SPECTROSCOPY 325 13.12 (b) The two methyl carbons of the isopropyl group are equivalent CH3 Four different types of carbons occur in the aromatic ring and two different types are present in the isopropyl group. TheC NMR spectrum of isopropylbenzene contains six signals (c) The methyl substituent at C-2 is different from those at C-I and C-3 The four nonequivalent ring carbons and the two different types of methyl carbons give rise to a"C NMR spectrum that contains six signals (d) The three methyl carbons of 1, 2, 4-trimethylbenzene are different from one another: C H,C Also, all the ring carbons are different from each other The nine different carbons give rise to nine separate signal (e) All three methyl carbons of 1, 3, 5-trimethy benzene are equivalet Because of its high symmetry 1, 3, 5-trimethylbenzene has only three signals in itsCNMR 13 13 sp-Hybridized carbons are more shielded than sp-hybridized ones. Carbon x is the most shielded, and has a chemical shift of 8 20 ppm. The oxygen of the OCH3 group decreased the shielding of carbon z: its chemical shift is 8 55 ppm. The least shielded is carbon y with a chemical shift of 20 H H OCH 13.14 The C NMR spectrum in Figure 13 22 shows nine signals and is the spectrum of benzene from part(d)of Problem 13.12. Six of the signals, in the range 8 127-138 ppm, are due to Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website13.12 (b) The two methyl carbons of the isopropyl group are equivalent. Four different types of carbons occur in the aromatic ring and two different types are present in the isopropyl group. The 13C NMR spectrum of isopropylbenzene contains six signals. (c) The methyl substituent at C-2 is different from those at C-1 and C-3: The four nonequivalent ring carbons and the two different types of methyl carbons give rise to a 13C NMR spectrum that contains six signals. (d) The three methyl carbons of 1,2,4-trimethylbenzene are different from one another: Also, all the ring carbons are different from each other. The nine different carbons give rise to nine separate signals. (e) All three methyl carbons of 1,3,5-trimethylbenzene are equivalent. Because of its high symmetry 1,3,5-trimethylbenzene has only three signals in its 13C NMR spectrum. 13.13 sp3 -Hybridized carbons are more shielded than sp2 -hybridized ones. Carbon x is the most shielded, and has a chemical shift of 20 ppm. The oxygen of the OCH3 group decreased the shielding of carbon z; its chemical shift is 55 ppm. The least shielded is carbon y with a chemical shift of 157 ppm. 13.14 The 13C NMR spectrum in Figure 13.22 shows nine signals and is the spectrum of 1,2,4-trimethyl￾benzene from part (d) of Problem 13.12. Six of the signals, in the range 127–138 ppm, are due to H3C OCH3 H H H H 20 ppm 55 ppm 157 ppm y z y z y H z 3C x CH3 x CH3 x H3C CH3 H3C CH3 x y x y w z n CH3 m CH3 m CH CH3 CH3 x y x y w z m n n SPECTROSCOPY 325 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website