Benzene and It s derivatives

From the previous lecture .
So far we have described three classes of hydrocarbons—alkanes, alkenes, and alkynes—called aliphatic hydrocarbons. More than 150 years ago, organic chemists realized that yet another class of hydrocarbons exists, one whose properties are quite different from those of aliphatic hydrocarbons. Because some of these new hydrocarbons have pleasant odors, they were called aromatic compounds. Today we know that not all aromatic compounds share this characteristic.
Some do have pleasant odors, but some have no odor at all, and others have downright unpleasant odors. A more appropriate definition of an aromatic compound is any compound that has one or more benzene-like rings.

Benzene is an important compound in both the chemical industry and the laboratory, but it must be handled carefully. Not only is it poisonous if ingested in liquid form, but its vapor is also toxic and can be absorbed either by breathing or through the skin. Long-term inhalation can cause liver damage and cancer diseases.
Benzene, the simplest aromatic hydrocarbon, was discovered in 1825 by Michael Faraday (1791–1867). Benzene has the molecular formula C6H6 and a compound with so few hydrogens for its six carbons (compare hexane, C6H14 and cyclohexane C6H12). benzene does not behave like an alkene (the only class of unsaturated hydrocarbons known at that time). Whereas 1-hexene, for example, reacts instantly with Br2.