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The H-1
hydrogen-1 (proton) NMR spectrum of 2,3-dimethylbutane
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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 2,3-dimethylbutane
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H-1 proton NMR spectroscopy -
spectra index
See also
comparing infrared, mass, 1H NMR & 13C NMR
spectra of the structural alkane isomers of C6H14
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 - 2,3-dimethylbutane here.
The chemical shifts quoted in ppm on the diagram of
the H-1 NMR spectrum of 2,3-dimethylbutane 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 2,3-dimethylbutane molecule.
2,3-dimethylbutane C6H14
 ,
,
For more
see The molecular structure,
classification and
naming of alkanes
Interpreting the
H-1 NMR spectrum of
2,3-dimethylbutane
For relatively simple molecules, the low
resolution H-1 NMR spectrum of 2,3-dimethylbutane is a good starting point
(low resolution diagram above).
The hydrogen atoms (protons) of
2,3-dimethylbutane occupy only two
different chemical environments so that the low resolution NMR
spectra should show two peaks of different H-1 NMR chemical shifts (diagram above for
2,3-dimethylbutane).
(CH3)2CHCH(CH3)2
Note the ratio
12:2
(structural formula) = 6:1
(1H spectrum) ratio of the
two colours of the protons
in the two chemically different environments
Although there are 14 hydrogen atoms in the molecule,
there only 2 possible chemical
environments for the hydrogen atoms in 2,3-dimethylbutane molecule.
The integrated signal proton ratio 6:1 observed, corresponds with
the structural formula of 2,3-dimethylbutane.
The high resolution H-1 NMR
spectrum of 2,3-dimethylbutane
In terms of spin-spin coupling from the possible proton magnetic orientations,
for 2,3-dimethylbutane I
have only considered the interactions of
non-equivalent protons on adjacent carbon atoms
e.g. -CH2-CH3, -CH-CH2- protons
etc.
All low and high resolution spectra of
2,3-dimethylbutane
show two groups of proton resonances and in the ratio expected from the
formula of 2,3-dimethylbutane.
The ppm quoted on the diagram represent the peak
of resonance intensity for a particular proton group in the
molecule of 2,3-dimethylbutane - since the peak' is at the apex of a band of
H-1 NMR resonances due to spin - spin coupling field splitting effects - see high resolution
notes on 2,3-dimethylbutane below.
So, using the chemical shifts and applying the
n+1 rule
to 2,3-dimethylbutane and make some predictions using some
colour coding! (In problem solving you work the other way
round!)
δ (a) 1H
Chemical shift 0.84 ppm of the 12 'blue' CH3 protons
(CH3)2CHCH(CH3)2
The CH3 proton resonance is
split into a 1:1 doublet by the adjacent CH protons (n+1 = 2).
These 12 protons are all equivalent to each other - all
in the same 1H chemical environment.
Evidence for the presence of a CH group
in the molecule of 2,3-dimethylbutane
δ (b) 1H
Chemical shift 1.39 ppm of the CH protons
(CH3)2CHCH(CH3)2
At first sight you would think this CH proton resonance is split into an
octet of resonance lines by the 2 x CH3 and CH protons
on either side (n+1 = 8).
However, theoretically, these particular
pair of CH protons are equivalent to each other due to
their central symmetry in the symmetrical
2,3-dimethylbutane molecule and should not split each
other's resonance.
These two protons are in the
same 1H chemical environment, but unlike
the methyl protons, they are adjacent to each other.
Therefore, theoretically, the CH proton
resonance is only split by the methyl protons into a
1:6:15:20:15:6:1 septet.
I would appreciate a second
opinion on this interpretation.
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Comparing the infrared, mass, 1H NMR and 13C NMR
spectra of the five structural alkane isomers of C6H14
NOTE: The images are linked to their
original detailed spectral analysis pages AND can be doubled in
size with touch screens to
increase the definition to the original hexane,
2-methylpentane, 3-methylpentane, 2,2-dimethylbutane and
2,3-dimethylbutane image sizes. These five molecules
are structural isomers of saturated alkanes of molecular formula C6H14
and
exemplify the infrared, mass, 1H NMR and 13C NMR spectra of lower
aliphatic alkanes (non-cyclic alkanes). |
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Infrared spectra below. |
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INFRARED SPECTRA:
Apart from the significant differences in the fingerprint region at
wavenumbers 1500 to 400 cm-1, there are no other
great striking differences, but each could be identified from
its infrared spectrum. All the absorption
bands are typical of molecules containing saturated alkyl structure and
there are no characteristic infrared absorptions due to a specific
functional group. |
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Infrared spectra above, mass spectra below. |
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MASS SPECTRA: Base ion
peaks plus m/z comments.
