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Chapter 18 Notes - CHAPTER 18 Reactions of Benzene and Its...

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CHAPTER 18 Reactions of Benzene and Its Derivatives 18.1 Introduction The most characteristic reaction of aromatic compounds is electrophilic aromatic substitution (one of the ring hydrogens is substituted with an electrophile). A less common reaction of aromatic compounds is nucleophilic aromatic substitution (generally a halogen on an aromatic ring is replaced by another nucleophile). In this chapter we’ll review electrophilic aromatic substitution and nucleophilic aromatic substitution . 18.2 Electrophilic Aromatic Substitution Benzene and its derivatives react by electrophilic aromatic substitution in which an electrophile (E + ) substitutes for H + . Before the reaction can proceed, the electrophile (E + ) must be generated by the use of certain catalysts. Generally, electrophilic substitution reaction on the benzene ring is represented as follows: Mechanism: Using a generic electrophile, the mechanism is a 2 step process as shown below. Step 1: the electrophile accepts an electron pair from the aromatic ring. Done by Dr. Felix N. Ngassa for Chemistry 242: Organic Chemistry for Life Sciences 2, GVSU, Spring/Summer 2011. 1
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Step 2: a nucleophile (acting as a base) removes the proton bonded to the same carbon atom as the newly arrived electrophile restoring the aromatic π system. 18.3 Examples of Electrophilic Aromatic Substitution Type Reagent Electrophile Example of Product Halogenation Nitration Sulfonation Friedel-Crafts Alkylation Friedel-Crafts Acylation Carbocations from Alkenes Carbocations from Alcohols A. Halogenation: Done by Dr. Felix N. Ngassa for Chemistry 242: Organic Chemistry for Life Sciences 2, GVSU, Spring/Summer 2011. 2
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The electrophile is a halogen atom (Cl + or Br + ). A Lewis acid is needed to generate the electrophile. Mechanism: B. Nitration: The electrophile is NO 2 + generated from HNO 3 and H 2 SO 4 (catalyst). Mechanism: C. Sulfonation: Done by Dr. Felix N. Ngassa for Chemistry 242: Organic Chemistry for Life Sciences 2, GVSU, Spring/Summer 2011. 3
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The electrophile is HSO 3 + generated from H 2 SO 4 /H + . Sulfonation is mostly carried out using a mixture of SO 3 and H 2 SO 4 ( fuming sulfuric acid ). Mechanism: D. Friedel-Crafts Alkylation: Reaction requires an alkyl halide with a corresponding aluminum trihalide as the catalyst. The catalyst helps to produce an alkyl carbocation, which is the electrophile. Mechanism: It is hard to stop the reaction at monoalkylation because the product is more reactive than the starting material. Isomerization of carbocations occurs in Friedel-Crafts alkylation. Some isomerizations occur by hydride (H: - ) shift , in which a less stable carbocation is converted into a more stable carbocation.
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