Double bond additions carbon atom-aromatic compound reactivity

The delocalization of the Jt-electrons is energetically favorable, and this affects the reactivity of aromatic compounds There is a tendency towards restoring aromaticity. This is why aromatic compounds, in contrast to regular alkenes (linear chains of carbon atoms containing at least one double bond), do not easily undergo addition reactions, whereby a double bond is replaced by two single bonds. Aromatic compounds show a preference for substitution reactions, which means that atoms are replaced. 

The first so-called aromatic compounds to be studied seriously, such as vanillin (derived from vanilla), had two obvious properties. They had a sweet smell and were remarkably stable. This last property was the reef on which many of the early theories of chemical bonding foundered. Consider benzene. Kekule knew that its molecular formula was C H. The only way he could rationalise this formula with the known properties of benzene, was to imagine the six carbon atoms joined in a ring and connected by three alternate double bonds. This is where the trouble started because double bonds are supposed to confer reactivity on an organic molecule benzene is stable. Double bonds can readily be added to for example, they will undergo fast reactions with bromine and sulphuric acid to give simple "addition" compounds. The reagents simply "add" across the double bond. 

Owing to the convex shape of the wall of Cgo, all the 30 double bonds, located exocyclic to the pentagons, are strained and thus more reactive than formal double bonds in aromatic compounds. Because the electron density is much higher on the [6,6] ring junctions than on the [5,6] jnnctions, most of the chemical reactions of fullerenes tend to occur across these sites. Cycloaddition reactions (widely applied for functionalization of fullerenes), as well as additions of nucleophiles and free radicals, provoke a hybridization change in the carbon atoms involved from a trigonal sp to a tetrahedral sp. This release of the double bond strain is the driving force of such reactions. 

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