Chemical Shift

– magnetic induction of the pi electrons in an

aromatic ring (Fig. 13.11)

Anisotropy of Aromatic compounds: in plane and above

CH3

CH3

H

H

H

H

H

H

H

H

H

H

δδδδring ���� 8.14-8.64 ppm

δδδδMe ���� -4.25 ppm

H H

δδδδring ���� 7.27-6.95 ppm

δδδδCH2���� -0.51 ppm

δδδδOUTSIDE ���� 9.28 ppm

δδδδINSIDE ���� -2.99 ppm

H

H

H

H

H

H

H

H H

H

H

H

H

HH

H

H

H

Anisotropy: Aromatic

Electronic effectsCH2

O

CH3

CH2

+

O-

CH3

Deshielded

O

O

O

H

H

7.10 ppm

COOEt

COOEt

H

H

COOEt

H

H

EtOOC

6.83 ppm6.28 ppm

O

H

H7.71 ppm

6.10 ppm

O

H

H

H

H7.07 ppm 6.38 ppm

6.28 ppm

5.93 ppm

8 7 6 5 4 3

O12

3

4

5

6

O7

Electronic effects: conjugation with carbonyl

8 7 6 5 4 3 2

O1

2

34

56

7.75

6.20

Electronic effects: conjugation with carbonyl

deshielded

Electronic effects: conjugation with heteroatom

OH

H

O+

C-

H

H

SH

H

6.06 ppm

5.48 ppm

S

H H

5.81 ppm

O

H H5.78 ppm

OH

H4.82 ppm

6.22 ppm

shielded

6.5 6.0 5.5 5.0 4.5 4.0 3.5

12

34

O5

Electronic effects: no conjugation with heteroatom

7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5

12

34

O5

2.65 2.60 2.55

6.35 6.30 4.95 4.90

shielded

Electronic effects: conjugation with heteroatom

O CH3

8 7 6 5 4 3 2

8.0 7.5

Electronic effects: conjugation with carbonyl

deshielded

deshielded

o

p

m

7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5

O

CH3

7.3 7.2 7.1 7.0 6.9 6.8

Electronic effects: conjugation with heteroatom

Shielded

shielded

o

pm

NH

CH3

7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5

7.0 6.5

Electronic effects: conjugation with heteroatom

Shielded

shielded

op

m

F

7.5 7.4 7.3 7.2 7.1 7.0 6.9

Cl

7.4 7.3 7.2 7.1

Br

7.6 7.5 7.4 7.3 7.2 7.1

Aromatic: inductive effect and resonance effect

Calculating Shifts

for aromatic

compounds

NMR

� Common Aromatic Patterns

6/9/2011 15

ortho

6-10 Hz

para

1-4 Hz

meta

0-2 Hz8-10 Hz

H

H

H

H

H

H

H

H

NMR

� “Activating” and “Deactivating” groups and the impact of the changing electron density in the Benzene ring on Chemical Shift of ortho, meta, para protons

6/9/2011 17

Ha Ha’

Hb Hb’

Ha&Ha’Hb&Hb’

2 Amino Protons

The Molecular Ion peakfrom Mass Spectrometry would have indicated the presence of the single Chlorine atom and Nitrogen.

� Para Di-Substituted Benzene ring

� Ha & Ha’ have same Chemical Shift

� Hb & Hb’ have same Chemical Shift

� Ha is split into doublet by Hb

� Hb is split into doublet by Ha

� Two sets of peaks produced by relativeelectronegativity of Amino & Cl groups

P-Chloroaniline (C6H6ClN)

Hydrogen bond

Aromatic substitution pattern: ortho

8.00 7.90 7.80 7.70 7.60 7.50 7.40

CH3

CH3

O

O

AA’ XX’

Typical spectra for ortho (symmetrical)

8.5 8.0 7.5

Br

N+

O-

O

HA

HB

HC

HD

Calculated shifts

δHA=8.44 δHB=7.82 δHC=7.31 δHD=8.19

HA

HB

HCHD

meta bromo nitro benzene

NMR

� “Activating” and “Deactivating” groups and the impact of the changing electron density in the Benzene ring on Chemical Shift of ortho, meta, para protons

6/9/2011 21

c

b a

� The Methoxy group is moderately activating, while the Nitro groups are strongly deactivating (electron withdrawing)

� Net effect is to Decrease the electron density about the ring protons

� The a & b protons are Ortho to the strongly deactivating Nitro groups, thus, they have reduced electron density and their Chemical Shift is down field relative to the “c” proton

� All protons interact to produce Spin-Spin Coupling.

ab c

3H

2,4-Dinitroanisole (C7H6N2O5)

AB-Spectra

AMX C6 H4 O5 N2

I = 6 - 4/2 + 2/2 +1

I = 6

Phenyl = 4 I

NO2 = 1 I

AMX

AFMX

δ

J

C5 H4 N Br

I = 5 – 4/2 – 1/2 +1/2 +1

I = 4 (aromatic ring)