circulution Location of Signals TABLE 13-2 Chemical Shifts of the Chloromethanes More electronegative Chemical Compound Shift Difference atoms deshield more and 80.2 give larger shift values. 2.8 ppm Effect decreases with 3.0 distance. 2.3 ppm Additional electronegative 65.3 1.9 ppm atoms cause increase in 872 chemical shift. => Note:Each chlorine atom added changes the chemical shift of the remaining methyl protons by 2 to 3 ppm.These changes are nearly additive. Chapter 13 16 Copyright 2005 Pearson Prentice Hall,Inc
Chapter 13 16 Location of Signals • More electronegative atoms deshield more and give larger shift values. • Effect decreases with distance. • Additional electronegative atoms cause increase in chemical shift. =>
circulition Typical Values TABLE 13-3 Typical Values of Chemical Shifts Type of Proton Approximateδ Type of Proton Approximateδ alkane (-CH) 0.9 >c=c< 1.7 alkane(-CH2-） 1.3 CH; Ph一H 7.2 alkane ( H-） 1.4 Ph一CH 2.3 R一CHO 9-10 2.1 R-COOH 10-12 一C=C一H 2.5 R一OH variable,about 2-5 R一CH,一X 3-4 Ar一OH variable,about 4-7 (X halogen.O) R-NH2 variable,about 1.5-4 5-6 Note:These values are approximate,as all chemical shifts are affected by neighboring substituents.The numbers given here assume that alkyl groups are the only other substituents present.A more complete = table of chemical shifts appears in Appendix 1. Copyright2005 Pearson Prentice Hall,Inc. 17
Chapter 13 17 Typical Values =>
circulution Aromatic Protons,δ7-δ8 circulation of electrons (ring current) induced field reinforces the H external field (deshielding) Binduced Bo induced magnetic field Copyright 2005 Pearson Prentice Hall.Inc. 18
Chapter 13 18 Aromatic Protons, 7-8 =>
Vinyl Protons,δ5-δ6 Binduced induced field …H reinforces the external field (deshielding) Bo Binduced Copyright 2005 Pearson Prentice Hall,Inc. => Chapter 13 19
Chapter 13 19 Vinyl Protons, 5-6 =>