1559T_ch15_276-29011/03/0520:24Pa9e282 EQA 282.Chapter 15 BENZENE AND AROMATICITY:ELECTROPHILC AROMATIC SUBSTITUTION Assign the signals at.6 ppm more precisely by looking at the resonance form 5-0-d tually (Compare Exercise 15-22) C(CHa):Friedel-Crafts alkylation involving H (Carctu (CH,CCH-C-ACt (CHC二GH-CH,Aa,4n Assign the signals at 136.9 and 186.6 ppm more precisely by looking at the resonance forms: In the resonance forms, positive charges are located at only three of the carbons: They should be the most deshielded. That explains the   178.1 chemical shift of the “bottom” carbon. The   186.6 signal must therefore correspond to the positively charged “end” carbons of the delocalized cation: 44. (a) (b) initially; eventually (Compare Exercise 15-22) (c), (e) (d) (f) (g) (h) CH3 C O CH3 CH3 CH3 CH3 (CH3)2C CH(CH3)2 (CH3)2CH CH3 CH3 C (CH3)2CH CH3 CH3 C H H (CH3)2C CH CH3 CH3 AlCl4 CH3 shift Careful! (CH3)3CCH CH2 Cl AlCl3 H H shift NO2 C(CH3)3 Friedel-Crafts alkylation involving (CH3)3C cation T T T T T T Cl T 186.6 So this is 136.9 H H H H H H 282 • Chapter 15 BENZENE AND AROMATICITY: ELECTROPHILIC AROMATIC SUBSTITUTION 1559T_ch15_276-290 11/03/05 20:24 Page 282