The H-1
hydrogen-1 (proton) NMR spectrum of butan-1-ol
(1-butanol)
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Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study
Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US
K12 grade 11 grade 12 organic chemistry courses involving molecular
spectroscopy analysing H-1 NMR spectra of butan-1-ol 1-butanol
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H-1 proton NMR spectroscopy -
spectra index
TMS is the acronym for tetramethylsilane, formula Si(CH3)4,
whose protons are arbitrarily given a chemical shift of 0.0 ppm.
This is the 'standard' in 1H NMR spectroscopy and all
other proton shifts, called chemical shifts, depend on the
individual (electronic) chemical environment of the hydrogen atoms
in an organic molecule - butan-1-ol here.
The chemical shifts quoted in ppm on the diagram of
the H-1 NMR spectrum of butan-1-ol represent the peaks of the
intensity of the chemical shifts of (which are often groups of split
lines at high resolution) AND the relative integrated areas under
the peaks gives you the ratio of protons in the different chemical
environments of the butan-1-ol molecule i.e. 3 : 2 : 2 : 2 :1.
Butan-1-ol C4H10O
,
.
Interpreting the
H-1 NMR spectrum of
butan-1-ol (1-butanol, n-butyl alcohol)
For relatively simple molecules, the low
resolution H-1 NMR spectrum of butan-1-ol is a good starting point.
The hydrogen atoms (protons) of butan-1-ol occupy
5
different chemical environments so that the low resolution NMR
spectra should show 5 peaks of different H-1 NMR chemical shifts (diagram above for
butan-1-ol).
CH3CH2CH2CH2OH
Note the ratio 3:2:2:2:1 of the 5 colours of the protons
in the 5 chemically different environments
Although there are 10 hydrogen atoms in the molecule,
there only 5 possible chemical
environments for the hydrogen atoms in butan-1-ol molecule.
The proton ratio
3:2:2:2:1
observed, corresponds with
the structural formula of butan-1-ol.
The high resolution spectrum
of butan-1-ol
BUT, an important note about the hydroxyl group on butan-2-ol (for
pre-university students):
Unless the alcohol is completely free of
water (difficult), the hydrogen on the -O-H
hydroxyl group and any hydrogens on the adjacent carbon
don't interact to produce any spin-spin splitting. Therefore
the -OH peak shows up as a singlet and you don't usually
have to consider its effect on any hydrogen atoms, if
present on the adjacent carbon atom (C-OH),
and, neither do you have to consider the splitting effect of
adjacent C-H protons on the hydrogen of the OH group.
The low and high resolution spectra of butanal
show 5 groups of protons and in the ratio expected from the
formula of butan-1-ol, namely
3:2:2:2:1, but the high resolution spectrum is very complex.
The ppm quoted on the diagram represent the peak
of resonance intensity for a particular proton group in the
molecule of butan-1-ol - since the peak' is at the apex of a
band of H-1 NMR resonances due to spin - spin filed splitting
effects - see high resolution notes on butan-1-ol below.
CH3CH2CH2CH2OH
(below I will refer to the 1st, 2nd and 3rd CH2 groups from
left to right)
So, using the chemical shifts and applying the
n+1 rule to
butan-1-ol
Chemical shift 0.94 ppm CH3
protons
The end methyl group proton resonance is
split into a 1:2:1 triplet by the first CH2
group (2 protons, n+1 = 3 = triplet).
Evidence for the presence of a CH2 group
in the molecule of butan-1-ol.
Chemical shift 1.39 ppm 1st CH2
protons
The first CH2 proton
resonance is split by the CH3 protons AND the
2nd CH2 protons into a 1:5:10:10:5:1 sextet
(5 protons, n+1 = 6 = sextet).
Evidence for the presence of a CH3CH2CH2 group
in the molecule of butan-1-ol
Chemical shift 1.53 ppm 2nd CH2
protons
The 2nd CH2 proton resonance
is split on either side by the 1st and 3rd CH2
protons into a 1:4:6:4:1 quintet (4 protons, n+1 =5 =
quintet).
Evidence for the presence of a CH2CH2CH2 group
in the molecule of butan-1-ol.
