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Organic ChemistryReactions of Aromatic CompoundsDiah Agustina Puspitasari, ST., MTChemical Engineering DepartmentEngineering FacultyBrawijaya University
Electrophilic Aromatic SubstitutionElectrophile substitutes for a hydrogen on the benzene ring.*
Mechanism*
Example*
Bromination of BenzeneRequires a stronger electrophile than Br2.Use a strong Lewis acid catalyst, FeBr3.*
Energy Diagramfor Bromination*
Chlorination and IodinationChlorination is similar to bromination. Use AlCl3 as the Lewis acid catalyst.Iodination requires an acidic oxidizing agent, like nitric acid, which oxidizes the iodine to an iodonium ion.*
Nitration of BenzeneUse sulfuric acid with nitric acid to form the nitronium ion electrophile.*
Example*
Aromatic nitro groups are easly reduced to amino (-NH2) groups by trestment with an active metal suach as tin, zinc or iron in dilute acid.*
SulfonationSulfur trioxide, SO3, in fuming sulfuric acid is the electrophile. *
Example*
*
DesulfonationAll steps are reversible, so sulfonic acid group can be removed by heating in dilute sulfuric acid.
*
Nitration of TolueneToluene reacts 25 times faster than benzene. The methyl group is an activator.The product mix contains mostly ortho and para substituted molecules.*
Sigma ComplexIntermediate is more stable if nitration occurs at the ortho or para position.*
Energy Diagram*
Activating, O-, P-Directing SubstituentsAlkyl groups stabilize the sigma complex by induction, donating electron density through the sigma bond.Substituents with a lone pair of electrons stabilize the sigma complex by resonance.*
Friedel-Crafts AlkylationSynthesis of alkyl benzenes from alkyl halides and a Lewis acid, usually AlCl3.Reactions of alkyl halide with Lewis acid produces a carbocation which is the electrophile.Other sources of carbocations: alkenes + HF or alcohols + BF3. *
Examples ofCarbocation Formation*
*
Limitations ofFriedel-CraftsReaction fails if benzene has a substituent that is more deactivating than halogen.Carbocations rearrange. Reaction of benzene with n-propyl chloride and AlCl3 produces isopropylbenzene.The alkylbenzene product is more reactive than benzene, so poly alkylation occurs. *
Friedel-CraftsAcylationAcyl chloride is used in place of alkyl chloride.The acylium ion intermediate is resonance stabilized and does not rearrange like a carbocation.The product is a phenyl ketone that is less reactive than benzene. *
Mechanism of Acylation*