13.5 Effects of molecular Structure on 'H Chemical Shifts much as the shielding is the electrons, it isnt surprising to find that the chemi- cal shift depends on the de to which X draws electrons away from the methyl group Increased shielding of methyl protons Decreasing electronegativity of attached atom CH3OCH3 (CH3)3N CH3CH Methyl Dimethyl Trimethylamine Ethane fluoride Chemical shift of methyl protons (6),ppm: A similar trend is seen in the methyl halides, in which the protons in CH3 F are the least shielded(8 4.3 ppm) and those of CH3I (8 2. 2 ppm) are the most The deshielding effects of electronegative substituents are cumulative, as the chem- ical shifts for various chlorinated derivatives of methane indicate CH,CI CH3CI Chloroform Methylene chloride Methyl chloride Chemical shift (6),ppm: 7.3 PROBLEM 13. 4 There is a difference of 4.6 ppm in the 'H chemical shifts CHCI3 and CH3CCl3 What is the chemical shift for the protons in CH3 CCl3? Exp your reasonIng inyl protons in alkenes and aryl protons in arenes are substantially less shielded Ethylene Chemical shift 7.3 One reason for the decreased shielding of vinyl and aryl protons is related to the directional properties of the induced magnetic field of the TT electrons. As Figure 13.8 shows, the induced magnetic field due to the T electrons is just like that due to elec trons in o bonds; it opposes the applied magnetic field. However, all magnetic fields close upon themselves, and protons attached to a carbon-carbon double bond or ar FIGURE 13. 8 The induced matic ring lie in a region where the induced field reinforces the applied field, which magnetic field of the elec- decreases the shielding of vinyl and aryl protons trons of (a) an alkene and A similar, although much smaller, effect of T electron systems is seen in the chem- (b)an arene reinforces the ical shifts of benzylic and allylic hydrogens. The methyl hydrogens in hexamethylben- where vinyl and aryl protons zene and in 2, 3-dimethyl-2-butene are less shielded than those in ethane are located Back Forward Main MenuToc Study Guide ToC Student o MHHE Websitemuch as the shielding is due to the electrons, it isn’t surprising to find that the chemi￾cal shift depends on the degree to which X draws electrons away from the methyl group. A similar trend is seen in the methyl halides, in which the protons in CH3F are the least shielded ( 4.3 ppm) and those of CH3I ( 2.2 ppm) are the most. The deshielding effects of electronegative substituents are cumulative, as the chem￾ical shifts for various chlorinated derivatives of methane indicate: PROBLEM 13.4 There is a difference of 4.6 ppm in the 1 H chemical shifts of CHCl3 and CH3CCl3. What is the chemical shift for the protons in CH3CCl3? Explain your reasoning. Vinyl protons in alkenes and aryl protons in arenes are substantially less shielded than protons in alkanes: One reason for the decreased shielding of vinyl and aryl protons is related to the directional properties of the induced magnetic field of the electrons. As Figure 13.8 shows, the induced magnetic field due to the electrons is just like that due to elec￾trons in  bonds; it opposes the applied magnetic field. However, all magnetic fields close upon themselves, and protons attached to a carbon–carbon double bond or an aro￾matic ring lie in a region where the induced field reinforces the applied field, which decreases the shielding of vinyl and aryl protons. A similar, although much smaller, effect of electron systems is seen in the chem￾ical shifts of benzylic and allylic hydrogens. The methyl hydrogens in hexamethylben￾zene and in 2,3-dimethyl-2-butene are less shielded than those in ethane. Chemical shift (), ppm: H H H H H H Benzene 7.3 C H H H H C Ethylene 5.3 CH3CH3 Ethane 0.9 Chemical shift (), ppm: CHCl3 Chloroform (trichloromethane) 7.3 CH2Cl2 Methylene chloride (dichloromethane) 5.3 CH3Cl Methyl chloride (chloromethane) 3.1 Increased shielding of methyl protons Decreasing electronegativity of attached atom Chemical shift of methyl protons (), ppm: CH3F Methyl fluoride 4.3 CH3OCH3 Dimethyl ether 3.2 (CH3)3N Trimethylamine 2.2 CH3CH3 Ethane 0.9 13.5 Effects of Molecular Structure on 1 H Chemical Shifts 495 C C H H H H H H H H H H 0 (a) (b) 0 FIGURE 13.8 The induced magnetic field of the elec￾trons of (a) an alkene and (b) an arene reinforces the applied fields in the regions where vinyl and aryl protons are located. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website