Exercise plus Solution – Quick overview
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1
H NMR spectrum
recorded at 250.13 MHz
C
3
H
9
N measured in CDCl
3
Get the constitution.
Solution
C
3
H
9
N
- no degree of unsaturation (double
bond equivalents)
- four signal group
- integral ratio: 2 : 2 : 2 : 3
- there are only 4 possible structures
2H 2H2H 3H
2H 2H2H 3H
N CH
3
H
3
C
H
3
C
N CH
3
H
CH
2
H
3
C
NH
2
H
2
C
CH
2
H
3
C
CH NH
2
H
3
C
H
3
C
Isopropylamine
n-Propylamine
N-Ethylmethylamine
Trimethylamine
9H
6H
2H
1H
3H
3H
2H
1H
3H
2H
2H
2H
1
H distribution
expected measured
3H (triplet)
2H (singlet)
2H (sextet ?)
2H (triplet)
In which of the four possible isomers do four
chemically distinguishable protons occur in the
measured integral ratio?
One signal group only.
Only three signal groups.
Four signal groups but
not the measured
integral ratio.
Four signal groups,
integral ratio as
measured.
2H 2H2H 3H
NH
2
H
2
C
CH
2
H
3
C
NH
2
H
2
C
CH
2
H
3
C
The constitution could be deduced solely from the number of
signal groups and the integral ratio between the signal
groups.
The protons and the corresponding multiplets are labelled
here with the same colours. But how did we made this
assignment?
How do you explain the multiplet at about 1.4 ppm?
NH
2
H
2
C
CH
2
H
3
C
2H 2H2H 3H
Because of the integral of 3 the methyl group
is easiest to assign.
≈ 0.84 ppm
The two protons of the neighbouring
methylene group cause the splitting to form
the triplet.
NH
2
H
2
C
CH
2
H
3
C
2H 2H2H 3H
The distance between two adjacent lines of
the triplet is the coupling constant J
1
between
the methyl and the methylene protons.
≈ 0.84 ppm
J
1
J
1
NH
2
H
2
C
CH
2
H
3
C
2H 2H2H 3H
The coupling constant J
1
should now naturally also appear in
the multiplet of the methylene protons at approx. 1.4 ppm.
J
1
J
2
At a distance of three bonds, there are two
more equivalent methylene protons. The
vicinal coupling constant J
2
should not differ
much from J
1
. However, J
1
and J
2
are not
identical.
NH
2
H
2
C
CH
2
H
3
C
2H 2H2H 3H
Theoretically, we expect a triplet of quartets with a
total of 12 lines for the multiplet at 1.4 ppm. Six lines
only are visible.
J
1
J
2
When simulating the multiplet, it is a little
clearer to start with the triplet splitting by
the second methylene group. For J
2
, a
coupling constant of 7.3 Hz is assumed in
this simulation.
NH
2
H
2
C
CH
2
H
3
C
2H 2H2H 3H
The colours of the three lines have been slightly
modified to distinguish them. Let's now shift them a
little bit in the vertical direction to provide space for the
splitting caused by the methyl protons.
J
1
J
2
NH
2
H
2
C
CH
2
H
3
C
2H 2H2H 3H
The three equivalent methyl protons now create a quartet
from each line of the triplet. The coupling constant J
1
was
assumed here to be 7.1 Hz, a little bit smaller than J
2
.
J
1
J
2
NH
2
H
2
C
CH
2
H
3
C
2H 2H2H 3H
There are actually 12 lines in total. Let's add lines with – nearly – identical
chemical shifts.
J
1
Because of the different values for J
1
and J
2
, the addition is not perfect.
But the small differences cannot be resolved within the natural
linewidth of the NMR signals and we get a pseudo-sextet in the integral
ratio 1 : 5 : 10 : 10 : 5 : 1.
J
2
NH
2
H
2
C
CH
2
H
3
C
2H 2H2H 3H
There are three bonds between the protons of the second methylene group
and the protons of the amino group. We actually expect a vicinal coupling
constant J
3
.
J
1
J
2
J
3
However, the protons of this methylene group appear as a
triplet, just like the protons of the methyl group. With the
assumption of J
3
= 0 Hz, the triplet can be explained well.
But why should we miss the vicinal coupling constant J
3
?
N
H
2
C
CH
2
H
3
C
H
H
N
H
H
CH
2
CH
3
H
2
C
H: m = ½
H: m = -½
If OH and NH groups are involved, there often seems to exist no coupling pathway. The reason is
the rapid chemical exchange of these protons.
Let us assume two separate molecules of n-propylamine. We focus on the magnetic orientation of
the protons of the amino group only.
Statistically, there are almost as many protons with m = ½ as with m = -½.
N
H
2
C
CH
2
H
3
C
H
H
N
H
H
CH
2
CH
3
H
2
C
N
H
2
C
CH
2
H
3
C
H
H
N
H
H
CH
2
CH
3
H
2
C
Due to Brownian molecular motion, the amino groups of two molecules occasionally come into
contact.
H: m = ½
H: m = -½
H
N
H
2
C
CH
2
H
3
C
H
H
N
H
CH
2
CH
3
H
2
C
After a small shift of the binding electrons, there are protons with other magnetic orientation
bound to the nitrogen. The magnetic orientation of each individual proton remains unchanged.
H: m = ½
H: m = -½
N
H
2
C
CH
2
H
3
C
H
H
H
N
H
CH
2
CH
3
H
2
C
After the molecules have separated again, the magnetic orientation of the protons of the amino
group is different compared to the initial state.
H: m = ½
H: m = -½
After the molecules have separated again, the magnetic orientation of the protons of the amino
group is different compared to the initial state.
H: m = ½
H: m = -½
Repeating the chemical exchange fast enough results in permanent
random change of m of the amino group protons. The effect is identical
to broadband decoupling used to suppress carbon-proton-coupling
when measuring carbon spectra.
The coupling between the amino group and the neighbouring
methylene group becomes undetectable.
N
H
2
C
CH
2
H
3
C
H
H
H
N
H
CH
2
CH
3
H
2
C
Have a look. The intensity ratio of this triplet is not perfectly
1 : 2 : 1.
We must ignore the underlying effect here in the interest of a simple
explanation of the multiplets. A detailed explanation can be found in
the “Problem of the month February 2021”.
Generally, the very difficult to explain rule on the next page applies.
Don’t worry for the moment if the reasons behind it are not apparent.
A final remark
The reality is more complex
C
H
H
H
H
C Y
X
As soon as an asymmetrically
substituted ethane is recognized as a
structural fragment within an achiral
compound, the methylene protons of
this ethane fragment are always
chemically equivalent and always
magnetically non-equivalent.
Why achiral?
That‘s very simple. Within chiral compounds
the methylene protons are chemically non-
equivalent, which means, the question of
magnetic equivalence doesn‘t appear.
A
A`
X
X`
A final remark
The reality is more complex
