KMnO 4 tests for the presence of alkenes or alkynes. The brownish color of bromine is discharged when the bromine adds to the p bond and the purple color of KMnO 4 is discharged when hydroxyls are added to the p bond. The reaction mechanism follows the typical pattern for nucleophilic addition to a carbonyl group but since nitrogen is trivalent a stable carbon-nitrogen double bond can form by elimination of water. Notice that the reaction is faster when the carbonyl oxygen is protonated but notice also that there can be no reaction if the nitrogen is protonated so the proton concentration is critical. The Tollens test distinguishes aldehydes from ketones.
Aldehydes produce a silver mirror with the reagent.
The orange color of chromic acid is discharged by aldehydes and 1 o and 2 o alcohols. The idea of false positive can be illustrated with the iodoform test. Methyl carbinols can be oxidized by iodine to to methyl ketones and thus masquerade as methyl ketones. Resonance, of course, explains it. The nucleophilic a -carbon can be halogenated.