CHEM302 S09 CA4a key

CHEM302 S09 CA4a key - CHEM302, Spring 2009 Dr. Ruder 1...

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CHEM302, Spring 2009 Dr. Ruder 1 Class Activity 4A key Electrophilic Aromatic Substitution (EAS) Part A: Reaction of Benzene An electrophile replaces hydrogen on an aromatic ring. Model 1 : Electrophilic Addition of HCl to Cyclohexene and Benzene + H Cl H Cl (Rxn 1) + H Cl H Cl (Rxn 2) X Questions: 1. Based on the information found in Model 1: (a). For Rxn 1 draw curved arrows to show the mechanism for addition of HCl to cyclohexene. Make sure to draw the carbocation intermediate formed. + H Cl H Cl (Rxn 1) H (b). Draw the carbocation that would form in Rxn 2. Why does this cation revert backwards to the starting material instead of forming the addition product? It reverts back to the starting material because the starting material is aromatic and more stable than the product. + H Cl H Cl (Rxn 2) X H (c). Why is Rxn 2 up-hill (endothermic)? The product of Rxn 2 is not aromatic so is much higher in energy than the starting material. Model 2 : General Electrophilic Aromatic Substitution (EAS) Reaction
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CHEM302, Spring 2009 Dr. Ruder 2 + E SLOW H H H H E H E + BH B sigma complex (arenium ion) Questions: 2. For the EAS reaction in Model 2: (a). Draw curved arrows to show electron movement for this mechanism. + E SLOW H H H H E H E + BH B sigma complex (arenium ion) (b). What is the role (Nu-, E+, acid, base) of benzene? What is the role of the sigma complex? benzene is the nucleophile, sigma complex is the acid (c). Draw all the resonance contributors for the sigma complex in Model 2. Use curved arrows to show electron movement. H
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CHEM302 S09 CA4a key - CHEM302, Spring 2009 Dr. Ruder 1...

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