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*

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DesulfonationAll steps are reversible, so sulfonic acid group can be removed by heating in dilute sulfuric acid.

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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*

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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*