The complete hydrogenation of benzene gives

Analyze the question: We need the product when benzene (C₆H₆) reacts completely with hydrogen (hydrogenation).

Concept: Benzene is an aromatic compound with a stable ring of six carbon atoms. It has three C=C double bonds, delocalised around the ring. Hydrogenation means adding hydrogen across the unsaturated bonds. Benzene is very stable, so this needs harsh conditions: a nickel catalyst, about 150–200 °C, and high pressure.

Solution:

1. Benzene has the formula C₆H₆ and is a ring of 6 carbons.
2. Complete hydrogenation saturates all the double bonds in the ring. Each of the three "double bonds" takes up one H₂, so 3 H₂ are added in total.
3. The equation is:

C₆H₆ + 3H₂ → C₆H₁₂ (Ni catalyst, heat, pressure)

4. C₆H₁₂ with all single bonds in a ring is cyclohexane.

Why the other options are wrong:

• A. Cyclohexene (C₆H₁₀) is only partial hydrogenation, because one double bond remains.
• C. Hexene and D. Hexane are open-chain compounds. Hydrogenation does not break the ring, so the ring structure remains.

Answer: B. Cyclohexane.

Exam tip: "Complete hydrogenation" of benzene means adding 3 H₂ to give cyclohexane (C₆H₁₂). The ring stays intact, so any option without "cyclo" can be eliminated immediately. Also remember that benzene with a Ni catalyst gives cyclohexane, while benzene with Cl₂ in sunlight gives benzene hexachloride (an addition reaction).