Location of Signals TABLE 13-2 Chemical Shifts of the Chloromethanes · More electronegative Chemical Compound Shif? Difference atoms deshield more and HS0.2 give larger shift values Effect decreases with H-C-C1S3.0 distance H Additional electronegative C185.3 H atoms cause increase in H-C-187.2 chemical shift Note: Each chlorine atom added hanges the chemical shift of the remaning methyl protons by about 2 to 3 ppm. These changes are nearly additive Chapter 13 16
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. =>
ypical values Type of Proton Approximate 8 Type of Proton Approximate 8 alkane(一CH3) 0.9 C-C alkane(-CH2) 1.3 CH3 Ph—H 7.2 alkane(-CH 1.4 Ph--CH, R一CHO 9-10 C一CH3 R—COOH 10-12 一C≡C-H 2.5 R-OH variable. about 2-5 R-CH -X Ar-OH variable. about 4-7 (x- halogen, O) R一NH variable. about 1.5-4 C=C H 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 I 7
Chapter 13 17 Typical Values =>
Aromatic Protons, 87-88 circulation of electrons ring current induced field reinforces the H H external field (deshielding) B induced nduced magnetic field 18
Chapter 13 18 Aromatic Protons, 7-8 =>
Vinyl Protons, 85-86 eB induce d induced field reinforces the C- C external field H (deshielding) B B 0 induced Chapter 13 19
Chapter 13 19 Vinyl Protons, 5-6 =>