THE COVALENT STRUCTURE OF BENZENE

We will often show the structure of benzene as a Lewis structure with a hexagon that contains three double bonds. However, the double bonds in the Lewis structure are misleading because the chemistry of benzene does not in the least resemble a triene. For example, benzene does not react with bromine, HBr, or aqueous acid, and does not react with the powerful oxidizing agent potassium permanganate. In short, benzene is remarkably unreactive. 

The low reactivity of benzene contradicts what we know about unsaturated compounds. Benzene does not react with most reagents that attack n bonds to form addition products. That is, it does not behave like 1,3,5-cyclohexatriene. Benzene does react with bromine to give a substitution product in which a bromine atom replaces a hydrogen atom, but the reaction requires iron(III) bromide as a catalyst. Only one compound forms, CgHjBr. 

Converting benzene to bromobenzene gives only one product. Therefore, all six positions of the ring must be equivalent. 

Converting benzene to dibromobenzene gives three isomers. 

Although it might seem obvious to us today that benzene consists of a single ring with six identical carbon atoms and therefore six identical hydrogen atoms, this was by no means obvious to chemists at the time Kekule made his original proposal. 

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