Ring Current in Benzene

1
UNEQUAL COUPLING TREE DIAGRAMS
SPLITTING DIAGRAMS
aka TREE DIAGRAMS
2
WHERE DOES THE N1 RULE WORK ?
The n1 rule works only for protons in aliphatic
chains and rings, and then under special
conditions.
There are two requirements for the n1 rule to
work
1) All 3J values must be the same all along the
chain.
2) There must be free rotation or inversion
(rings) to make all of the hydrogens on a
single carbon be nearly equivalent.
3
THE TYPICAL SITUATION WHERE THE n1 RULE APPLIES
Hydrogens can interchange their positions
by rotations about the C-C bonds.
This makes all the hydrogens on each of the
carbon atoms equivalent.
All the couplings along the chain have the same
J value.
4
WHAT HAPPENS WHEN THE J VALUES ARE NOT EQUAL ?
H
H
H
3Ja 3Jb
C
C
C
H
H
H
3Ja 3Jb
In this situation each coupling must be
considered independently of the other.
A splitting tree is constructed
5
USE THESE VALUES
6
CONSTRUCTING A TREE DIAGRAM
SPLITTING FROM HYDROGENS TO THE LEFT
H
H
H
-CH2-CH2-CH2-
LEVEL ONE
C
C
C
The largest J value is usually used first.
H
H
H
Two neighbors gives a triplet.
3Ja 7
The next splittings will be added to each leg of
the first splitting.
Each level of the splitting uses the n1 rule.
7
CONSTRUCTING A TREE DIAGRAM
ADD SPLITTING FROM HYDROGENS TO THE RIGHT
H
H
H
-CH2-CH2-CH2-
C
C
C
H
H
H
3Ja 7
3Jb 3
FIRST LEVEL
SECOND LEVEL
LEVEL TWO
triplet of triplets
The smaller splitting is used second.
EACH LEG OF LEVEL ONE IS SPLIT
It is also a triplet.
8
WHEN BOTH 3J VALUES ARE THE SAME
The n1 rule is followed ..
LEVEL ONE
-CH2-CH2-CH2-
Splitting from hydrogens on the left
INTENSITIES
n1 (4 1) 5
121
121
LEVEL TWO
121
Splitting from hydrogens on the right
121

14641
Splittings overlap
.. because of overlapping legs. You get the
quintet predicted by the n1 rule.
WHEN THE n1 RULE APPLIES WE CAN JUMP TO THE
FINAL RESULT - NO TREE NEEDED
9
2-PHENYLPROPANAL
A case where there are unequal J values.
10
Spectrum of 2-Phenylpropanal
a
b
d
TMS
c
a
c
d
b
11
7 Hz
2 Hz
Rather than the expected quintet ..
the methine hydrogen is split by two
different 3J values.
quartet by -CH3
3J1 7 Hz
3J2 2 Hz
doublet by -CHO
ANALYSIS OF METHINE HYDROGENS SPLITTING
quartet of doublets
12
PURE ETHANOL
13
ETHANOL
400 MHz
Old sample Rapid exchange catalyzed by impurities
hydrogen on OH is decoupled
HO-CH2-CH3
triplet
broad singlet
quartet
14
expansion
expansion
doublet of quartets
triplet
ETHANOL
Ultrapure sample (new) Slow or no exchange
400 MHz
triplet
15
J 7
J 5
J 7
J 5
quartet of doublets
triplet
triplet
16
VINYL ACETATE
ALKENE HYDROGENS
17
COUPLING CONSTANTS
PROTONS ON CC DOUBLE BONDS

• 3J-cis 8-10 Hz
• 3J-trans 16-18 Hz
• protons on the same carbon
• 2J-geminal 0-2 Hz

For protons on saturated aliphatic chains 3J 8
Hz
18
NMR Spectrum of Vinyl Acetate
60 MHz
19
Analysis of Vinyl Acetate
3J-trans gt 3J-cis gt 2J-gem
HC
HB
HA
3JAC
3JBC
3JBC
cis
trans
trans
3JAC
2JAB
2JAB
cis
gem
gem
20
2,4-DINITROANISOLE
BENZENE HYDROGENS
21
2,4-DINITROANISOLE
400 MHz
9.0 8.0
7.0
22
2,4-DINITROANISOLE
8.72 ppm 8.43 ppm
7.25 ppm
23
END