Download Sect 5 NMR Trends

ORGANIC NMR INTERPRETATION
p. 101
ALKANES AND ALKYL HALIDES
d
p. 101
CH3—CH2—CH2—C
0.9 1.3
CH3—F 4.3
CH3—O- 3.5
CH3—Cl 3.0
CH3—Br 2.7
CH3—I
2.2
Dd ~ 2 ppm downfield
CH3—CH2-
CH3—X
p. 102
Inductive
effects
d
CH3—CH2—CH2—C
0.9 1.3
d
CH3—CH2—CH2Cl
1.0
1.8
3.5
CH3—CH2—CH2Br
d
1.0
1.8
3.4
CH3—CH2—CH2I
d
1.0
1.8
3.2
Dd
0
½
2
Effect falls off
with distance
and is ~ 0 two
C away
p. 102
CH3—CH2—CH2Cl
1.0
1.8
3.5
CH3—CH2—CH2Br
1.0
1.8
3.4
CH3—CH2—CH2I
1.0
1.8
3.2
Dd 0
½
2
d
CH3—CH2—
0.9 1.3
p. 103
d
CH3—CH2—
0.9 1.3
CH3—CH2—CH2X
Dd
0
½
2
Each extra X adds ~ 2 ppm
CH3—X
d 3
CH2X2
5
CHX3
7
Ballpark ONLY!!
Inductive effects are more or less additive
p. 103
CH3—CH2—I
d 1.8 3.2
I—CH2—CH2—I
d
3.6
Additional X next door
has added about ½
ppm
p. 103
In general, the more substituted, the more downfield
CH3—I
d 2.2
CH3—CH2—I
3.2
(CH3)2CH—I
4.2
but additional alkyl groups are not as strong as –X
CH3CH2Br = 3.4
(CH3)2CHBr =4.3
split by 6
= 6+1
6H
1H
CH3CHBr2 = 5.5
split by 1
= 1+1
I—CH(CH3)2
p. 104
More complex splittings
I—CH2—CH2—CH2—CH3
t
?
?
t
What about when neighbors are chemically different?
If J’s are same, then can use
splitting (# of lines) = total # of H neighbors + 1
3
2
1
H
3
H
J=7
4
H
J=7
J=0
Characteristic chain
splitting in alkane
chains
More complex splittings
I—CH2—CH2—CH2—CH3
t
5
2+2+1
6
t
2+3+1
p. 104
p. 105
ANISOTROPIC EFFECTS
Spherical atoms have same effect
in all directions
F
p-electrons
H
H
H
H
are above and
below the plane
of molecule
so electron density is different
above or below molecule than
in plane
p. 105
so alkenes and aromatics (and other p-bonds)
are not isotropic – they have effects that are different in
different directions – we call them ANISOTROPIC
Circulation of
p
the -electrons
around the ring
produce a field,
Blocal
= Bp-electrons
= Blocal
Bo + Blocal
H
H
(deshielded)
increased chemical
shift in the plane of
the ring
Bo - Blocal
Bo
(shielded)
decreased
chemical shift
above and
below the plane
of the ring
so H feels B0 + Blocal
so appear at low field
aromatics &
alkenes appear at
H
aromatics
d6-8
-
H
alkenes
5 - 7 ppm
LOW FIELD
- is shielded
(to lower ppm)
+
-
+ is deshielded
(to higher ppm)
p. 105
aromatic hydrogens ~ d 7
p. 106
7.2
2.3
7.0
2.3
6.8
2.3
X=C—CH3
methyl on an aromatic ring, double bond or carbonyl ~2.3
p. 107
Ph—CH2—CH2—(CH2)4—CH2—CH3
7.1-7.3 2.6 1.6
1.3
1.3 0.9
Ph—CH2—CH2—(CH2)6—CH2—CH3
p. 107
Clearing up some terminology:
Downfield
Deshielded
Low field
Greater d
Upfield
Shielded
High field
Smaller d
p. 108
EWG
Electron Withdrawing Groups (EWG)
deshield the ortho & para H’s,
o>p
Benzene = 7.3
7.8
7.3
7.6
-CHO
-COR
-COOH
-COOR
O-
O
-CN
-NO2
-SO2
resonance effects
+
etc
+ve charge deshields: less electron density at the C and H
2:1:2
d 10
CHO
EWG
deshield
7.8
7.3
7.6
p. 108
p. 109
D
Electron Donating groups (D:)
6.8
7.2
SHIELD the ortho and para protons
o>p
7.0
Donating groups are X: (atoms with lone pairs
but not halogens)
e.g. -OH -OR -NH2 -NHR -NR2 -SR -R
R
R
O:
-
O+
resonance effect
dominates inductive
effect
negative charge
shields: more
etc
electron density
at C and H
p. 109
OCH3
6.8-6.9
7.2
6.9-7.0
CH3 on O + Ar ring
~ 3.8
HALOGENS
p. 110
Not easy to predict:
Lone pairs shield by resonance but deshield because
of high electronegativity
Part of Table on manual page 110
Increments add to d 7.27 to predict shifts, e.g.
