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Interpreting
and explaining the
1H NMR spectrum of 1-bromobutane
[Author
©
Dr Phil Brown GRIC, 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
1-bromobutane (1H NMR spectra)
[updated
RE-EDIT]
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Re-edit 1H NMR spectrum of CH3CH2CH2CH2Br
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Links associated
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H-1 proton NMR spectroscopy -
spectra index
See also
comparing
infrared, mass, 1H NMR & 13C NMR spectra of 4 halogenoalkane isomers of C4H9Br
and
Isomers of molecular formula
C4H9X (where
X =
F, Cl, Br or I and basic data on NMR chemical shifts)
Practise exam questions based on the 1H NMR
spectrum of 1-bromobutane
Introductory note on the 1H NMR spectra of 1-bromobutane
Students and teachers please note my explanation of the
proton NMR spectrum of 1-bromobutane is designed for advanced, but
pre-university, chemistry courses.
The chemical shift
δ splitting pattern effects for
1-bromobutane 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 1-bromobutane molecule).
It is assumed that the integrated intensities of the
δ chemical
shifts give the ratio of the protons in the different non-equivalent chemical
environments in the 1-bromobutane molecule.
The most common solvent used for investigating the 1H NMR
spectrum of compounds like 1-bromobutane, is CDCl3 and
other deuterated solvents to avoid confusion with a 1H
NMR signal, 2D (2H) has a different 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 resonances, called chemical shifts, are measured
with respect to the TMS, and depend on the
individual (electronic) chemical environment of the hydrogen atoms
in an organic molecule - 1-bromobutane here.
The chemical shifts quoted in ppm on the diagram of
the H-1 NMR spectrum of 1-bromobutane 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 1-bromobutane molecule.
1-bromobutane,
C4H9Br,
CH3CH2CH2CH2Br,
CH3-CH2-CH2-CH2-Br
The molecular structure and naming of haloalkanes
Interpreting the
H-1 NMR spectrum of
1-bromobutane
In terms of spin-spin coupling from the possible proton magnetic orientations,
for 1-bromobutane I
have only considered the interactions of
non-equivalent protons on adjacent carbon atoms
e.g. -CH2-CH3,
R-CH2-CH2-X, protons
etc.
For relatively simple molecules, the low
resolution H-1 NMR spectrum of 1-bromobutane is a good starting point
(low resolution diagram above).
The 9 hydrogen atoms (protons) of 1-bromobutane occupy
4
different chemical environments so that the low resolution NMR
spectra should show
4 principal peaks of different H-1 NMR chemical shifts
(diagram above for
1-bromobutane).
CH3CH2CH2CH2Br
Note the
proton ratio 3:2:2:2
of the 4 colours of the
9 protons of 1-bromobutane
in the 4 chemically different proton environments
Chemical shifts (a) to (d) on the H-1 NMR
spectrum diagram for 1-bromobutane.
Although there are 9 hydrogen atoms in the molecule, the
proton NMR spectrum shows there are only
4 possible different chemical
environments for the hydrogen atoms in 1-bromobutane molecule.
The integrated signal
proton ratio 3:2:2:2 observed
in the high resolution H-1 NMR spectrum, corresponds with
the structural formula of 1-bromobutane.
The high resolution 1H NMR
spectrum of 1-bromobutane
The high resolution spectra of 1-bromobutane
also
shows 4 groups of proton resonances and in the 3:2:2:2 ratio expected from the
structural
formula of 1-bromobutane, but we can now consider the splitting of
resonance lines from the spin-spin coupling in the molecule of
1-bromobutane.
The ppm quoted on the diagram represent the peak
of resonance intensity for a particular proton group in the
molecule of 1-bromobutane - 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 1-bromobutane below.
So, using the chemical shifts and applying the
n+1 rule to
1-bromobutane
and make some predictions using some colour coding! (In problem
solving you work the other way round!)
1H NMR resonance
(a) 1H
Chemical shift 0.90: CH3-CH2-CH2-CH2-Br
The 1H resonance is split into a 1:2:1
triplet by the adjacent CH2 protons (n+1 =
3).
