- Question Answer
- Explain the aldehydes and ketones which possess the carbonyl functional group, they undergo similar oxidation reactions
. Explain the aldehydes and ketones which possess the carbonyl functional group, they undergo similar oxidation reactions
Aldehydes are easily oxidised to carboxylic acids but ketones are not. Aldehydes are oxidised not only by the same reagents, which oxidizes primary and secondary alcohols (like acidified KMnO4, K2Cr2O7) but also by mild oxidising agents like Tollen’s reagent, Fehling’s solution and Benedict’s solution. Aldehydes are very easily oxidised and thus are very powerful reducing agents.
Tollen’s reagent contains diamminesilver(I) ion, which is obtained through ammoniacal AgNO3 solution. Tollen’s reagent oxidises aldehydes to acid salt and they reduces to free silver in the form of silver mirror
Tollen’s reagent is useful in differentiating aldehydes from ketones because ketones do not react with them. Tollen’s reagent is a mild and selective oxidising agent, attacking only aldehydic group, keeping other groups untouched. Unsaturated aldehydes can be converted to unsaturated acid using Tollen’s reagent.
Aldehydes restore the magenta colour of the Schiff’s reagent (rosaniline hydrochloride is dissolved in H2O and SO2 is passed till the magenta colour is decolourised). Ketones do not restore the colour of Schiff’s reagent except acetone, which restores the colour very slowly.
Oxidation of ketones requires breaking of carbon-carbon bonds, which requires vigorous conditions. Cleavage involves the double bond of the enol form and wherever the structure permits, occurs on both sides of carbonyl group. Thus, in general, ketones on oxidation give a mixture of carboxylic acids.