The compound which does not decolourise Baeyer’s reagent is

Baeyer’s reagent, which is a cold dilute solution of potassium permanganate (KMnO₄), is commonly used to test for unsaturation in organic compounds, particularly for the presence of double and triple bonds. When a compound contains such unsaturations, it reacts with Baeyer’s reagent, causing the purple color of KMnO₄ to fade or decolorize. This decolorization is often considered a positive test for the presence of unsaturation.

However, some compounds do not undergo this reaction with Baeyer’s reagent due to their structure. A key example of such a compound is benzene, an aromatic compound that does not have typical carbon-carbon double bonds like alkenes or alkynes. Instead, the bonding in benzene is delocalized, forming a stable resonance structure that does not readily react with Baeyer’s reagent.

In summary, the compound that does not decolorise Baeyer’s reagent is benzene. Benzene is an aromatic compound and its resonance structure prevents it from undergoing the typical reaction with Baeyer’s reagent, which is primarily used to detect unsaturation in alkenes and alkynes.

To conclude, compounds like benzene, which do not possess conventional double or triple bonds, will not show decolorization when treated with Baeyer’s reagent. This makes Baeyer’s reagent an effective tool for distinguishing between unsaturated hydrocarbons (like alkenes and alkynes) and aromatic compounds.