to form a brick red precipitate, with the reaction 2CuO + 2e- + 2H+ → Cu2O + H2O
occurring. Aldehydes also cause Tollen’s Reagent to form a silver mirror, with the reaction [Ag(NH3)2]
+ e- → Ag + 2NH3 occurring. Ketone causes no visible change with all reactants.
Aldehydes and ketones can be reduced to alcohols using NaBH4 in aqueous solution. These reduction
reactions are examples of nucleophilic addition. The hydride nucleophile is attracted to the partially
positive charge of the carbon in the C=O bond. RCHO + 2[H] → RCH2OH
Aldehydes and ketones undergo nucleophilic addition, as they are unsaturated with a strongly polar
δ+=Oδ- bond. KCN in aqueous alcohol is used as a source of cyanide ions, followed by addition of dilute
HCl(aq). The product is a hydroxynitrile; this molecule has two reactive groups and therefore this is a
useful step in synthesis, particularly as the carbon chain length has increased. A racemic mixture of the
two enantiomers is produced as there is equal chance of nucleophilic attack above and below the planar
carbonyl group.
Potassium cyanide releases hydrogen cyanide gas, a highly toxic chemical that interferes with the body’s
ability to use oxygen. HCN is very difficult to contain. KCN itself can cause nausea or sores due to its
ability to change red blood cell count.