Benzene possesses a chemical formula of C6H6. Structurally, benzene molecules consist of six carbon atoms in a hexagonal six-membered ring with alternating single and double bonds between the carbon atoms. Cyclohexane--chemical formula C6H12--also consists of molecules with a hexagonal six-membered carbon ring, but with single bonds only. Chemists refer to certain compounds with alternating single and double bonds between carbon atoms, like benzene, as “aromatic” compounds. Organic-chemistry students learn to test for aromaticity in hydrocarbons by the ignition test (See References 2). When burnt, aromatic hydrocarbons produce a sooty, yellow flame due to incomplete combustion, whereas nonaromatic hydrocarbons tend to burn more cleanly.
Set up a propane torch or Bunsen burner and adjust the flame until it exhibits a blue-coloured cone.
Dip the tip of a metal wire or metal spatula into the liquid being tested.
Hold the wire or spatula in the tip of the flame and observe the colour of both the flame and the smoke produced by the combustion. A yellow flame with black, sooty smoke indicates benzene. A yellow flame with white smoke indicates cyclohexane.