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Interpreting the H-1
hydrogen-1 (proton) NMR spectrum of octane
[Author
©
Dr Phil Brown PhD:
Doc Brown's advanced level organic chemistry exam revision notes
suitable for students of UK A level chemistry courses & US K12 grade
11, grade 12 and AP honors chemistry courses:
Molecular
spectroscopy analysis of
octane
[spectra page updated
Mar 25th 2026
*]
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1H NMR spectrum of
H3C(CH2)6CH3
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Associated links for octane
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H-1 proton NMR spectroscopy -
spectra index
Introductory note on the 1H NMR spectra of octane
Students and teachers please note my explanation of the
proton NMR spectrum of octane is designed for advanced, but
pre-university, chemistry courses.
The chemical shift
δ splitting pattern effects for
octane are
confined to a proton spin-spin
coupling effects analysed using the n+1 rule for adjacent
non-equivalent proton fields (n is the number of neighbouring
protons in a non-equivalent different chemical environment for the
octane molecule).
It is assumed that the integrated intensities of the
1H
NMR δ
chemical shifts give the ratio of the protons in the different
non-equivalent chemical environments of the octane molecule.
The most common solvent used for investigating the 1H NMR
spectrum of compounds like octane, is CDCl3 and other
deuterated solvents to avoid confusion with a 1H NMR
signal, 2D (2H) has a different NMR chemical
shift.
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 - octane here.
The chemical shifts quoted in ppm on the diagram of
the H-1 NMR spectrum of octane 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 octane molecule.
Octane, C8H18,
CH3(CH2)6CH3, CH3CH2CH2CH2CH2CH2CH2CH3,
an alkane
The molecular structure and
naming of alkanes
Interpreting the
H-1 NMR spectrum of
octane
For relatively simple molecules, the low
resolution H-1 NMR spectrum of octane is a good starting point
(low resolution diagram above).
Theoretically the hydrogen atoms (protons) of octane occupy
4
different chemical environments, but the low resolution NMR
spectra only shows two peaks of different H-1 NMR chemical shifts (diagram above for
octane).
These would be in the integrated proton ratio of
(2:1) 6:12 for the CH3:CH2 proton
groups ratio.
CH3CH2CH2CH2CH2CH2CH2CH3
Note the theoretical proton ratio of (3:2:2:2)
6:4:4:4 of the 4 colours of the protons
in the 4 chemically different environments because of the
symmetry of the octane molecule.
Chemical shifts
(a) to (d) on the H-1 NMR
spectrum diagram for octane.
Although there are 18 hydrogen atoms in the molecule,
theoretically
there are only 4 possible different chemical
environments for the hydrogen atoms in octane molecule.
The high resolution 1H NMR
spectrum of octane
The ppm quoted on the diagram represent the peak
of resonance intensity for a particular proton group in the
molecule of octane - 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 octane below.
So, using the chemical shifts and applying the
n+1 rule to
octane
and make some predictions using some colour coding! (In problem
solving you work the other way round!)
CH3CH2CH2CH2CH2CH2CH2CH3
has four different proton chemical environments
(a)
1H
Chemical shift 0.88 ppm for the two methyl groups.
In each case the CH3 group
proton resonance is split into a 1:2:1 triplet by the
neighbouring CH2 group.
Evidence for the presence of a CH2 group
in the molecule of octane
(b) to (d) 1H
Chemical shift ~1.26 ppm
All the proton resonances for the six CH2
groups are very close together - three pairs of
equivalent protons in three different chemical
environments.
Theoretically, at very high resolution,
the spectrum for octane would reveal these three
different resonances split into:
(b)
would be split into a 1:5:10:10:5:1 sextet from CH2-CH2-CH3
(c)
would be split into a 1:4:6:4:1 quintet from
CH2-CH2-CH2
(d)
would be only split into a 1:2:1 triplet from CH2-CH2-CH2
because the (d) CH2 protons are
chemically equivalent to each other and their proton
fields cannot split each other.
Evidence for the presence of CH2-CH2-CH2
and CH3-CH2-CH2 groups
in the molecule of octane.
The splitting pattern from proton
spin-spin coupling effects is analysed using the n+1 rule for
adjacent non-equivalent proton fields (n is the number of
neighbouring protons in a non-equivalent different chemical
environment).
|
Number of directly adjacent protons 1H
causing splitting |
Splitting pattern produced from the
n+1 rule on spin-spin coupling 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:
C8H18
Interpreting the proton H-1 NMR spectra of octane, low resolution & high resolution proton
nmr spectra of octane, H-1 nmr spectrum of octane, understanding the
hydrogen-1 nmr spectrum of octane, explaining the line splitting patterns in the
high resolution H-1 nmr spectra of octane, revising the H-1 nmr spectrum of
octane,
proton nmr of octane, ppm chemical shifts of the H-1 nmr spectrum of octane,
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 octane, how to work out the
number of chemically different protons in the structure of the octane organic
molecule, how to analyse the chemical shifts in the hydrogen-1 H-1 proton NMR
spectrum of octane using the n+1 rule to explain the spin - spin coupling ine
splitting in the proton nmr spectrum of octane deducing the nature of the protons
from the chemical shifts ppm in the H-1 nmr spectrum of octane
examining the 1H nmr spectrum of octane analysing the 1-H nmr spectrum of
octane how do you sketch and interpret the H-1 NMR spectrum of octane
interpreting interpretation of the 1H proton NMR spectrum of n-octane
How do you interpret the H-1 NMR spectrum of n-octane How to interpret
the H-1 NMR spectrum of n-octane Explanatory diagram of the chemical
shifts of the 1H H-1 proton NMR spectrum of the n-octane
molecule in terms of its molecular structure. Listing data of all the chemical shift peaks in ppm in the
proton NMR spectrum of n-octane. How to explain the H-1 NMR spectrum of
n-octane. The chemical shifts and integrated values of the proton ratios in the 1-H NMR
spectrum of the n-octane molecule. How to work out the molecular
structure of the n-octane molecule from its proton NMR spectrum. The uses
and distinctive features of the proton NMR spectrum of the n-octane
molecule explained. What does the H-1 proton NMR spectrum chemical
shifts tell us about the
structure and properties of the n-octane
molecule? explaining the spin-spin proton coupling effects in the 1H
NMR spectrum of n-octane. interpretation
diagram explaining the proton splitting pattern produced from the
n+1 rule and the theoretical ratio of chemical shift
δ and values of
intensities for the proton NMR spectrum lines of n-octane
What next?
links associated
with
octane
The infrared
spectrum of octane
The mass spectrum
of octane
The C-13 NMR spectrum of octane
The chemistry of ALKANES
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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