CHEM302 S09 CA4Bkey

CHEM302 S09 CA4Bkey - CHEM302, Spring 2009 Dr. Ruder 1...

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CHEM302, Spring 2009 Dr. Ruder 1 Class Activity 4B key Electrophilic Aromatic Substitution Part B: Substituent Effects Model 1: Nitration of Methyl benzene (Toluene) Shown below is the nitration of methylbenzene (toluene) to form the ortho (Rxn 1) and meta (Rxn 2) products. The para product will be considered later. + (Rxn 1) + CH 3 (Rxn 2) CH 3 CH 3 NO 2 NO 2 CH 3 NO 2 CH 3 NO 2 CH 3 H 2 O H 2 O N O O N O O Questions : 1. (a). Draw curved arrows in both Rxn 1 and Rxn 2 above to show electron movement. + (Rxn 1) + CH 3 (Rxn 2) CH 3 CH 3 NO 2 NO 2 CH 3 NO 2 CH 3 NO 2 CH 3 H 2 O H 2 O N O O N O O H H (b). Draw any additional resonance forms for the cationic intermediate (sigma complex) formed in each of the above reactions (one resonance form for each is redrawn below).
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Dr. Ruder 2 (Rxn 1) (Rxn 2) CH 3 NO 2 CH 3 NO 2 CH 3 NO 2 CH 3 NO 2 CH 3 NO 2 CH 3 NO 2 (c). In Rxn 2, none of the three resonance structures stand out as being most important, but in Rxn 1, one resonance structure stands out as being most important . As a group discuss why this statement is true and circle the most important resonance structure. . In Rxn 2 all the resonance forms are secondary, while in Rxn 1 two resonance forms are secondary and one is tertiary. The tertiary cation is more stable thus is the more important form. (d). Of the two products in Scheme 1, one forms and the other does not. Circle the product that forms and explain your reasoning . The ortho product forms because the intermediate sigma complex leading to the ortho product is of lower energy than the sigma complex leading to the meta product. 2. (a). Draw the three resonance forms of the cationic intermediate (sigma complex) that forms when the nitronium ion adds to the para position, shown below. + NO 2 CH 3 CH 3 O 2 N CH 3 O 2 N CH 3 O 2 N CH 3 O 2 N (b). The cationionic intermediate above is very close in potential energy to the intermediate in the ortho set (Rxn 1 above). In what way are these two sets of resonance structures similar? The two sets of intermediates both have two secondary and one tertiary cation. (c).
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This note was uploaded on 02/17/2011 for the course CHEM 302 taught by Professor Ruder during the Spring '09 term at VCU.

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CHEM302 S09 CA4Bkey - CHEM302, Spring 2009 Dr. Ruder 1...

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