The chemical structure of benzene was determined in 1834 by the German chemist Eilhardt Mitscherlich from the University of Berlin. Benzene was found to be a cyclic hydrocarbon with a molecular formula of C6H6, making it a highly unsaturated compound with an index of hydrogen deficiency equal to four. Benzene is rather stable and reacts preferentially by substitution of hydrogen for another group, such as a hydroxyl, rather than by addition reactions. Substituted benzene derivatives maintain their aromatic character, presumably due to the retention of their resonance distribution of electrons between the ring carbons. Benzene derivatives were originally termed aromatic hydrocarbons, due to the fragrant nature of some of these compounds. In physical appearance, benzene is a colorless liquid which has a characteristic sweet odor. Most humans can smell benzene at a threshold of 1.5-5 ppm in air and taste the compound at concentrations as low as 0.5-4.5 ppm
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