A colourless hydrocarbon with a sweet smell undergoes substitution reaction. The hydrocarbon is likely to be

Analyze the question: We need a hydrocarbon (contains only carbon and hydrogen) that is colourless, has a sweet (aromatic) smell, and undergoes substitution reactions.

Concept:

• Hydrocarbons contain only C and H. Alkanols (alcohols) contain –OH, and esters contain –COO–, so neither is a hydrocarbon. This eliminates A and C.
• Methane is a hydrocarbon that undergoes substitution (e.g., with chlorine in sunlight), but it is an odourless gas.
• Benzene is a colourless liquid with a characteristic sweet, aromatic smell. Because of its stable delocalised π-electron ring, it prefers substitution to addition. Examples are nitration, halogenation (with a catalyst such as FeBr₃), and Friedel-Crafts alkylation.

Step-by-step:

1. Is it a hydrocarbon? Alkanol and ester are not, so we remove A and C.
2. Does it have a sweet smell? Methane is odourless, so we remove D.
3. Does it undergo substitution? Benzene does, e.g. C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O (conc. H₂SO₄ catalyst).

Answer: B. benzene

Exam tip: Link "sweet/aromatic smell + hydrocarbon" with benzene (arene). Remember that alkenes and alkynes favour addition, alkanes favour substitution but are odourless, and benzene is the special one that has an aromatic smell and favours substitution.