BENZENE AROMATIC COMPOUNDS Aliphatic Compounds Open chain acyclic and those cyclic compounds with similar chemical properties A typical reaction type of unsaturated aliphatic compounds electrophilic addition Aromatic Compounds Benzene C6H6 and other unsaturated compounds that resemble it in chemical behavior The aromatic properties of benzene are those that distinguish it from aliphatic hydrocarbons What are these properties or Benzene Properties of Aromatic Compounds Cyclic and each atom in the ring is a p center uses a p atomic orbital to form p type bonds ie sp2 or sp Ring is flat or nearly so 1 High degree of unsaturation but resistant to addition reactions generally undergo electrophilic substitution an electrophilic reagent replaces a hydrogen usually attached to the ring Unusually stable p Electrons delocalized above and below plane of ring p Electron Delocalization Benzene 6 Y 1 through Y 3 are bonding orbitals Y Each carbon starts with a p atomic orbital containing one electron These orbitals are perpendicular to the ring but parallel to each other These atomic orbitals are shown in the figure below Each p orbital interacts overlaps with two neighbors This gives rise to six p type orbitals Y through Y 6 are antibonding It turns out that Y orbital Y energy and it is higher than the energy of Y electrons in the three occupied bonding orbitals give rise to one doughnut of electron density above the ring and one below containing a total of 6 pp electrons This aromatic electronic delocalization results in considerable stabilization more than is observed in the case of conjugated aliphatic compounds 4 through 1 is the lowest energy 3 are degenerate i e they have the same 1 The 2 and Y 1 2 Y 3 Because of the p electrons benzene and other aromatic compounds frequently act as Lewis bases or nucleophiles thus they are suscepable to electrophilic attack Because of the stability associated with the delocalized electrons this feature tends to be retained in the products consequently these reactions are usually substitutions not additions E E H Sources of Aromatic Compounds Aromatic hydrocarbons may be obtained from 1 2 petroleum not rich in aromatics but has some reforming cycloalkanes from petroleum eg CH3 heat catalyst high pressure CH3 3 H2 4 3 coal tar and coal gas Coal derives from plants which have suffered partial decay and been subjected to heat and pressure plants peat lignite bituminous soft coal anthracite hard Bituminous coal is a good source of aromatic compounds Destructive distillation of it forms coal gas coal tar and coke Coal tar is rich in aromatic hydrocarbons bases eg pyridine and phenolic Ar OH compounds Ar is shorthand for an aromatic ring just as R is shorthand for an alkyl group bituminous coal heat no air non volatile volatile coke blast furnaces fuel coal gas and coal tar Rich in aromatic compounds benzene toluene xylenes etc 5
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