Evidence for the presence of a CH2 group
in the molecule of 1-bromobutane
1H NMR resonance
(b) 1H
Chemical shift 1.31 ppm: CH3-CH2-CH2-CH2-Br
This resonance is split into
1:5:10:10:5:1 sextet by the CH3 and CH2 protons
on either side (n+1 = 6).
Evidence for the presence of a CH3-CHx-CH2 grouping
in the molecule of 1-bromobutane (x can be 1 or 2, as in
this case).
1H NMR resonance
(c) 1H
Chemical shift 1.82 ppm: CH3-CH2-CH2-CH2-Br
This resonance is split into
1:4:6:4:1 quintet by the CH2 and CH2
protons on either side (n+1 = 6).
Evidence for the presence of a CH2-CHx-CH2 grouping
in the molecule of 1-bromobutane (x can be 1 or 2, as in
this case).
1H NMR resonance
(d) 1H
Chemical shift 3.51 ppm : CH3-CH2-CH2-CH2-Br
The 1H resonance is split into a 1:2:1
triplet by the adjacent CH2 protons (n+1 =
3).
Evidence for the presence of another CH2 group
in the molecule of 1-bromobutane.
Note the decreasing effect on the 1H chemical shift as the
proton is further from the more electronegative bromine atom in
1-bromobutane.
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QUESTIONS
Advanced A-level chemistry - practise exam questions on
the 1H NMR spectrum of 1-bromobutane
This is a joint AI-doc b experiment!
Jot
down your responses and check out the answers:
ANSWERS
If you think there are
any errors, please email me asap at
chem55555@hotmail.com
I don't mind if students/teachers do a selected printout
of these questions and answers.
Question 1
The structure of 1‑bromobutane is:
CH3–CH2–CH2–CH2–Br
In its ¹H NMR spectrum, how
many distinct proton signals are observed?
A. 2 B. 3
C. 4 D. 5
Question 2 –
integration pattern
Which integration pattern best
matches the ¹H NMR of 1‑bromobutane?
A. 3H : 2H : 2H : 2H
B. 2H : 2H : 2H : 2H
C. 3H : 3H : 2H : 2H
D. 1H : 2H : 3H : 2H
Question 3
Which group in
1‑bromobutane gives the most downfield
(highest ppm) ¹H NMR signal?
A.
CH3–
B.
CH3–CH2–
C.
–CH2–CH2–
(middle CH2) D.
–CH2–Br
Question 4
In 1‑bromobutane,
what is the expected splitting pattern
for the terminal
CH3–
group in the ¹H NMR spectrum?
A. Singlet B. Doublet
C. Triplet D. Quartet
Question 5 –
splitting of the CH2–Br group
What is the expected splitting pattern
for the
–CH2–Br
group in 1‑bromobutane?
Question 6
You are given two
spectra: one for 1‑bromobutane and one for
2‑bromobutane. Which feature would most
clearly indicate the spectrum is
1‑bromobutane?
A. A single ¹H NMR signal integrating to 10H
B. A downfield triplet (2H) around 3–4 ppm from
CH2–Br
C. A singlet (1H) around 2 ppm from a CH–Br
proton
D. No signals above 1 ppm
Correct answer: B
If you think there are
any errors, please email me asap at
chem55555@hotmail.com
Jot
down your responses and check out the answers:
ANSWERS
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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) and applied to the 1H NMR spectrum of
1-bromobutane.
