magnetic induction of the pi electrons in an aromatic ring ... · nmr “activating” and...
TRANSCRIPT
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
dring 8.14-8.64 ppm
dMe -4.25 ppm
H H
dring 7.27-6.95 ppm
dCH2 -0.51 ppm
dOUTSIDE 9.28 ppm
dINSIDE -2.99 ppm
H
H
H
H
H
H
H
H H
H
H
H
H
HH
H
H
H
Anisotropy: Aromatic
Electronic effects CH2
O
CH3
CH2
+
O-
CH3
Deshielded
O
O
O
H
H
7.10 ppm
COOEt
COOEt
H
H
COOEt
H
H
EtOOC
6.83 ppm 6.28 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
p m
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
o p
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
5/14/2013 15
H H
H
H
H
H
H
H
Ortho
6-10 Hz
para
1-4
Hz
meta
0-2 Hz
8-10 Hz
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
5/14/2013 17
Ha Ha’
Hb Hb’
Ha&Ha’ Hb&Hb’
2 Amino Protons
The Molecular Ion peak
from 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 relative electronegativity of Amino & Cl groups
P-Chloroaniline (C6H6ClN)
Hydrogen bond
8.5 8.0 7.5
Br
N+
O-
O
HA
HB
HC
HD
Calculated shifts
dHA=8.44 dHB=7.82 dHC=7.31 dHD=8.19
HA
HB HC HD
meta bromo nitro benzene
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)
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
5/14/2013 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.
a b 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
d
J
C5 H4 N Br
I = 5 – 4/2 – 1/2 +1/2 +1
I = 4 (aromatic ring)
Aromatic substituent pattern CH3O
NO2
CH3O
NO2
8.5 8.0
t
J=8.1
dt
J=7.7, 1.5
ddd
J=8.1, 2.2, 1.1
t
J=1.8
8.1 8.0 7.9 7.8 7.7 7.6 7.5 7.4
CH3O
NO2
CH3O
NO2
dd
J=7.7, 1.5
dd
J=8.1, 0.7
td
J=7.4, 1.1 ~td
J=8.1, 1.5
Aromatic substituent pattern