How many moles of hydrogen gas (H₂) react with benzene in the presence of a platinum catalyst?

Benzene (C₆H₆) is an aromatic compound with six carbon atoms connected in a ring structure, containing three delocalized π-bonds. These bonds provide exceptional stability due to resonance, making benzene less reactive than typical alkenes. When benzene undergoes hydrogenation, it reacts with hydrogen gas (H₂) in the presence of a metal catalyst such as platinum (Pt), palladium (Pd), or nickel (Ni).

In this process, hydrogen molecules add across the π-bonds of the benzene ring. Each π-bond requires one mole of hydrogen gas to be fully saturated. Since benzene contains three π-bonds, three moles of H₂ are needed to convert benzene into cyclohexane (C₆H₁₂). The balanced chemical equation is:
C₆H₆ + 3H₂ → C₆H₁₂

The reaction occurs at high temperature and pressure. The platinum catalyst accelerates the reaction by adsorbing hydrogen molecules onto its surface, breaking them into atomic hydrogen. These atoms then bond to the carbon atoms of benzene, replacing the delocalized π-system with single C–H and C–C bonds.

The product, cyclohexane, is a saturated hydrocarbon with no double bonds and is commonly used as an industrial solvent and as a precursor for the production of nylon. This hydrogenation reaction also demonstrates the stability of benzene’s aromatic ring; it requires more energy to hydrogenate compared to simple alkenes because the aromatic system must first lose its resonance stabilization.

Thus, under catalytic hydrogenation conditions, benzene reacts with three moles of hydrogen gas to form cyclohexane, showing a complete loss of aromaticity and formation of a fully saturated cyclic compound.