Combustion of Benzene

Like other hydrocarbons, benzene burns in oxygen to form carbon dioxide and water. For complete combustion:2C₆H₆ + 15O₂ → 12CO₂ + 6H₂Oor, per mole of benzene, C₆H₆ + 7½O₂ → 6CO₂ + 3H₂O.Observation: benzene and other arenes burn in air with a very smoky (sooty) yellow flame. This is because benzene has a high carbon to hydrogen ratio: 1 : 1, compared with about 1 : 2 for alkanes such as hexane, C₆H₁₄. There is not enough oxygen in the air around the flame to burn all the carbon, so combustion is incomplete. Unburnt carbon particles (soot) are released, together with carbon monoxide.C₆H₆ + 1½O₂ → 6C + 3H₂OA very smoky flame is a simple sign that a compound is aromatic: aliphatic compounds with the same number of carbon atoms, such as cyclohexane, burn with a much cleaner flame.Worked example: 3.90 g of benzene (M_r = 78.0) burns completely. Amount of benzene = 3.90 ÷ 78.0 = 0.0500 mol. From the equation, 1 mol of benzene gives 6 mol of CO₂, so 0.300 mol of CO₂ forms. The oxygen needed is 0.0500 × 7.5 = 0.375 mol.