Chapter 12 - Chapter 12 Reactions of Arenes I II III IV V...

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Chapter 12 - Reactions of Arenes I. Reaction of Arenes with electrophiles a. Substitution occurs rather than the addition as seen in alkenes b. Ar stands for Aryl group c. Electrophilic portion of the reagent replaces one of the Hs on the ring II. Electrophilic Aromatic Substitution a. The method by which substituted derivatives of benzene are prepared. b. Mechanism i. Electrophile accepts an electron pair from the pi system of benzene to form a carbocation. 1. Product is arenium ion or cyclohexadienyl cation 2. Also known as σ -complex 3. It is an allylic carbocation that can be stabilized by the electron delocalization in the resonance structures ii.Cyclohexadienyl cation rapidly loses a proton, restoring its aromaticity and giving the electrophilic aromatic substituted ring c. The electrophile must be very reactive in order to overcome the activation energy required i. Some electrophiles (Bromine) require a catalyst to begin the reaction III. Nitration of Benzene a. Electrophile is nitronium ion (::O=N + =O::) b. Mechanism i. Benzene reacts with NO 2 + to make cyclohexadienyl cation intermediate. RATE DETERMINING ii.Cyclohexadienyl cation loses a proton, restoring aromaticity c. Reaction must occur with Nitric Acid (NO 2 OH) to make the NO 2 + and Sulfuric Acid (H 2 SO 4 ) to react with Nitric Acid to mke the NO 2 + . IV. Sulfonation of Benzene a. Benzene reacts with sulfuric acid (H 2 SO 4 ) to produce bezenesulfonic acid (Ar-SO 2 OH) b. Benzene reacts with sulfur trioxide (SO 3 ) in SO 4 so make benzenesulfocinc acid. V. Halogenation of Benzene a. Bromination of benzene
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i. Br 2 is added to benzene in the presence of metallic iron and heat 1. Metalic Iron reacts rapidly with Br 2 to form iron(III) bromide (FeBr 3 ) 2. FeBr 3 is the actual catalyst in the reaction 3. The reaction of BR 2 and FeBr 3 creates a weak Lewis acid, which makes bromine more electrophilic and allows it to react with the benzene b. Chlorination i. Same reaction but uses iron(III) chloride rather than bromide as the catalyst VI. Friedel-Crafts Alkylation of Benzene a. Alkyl Halides react with benzene in the presence of aluminum chloride (AlCl 3 ) to yield alkylbenzenes i. AlCl 3 acts as a lewis acid catalyst to enhance the electrophilicity of the alkylating agent.
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