Proton ortho to –CHO will be 7.27 + 0.58 = 7.85
O
H
p. 110
C
AH
HA
BH
HB
OCH3
p-anisaldehyde
 d = 7.27 + 0.58 (ortho CHO) - 0.10 (meta OCH3)
d = 7.75 ppm predicted shift of HA
 d = 7.27 - 0.46 (ortho OCH3) + 0.20 (meta CHO)
d = 7.01 ppm predicted shift of HB
p. 110
Look carefully at peaks, they are doublets
CHO
CH3O
H1
H2
3J
What about to other protons?
~ 8 Hz
p. 111
H
3J
ORTHO
H
H
H
H-H = ~8Hz
4J
META
H-H = ~2Hz
5J
PARA
H-H = ~0 Hz
p. 111
At high fields, can use trees to get patterns, IF chemical
shifts are far apart (called 1st order spectrum if Dd >> J)
H= d (H) of d (H) H
8Hz
2Hz
H=
t (HH) of d (H)
8,8
2
H
CHO
H= t (HH) of d (H)
H
OCH3
H
H= d (H) of d (H)
p. 112
ALDEHYDES
O=C-H
- is shielded
(to lower ppm)
+ is deshielded
(to higher ppm)
d ~ 10
AND coupling constant to neighbors is small
H2C
3
JHH ~ 1 Hz
a
C
H
O
Karplus showed
relationship of
J and a
p. 112
p. 113
60MHz
60Hz
Spectrum looks
different on different
instruments
30Hz
J in Hertz is always
independent of field
p. 114
ALKENE COUPLING CONSTANTS
HB
HB
HA
HA
HC
HC
trans
typically 14-16 Hz
14-16 Hz
cis
typically 7-10 Hz
geminal
typically 2 Hz
14-16 Hz
7-10 Hz
7-10 Hz
2 Hz
2 Hz
16Hz
8Hz
2Hz
mono-substituted alkene
R--CH=CH2
p. 114
Always
12 lines:
d(JL)d(JM)
d(JL)d(JS)
d(JM)d(JS)
p. 115
Jcis
Jtrans
p. 116
Chemical shifts – much like aromatics
p. 117
‘normal’ = 5.25 ppm
d+
E=W d-
H
C
C
proton is deshielded,
signal shifts downfield
to higher ppm
d-
..
ED
H
d+
C
C
proton is shielded,
signal shifts upfield
to lower ppm
b to the substituent feels resonance effect
Geminal (same C)
always deshield
by ~ 1ppm
Manual, table page 117
H
dC
Rcis
= 5.25 + Dgem + Dcis + Dtrans
C
H
Always deshield
geminal
Shield b
Rgem
Deshield b
Rtrans
4J
= ~1Hz
H1
4J
= ~1Hz
H
2
H
H
4
3
3
2
1
p. 118
H
3J
= ~7Hz
Cis Me
Gem H
Trans Ph
H
d cis to Me = 5.25 + 0 - 0.22 – 0.07 = 4.96
d cis to Ph = 5.25 + 0 + 0.36 – 0.28 = 5.33
Cis Ph
Trans Me
H
H
See
yellow
pages A5
ALCOHOLS, AMINES, AMIDES AND ACIDS – exchangeable H’s
R
OH
Ar
O
OH
Ar-CONHR and Ar-CONH2
R
NH2
Ar
d 1-5
R
C
OH
NH2
d 5-10
d 10-16
Hydrogen is becoming more acidic.
More acidic
More acidic = better able to form hydrogen bonds. means more
Shifts further downfield (higher ppm).
d+ on H
[H3O+] + [-OH]
2 H2O
ROH + R*OH'
[ROHH']+ + [R*O]-
Just like water, alcohols are always swapping
their hydrogen partners. This happens rapidly:
105 times per second at room temperature.
NMR time scale is ~ 10-2 – 10-3 sec
fastest NMR can measure!
p. 119
p. 119
Since acidic H exchange between molecules occurs faster
than the NMR time scale they DO NOT show coupling to any
neighbours and are typically broadened
p. 119
To prove which is –OH peak, add D2O and shake
ROH + D2O => ROD + HOD d ~ 5.2
Shape of peak depends upon temperature
(rate of exchange is affected by temperature)
exchange
stopped
Coupling visible
slow
exchange
fast
exchange
HO—CH3
p. 120
p. 121
Only a
triplet
Amines: RNH2 d 1-5; ArNH2 d 5-10
p. 121
Amides: d 5-10
O-
O
Why 2?
+
R
NH2
R
N
H
H
p. 121
Acids: d 10-16
d = 9.85 + offset (2.0) = 11.85
You can now start
Assignment 5