(a) Oxidation reactions of aldehydes and ketones. Aldehydes can be easily oxidised to carboxylic acids on treatment with common oxidising agents like `KMnO_(4)` and `K_(2)Cr_(2)O_(7)`. The carboxylic acids formed contain the same number of carbon atoms as the aldehyde.
`R-CHO overset((O))to RCOOH`
e.g. `CH_(3)CHO overset((O))to underset("Ethanoic acid")(CH_(3)COOH)`
Ketones are not easily oxidised. However, under drastic conditions, cleavage of carbon-carbon bond takes place giving a mixture of carboxylic acids having lesser number of carbon atoms than the original ketones. For examples,
or `underset(("Cleavage of" C_(1)-C_(2) "bond"))(RCOOH+R.CH_(2)COOH)` or `underset(("Cleavage of" C_(2)-C_(1) "bond"))(RCH_(2)COOH+R.COOH)`
The oxidation reactions are very important and can be used to distinguish aldehydes from ketones by the following reagents :
(i) Tollen.s reagent. It is ammoniacal silver nitrate solution. When heated with Tollen.s reagent, aldehydes reduce silver ions to metallic silver and a bright silver mirror is produced on the inner side of the test tube.
`underset("Acetaldehyde")(CH_(3)CHO) + underset("Tollen.s reagent")(2[Ag(NH_(3))_(2)]^(+)) + 3OH^(-) to CH_(3)COO^(-) + underset("Silver mirror")(2 Ag)+ 4NH_(3)+ 2H_(2)O`
Ketones, however, do not give this test.
(ii) Fehling.s solution. Fehling solution is alkaline copper (II) ions complexed with sodium potassium tartarate. When an aldehyde is heated with Fehling solution, a red precipitate of cuprous oxide (`Cu_(2)O`) is formed.
`underset("Aldehyde")(R-overset(O)overset(||)C-H)+2Cu^(2+)+5OH^(-) to RCOO^(-) + underset("(Red ppt.) ")(Cu_(2)O)+ 3H_(2)O`
e.g. `underset("Acetaldehyde")(CH_(3) -overset(O)overset(||)C-H) + 2Cu^(2+) + 5OH ^(-) to CH_(3)COO^(-)+ underset("(Red ppt.) ")(Cu_(2)O) + 3H_(2)O`
Ketones do not give this test.
(b) Reduction. Aldehydes are reduced to primary alcohols and ketones to secondary alcohols by catalytic hydrogenation or with nascent hydrogen or with reducing agents such as lithium aluminium hydride (`LiAlH_(4)`). For example,
`underset("Acetaldehyde")(CH_(3)CHO) overset(LiAlH_(4))to underset("Ethyl alcohol" (1^(@))(CH_(3)CH_(2)OH)`
`underset("Acetone")(CH_(3)COCH_(3))+H_(2) overset(LiAlH_(4))to underset("Isopropyl alcohol" (2^(@))(CH_(3)CHOHCH_(3))`
Aldehydes and ketones are reduced to corresponding hydrocarbons by treatment with zinc amalgam and hydrochloric acid. This reaction is known as Clemmensen reduction.
For example