Chemical shift 2.24 ppm OH proton
The OH proton resonance is often seen
and 'portrayed' as a singlet.
Normally the O-H proton resonance is not
split by adjacent carbon atom protons and neither does it, in turn,
split the resonance of the same adjacent protons (see extra
note below).
Chemical shift 2.24 ppm 3rd CH2
protons
You can consider that the 3rd CH2
proton resonance is split by the 2nd CH2
protons into a 1:2:1 triplet (2 protons, n+1 = 3 =
triplet, as on diagram).
Note that no splitting due to OH proton
is detected.
Extra
note
on the OH proton resonance
If the alcohol is impure, containing water or
any source of labile protons, because water and the alcohol exchange protons
e.g.
R-O-H
+ H-O-H
R-O-H
+ H-O-H
This means the CH2 protons no
longer experience a 'simple' local field from one
singlet proton from two possible orientations, but, over
a finite period, experience the averaging effect of
exchanging protons.
This removes the spin - spin coupling effect and
the OH proton resonance just shows up as a singlet if the
butan-1-ol contains even a trace of water
(or acid).
This sort of exchange cannot happen with
the alkyl protons, but is common with molecules
containing a hydroxylic (OH) hydrogen atom like alcohols
and carboxylic acids.
Not only that, you also get proton transfer
between the alcohol molecules i.e.
R-O-H
+ H-O-R
R-O-H
+ H-O-R
which gives the same effect as traces of
water of acid.
So, in
butan-1-ol, all
you usually see in the H-1 NMR spectrum is the mutual splitting of the CH2
and CH3 proton resonances plus a singlet line
for the OH proton resonance.
Number of protons 1H
causing splitting |
Splitting pattern produced from the
n+1 rule and the theoretical ratio of line intensities |
0
means no splitting |
|
|
|
|
|
|
1 |
|
|
|
|
|
|
1
creates a doublet |
|
|
|
|
|
1 |
|
1 |
|
|
|
|
|
2
creates a triplet |
|
|
|
|
1 |
|
2 |
|
1 |
|
|
|
|
3
creates a quartet |
|
|
|
1 |
|
3 |
|
3 |
|
1 |
|
|
|
4
creates a quintet |
|
|
1 |
|
4 |
|
6 |
|
4 |
|
1 |
|
|
5
creates a sextet |
|
1 |
|
5 |
|
10 |
|
10 |
|
5 |
|
1 |
|
6
creates a septet |
1 |
|
6 |
|
15 |
|
20 |
|
15 |
|
6 |
|
1 |
Key words & phrases: 1-butanol n-butyl
alcohol
Interpreting the proton H-1 NMR spectra of
butan-1-ol, low resolution & high resolution proton
nmr spectra of butan-1-ol, H-1 nmr spectrum of butan-1-ol, understanding the
hydrogen-1 nmr spectrum of butan-1-ol, explaining the line splitting patterns in the
high resolution H-1 nmr spectra of butan-1-ol, revising the H-1 nmr spectrum of
butan-1-ol,
proton nmr of butan-1-ol, ppm chemical shifts of the H-1 nmr spectrum of
butan-1-ol,
explaining and analyzing spin spin line splitting in the H-1 nmr spectrum, how
to construct the diagram of the H-1 nmr spectrum of butan-1-ol, how to work out the
number of chemically different protons in the structure of the butan-1-ol organic
molecule, how to analyse the chemical shifts in the hydrogen-1 H-1 proton NMR
spectrum of butan-1-ol using the n+1 rule to explain the spin - spin coupling ine
splitting in the proton nmr spectrum of butan-1-ol deducing the nature of the
protons from the chemical shifts ppm in the H-1 nmr spectrum of butan-1-ol
examining the 1H nmr spectrum of butan-1-ol analysing the 1-H nmr spectrum
of butan-1-ol how do you sketch and interpret the H-1 NMR spectrum of butan-1-ol
1-butanol n-butyl alcohol
Links associated
with butan-1-ol
The chemistry of ALCOHOLS
revision notes INDEX
H-1 proton NMR spectroscopy index
(Please
read 8 points at the top of the 1H NMR index page)
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All Advanced Organic
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