Hexane: m/z 57, 42 and 56 prominent
2-methylpentane: m/z 43, 42 and 71 prominent
3-methylpentane: m/z 57, 41 and 56 prominent
2,2-dimethylbutane: m/z 43, 41, 57 and 71
prominent
2,3-dimethylbutane: m/z 43, 41, 42 and 71
prominent |
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Mass spectra above, 1H NMR spectra below. |
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1H NMR SPECTRA: They can
all be distinguished by their different integrated proton ratios -
need very high resolution.
Hexane:
3 1H
δ shifts, H ratio 3:2:2 (6:4:4 in formula)
2-methylpentane:
5 1H
δ shifts, H ratio 6:3:2:2:1
3-methylpentane:
4 1H
δ shifts, H ratio 6:4:3:1
2,2-dimethylbutane: 3 1H
δ shifts, H ratio 9:3:2
2,3-dimethylbutane: 2 1H
δ shifts, H ratio 6:1 (12:2 in formula) |
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1H NMR spectra above, 13C NMR spectra below. |
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13C NMR SPECTRA: From the
number of shifts, you can't distinguish (iii) and (iv) but you can
distinguish them from (i), (ii) and (v). (i) Hexane: 3 13C
δ shifts
(ii) 2-methylpentane: 5 13C
δ shifts
(iii) 3-methylpentane: 4 13C
δ shifts
(iv) 2,2-dimethylbutane: 4 13C
δ shifts
(v) 2,3-dimethylbutane: 2 13C
δ shifts |
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13C NMR spectra above. |
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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 |
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1 |
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| 1
creates a doublet |
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1 |
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1 |
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| 2
creates a triplet |
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1 |
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2 |
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1 |
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| 3
creates a quartet |
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1 |
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3 |
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3 |
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1 |
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| 4
creates a quintet |
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1 |
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4 |
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6 |
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4 |
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1 |
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| 5
creates a sextet |
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1 |
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5 |
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10 |
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10 |
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5 |
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1 |
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| 6
creates a septet |
1 |
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6 |
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15 |
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20 |
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15 |
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6 |
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1 |
Key words & phrases: Interpreting the proton H-1 NMR spectra of
2,3-dimethylbutane, low resolution & high resolution proton
nmr spectra of 2,3-dimethylbutane, H-1 nmr spectrum of 2,3-dimethylbutane, understanding the
hydrogen-1 nmr spectrum of 2,3-dimethylbutane, explaining the line splitting patterns in the
high resolution H-1 nmr spectra of 2,3-dimethylbutane, revising the H-1 nmr spectrum of
2,3-dimethylbutane,
proton nmr of 2,3-dimethylbutane, ppm chemical shifts of the H-1 nmr spectrum of
2,3-dimethylbutane,
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 2,3-dimethylbutane, how to work out the
number of chemically different protons in the structure of the
2,3-dimethylbutane organic
molecule, how to analyse the chemical shifts in the hydrogen-1 H-1 proton NMR
spectrum of 2,3-dimethylbutane using the n+1 rule to explain the spin - spin coupling ine
splitting in the proton nmr spectrum of 2,3-dimethylbutane deducing the nature of the protons
from the chemical shifts ppm in the H-1 nmr spectrum of 2,3-dimethylbutane
examining the 1H nmr spectrum of 2,3-dimethylbutane analysing the 1-H nmr spectrum of
2,3-dimethylbutane how do you sketch and interpret the H-1 NMR spectrum of
2,3-dimethylbutane interpreting interpretation of
the H-1 proton NMR spectrum of 2,3-dimethylbutane
Molecular structure diagram of the
proton NMR diagram for the 1H NMR spectrum of 2,3-dimethylbutane. The proton ratio in the
1H NMR spectrum of 2,3-dimethylbutane. Deducing the number of different chemical
environments of the protons in the 2,3-dimethylbutane molecule from the 1H chemical shifts
in the hydrogen-1 NMR spectrum of 2,3-dimethylbutane. Analysing the high resolution 1H NMR
spectrum of 2,3-dimethylbutane. Analysing the low resolution 1H NMR spectrum of
2,3-dimethylbutane. You
may need to know the relative molecular mass of 2,3-dimethylbutane to deduce the molecular
formula from the proton ratio of the 1H NMR spectrum of 2,3-dimethylbutane. Revision notes
on the proton NMR spectrum of 2,3-dimethylbutane. Matching and deducing the structure of
the 2,3-dimethylbutane molecule from its hydrogen-1 NMR spectrum.
Proton NMR spectroscopy of aliphatic
alkanes,
1H NMR spectra of 2,3-dimethylbutane, a structural isomer of molecular formula
C6H14
Links associated
with 2,3-dimethylbutane
The chemistry of ALKANES
revision notes INDEX
The infrared spectrum of
2,3-dimethylbutane
The mass spectrum of
2,3-dimethylbutane
The C-13 NMR spectrum of
2,3-dimethylbutane
H-1 proton NMR spectroscopy index
(Please
read 8 points at the top of the 1H NMR index page)
ALL SPECTROSCOPY INDEXES
All Advanced Organic
Chemistry Notes
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