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250 ChemActivity 29 Electrophilic Aromatic Substitution no ChemActivity 29 Part A: Electrophilic Aromatic Substitution (What products...


250 ChemActivity 29 Electrophilic Aromatic Substitution no ChemActivity 29 Part A: Electrophilic Aromatic Substitution (What
251 ChemActivity 29 Electrophilic Aromatic Substitution Model 2: Electrophilic Aromatic Substitution Recall that deuterium (D
252 ChemActivity 29 Electrophilic Aromatic Substitution 7. Draw a generalized mechanism for EAS (Electrophilie Aromatic Subst
ChemActivity 29 Electrophilic Aromatic Substitution 253 Part B: Substituent Effects (How do substituents on an aromatic ring
250 ChemActivity 29 Electrophilic Aromatic Substitution no ChemActivity 29 Part A: Electrophilic Aromatic Substitution (What products are formed when a strong electrophile is added to benzene?) Model 1: (review) Electrophilic Addition of HCI Rani o g cyclohexene carbocation intermediate Run 2 U X benzene This product carbocation intermediate DOES NOT Critical Thinking Questions 1 For Rxn I (above) draw curved arrows showing the mechanism of electrophilic addition of HCl. Include an appropriate carbocation intermediate in the box above. Figure 1: Reaction Diagrams for Electrophilic Addition of HCI V.E. (Potential Energy) V.E. H-CI Reaction Progress (Rxn 1) Reaction Progress (Rxn 2) 2. Rxn 1 is slightly down-hill in terms of energy. Rxn 2 is very up-hill in terms of energy (see Figure 1). Construct an explanation for the large difference in energy between the reactants and the product in Ran 2. 3. Draw the carbocation that would form in Rxn 2. Explain why this carbocation goes back to the starting material (H-Cl and benzene) instead of forming the product.
251 ChemActivity 29 Electrophilic Aromatic Substitution Model 2: Electrophilic Aromatic Substitution Recall that deuterium (D) has nearly identical reactivity as hydrogen (H). In the following reaction D C reacts the same way as would H-CI. When benzene is treated with D-Can His replaced with a D (With excess this continues until all the H's have been replaced and the product is CD- D-a Rxn 3 H-C1 Critical Thinking Questions 4. Use curved arrows to show a reasonable mechanism for the reaction in Model 2. (Hint: the first step is formation of a carbocation intermediate, just as m 3. The energy diagram for Rxn 2 (using D in place of HCD is shown below using doffed line. On this same set of axes drowolid line to show an energy diagram for Run 3 in Model 2. (Note: Runs 2 and 3 have the same carbocation intermediate) ma carbocation intermediate for Rxns 2 & 3 V.E. D-ci Reaction Progress (Rxn 2 with DCI) Reaction Progress (Rxn 3) 6. Construct an explanation for why Rxn 3 is much more likely to occur than Rxn 2. (Note: Rxn 2 does not occur under normal circumstances.)
252 ChemActivity 29 Electrophilic Aromatic Substitution 7. Draw a generalized mechanism for EAS (Electrophilie Aromatic Substitution) showing the substitution of an electrophile (E) for one of the H's of benzene in the presence of a mild base (B). (Note: in Rxn 3, E-D) Ε -B® 8+ Information: Not All Electrophiles are created Equal • A very strong electrophile (E) is required to break up an aromatic ring. • The H of a strong acid will work since it exists essentially as H. Know the following strong acids. 1 8 + 0 0 # 1 0S-OH H N Oo hydrochloric acid hydrobromic acid hydroiodic acid sulfuric acid nitric acid • Suitable electrophiles (E) for EAS (Electrophilic Aromatic Substitution) include an H or D donated by a strong acid, or the electrophiles (E) listed below. Table 2: Other Suitable Electrophiles (ET) for EAS Reactant E= Reagents Products NOZ — — HASO SO, oso HNO, anhydrous sulfuric acid Br2 and FeBry 8 Cl2 and FeCl2 benzenes R-X, AIX; or IR (X=Cl or Br) other aromatic R-alkyl group ring AIX; (X=Cl or Br) R-alkyl group "The last three rows of electrophiles are generated only in the presence of a Lewis acid catalyst such as FeX, or AIX3. A supplementary activity on Lewis acid catalysts is assigned for homework.
ChemActivity 29 Electrophilic Aromatic Substitution 253 Part B: Substituent Effects (How do substituents on an aromatic ring direct the placement of E' and rate of EAS?) Model 3: Electrophilic Aromatic Substitution with Toluene Critical Thinking Questions 8. Add curved arrows showing the movement of electrons in the two reactions above. a) Draw any missing important resonance structures for each of the two carbocation intermediates. (One from each set is re-drawn for you below.) -- ON b) In the meta set, none of the three resonance structures stand out as being most important, but in the para set one resonance structure stands out as being most important. Explain this statement and circle the most important res, structure. c) of the two products at the top of the page, one forms and the other does not. Circle the product that forms and explain your reasoning.
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1. Mechanism for the electrophilic addition along with carbocation intermediate is : + H-CI (Callocation Intermediate) 2. In

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