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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 |
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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 |
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Comparing the infrared, mass, 1H NMR and 13C NMR
spectra of the 4 halogenoalkane isomers of C4H9Br
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 1-bromobutane,
2-bromobutane, 1-bromo-2-methylpropane and 2-bromo-2-methylpropane
image sizes. These four molecules
are structural isomers of molecular formula C4H9Br
and
exemplify the infrared, mass, 1H NMR and 13C NMR spectra of lower
aliphatic halogenoalkanes (haloalkanes, alkyl halides,
bromoalkanes, alkyl bromides). |
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INFRARED SPECTRA
(above):
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. |
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MASS SPECTRA (above):
All four give the parent molecular ions of m/z 136 and 138, but it is
only a relatively tiny peak for 2-bromobutane and 2-bromo-2-methylpropane. All four
give the base ion peak of m/z 57. All four give prominent peaks
for m/z ions 27, 29, 39 and 41 and all give a tiny peak from an ionised
iodine atom at m/z 127. They look quite similar to me and lack a
clear fingerprint fragmentation pattern. There are small
differences in the relative abundances (peak heights) for pairs
of ions involving 79Br/81Br isotopes e.g.
m/z 93/95, 107/109 and 121/123. 1-bromo-2-methylpropane is the
only one of the four to have a prominent peak for the m/z 43
ion. |
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1H NMR SPECTRA
(above): The 1H NMR spectra of all four molecules give different
integrated proton ratios i.e.1-bromobutane
four peaks of ratio 3:2:2:2; 2-bromobutane four peaks of
ratio 3:3:2:1,
1-bromo-2-methylpropane three peaks of ratio 6:2:1 and
2-bromo-2-methylpropane gives just one peak '1' (effectively no ratio
involved), so all four molecular structures can be distinguished from each other by their
1H NMR spectra proton ratios, numbers of peaks and (n+1)
rule splitting patterns. |
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13C NMR SPECTRA
(above): The
13C NMR spectra of the four molecules show various numbers of
carbon-13 chemical environments i.e 1-bromobutane and
2-bromobutane show four 13C NMR resonances,
1-bromo-2-methylpropane three 13C NMR resonances and
2-bromo-2-methylpropane only two 13C resonances. Therefore
1-bromo-2-methylpropane and 2-bromo-2-methylpropane can be
distinguished from the other three by their number of resonances
in their 13C NMR spectra, but 1-bromobutane and 2-bromobutane
cannot be distinguished from each other from their number of 13C
NMR resonance lines - other data would be required. |
Key words & phrases:
C4H9Br
CH3CH2CH2CH2Br
Interpreting the proton H-1 NMR spectra of 1-bromobutane, low resolution & high resolution proton
nmr spectra of 1-bromobutane, H-1 nmr spectrum of 1-bromobutane, understanding the
hydrogen-1 nmr spectrum of 1-bromobutane, explaining the line splitting patterns from
spin-spin coupling in the
high resolution H-1 nmr spectra of 1-bromobutane, revising the H-1 nmr spectrum of
1-bromobutane,
proton nmr of 1-bromobutane, ppm chemical shifts of the H-1 nmr spectrum of
1-bromobutane,
explaining and analyzing spin line splitting in the H-1 nmr spectrum, how
to construct the diagram of the H-1 nmr spectrum of 1-bromobutane, how to work out the
number of chemically different protons in the structure of the 1-bromobutane organic
molecule, how to analyse the chemical shifts in the hydrogen-1 H-1 proton NMR
spectrum of 1-bromobutane using the n+1 rule to explain the spin - spin coupling ine
splitting in the proton nmr spectrum of 1-bromobutane deducing the nature of the protons
from the chemical shifts ppm in the H-1 nmr spectrum of 1-bromobutane
examining the 1H nmr spectrum of 1-bromobutane analysing the 1-H nmr spectrum of
1-bromobutane
how do you sketch and interpret the H-1 NMR spectrum of 1-bromobutane
interpreting interpretation of the 1H proton spin-spin coupling causing line
splitting in the NMR spectrum of 1-bromobutane
assignment of chemical shifts in the
proton 1H NMR spectrum of 1-bromobutane formula explaining spin-spin coupling
for line splitting for 1-bromobutane
n-butyl iodide alkyl halide
functional group haloalkane halogenoalkane
bromoalkane Stick diagram of the relative abundance
of ionised fragments in the fingerprint pattern of the mass spectrum of
1-bromobutane. Table of the m/e m/z values and formula of the ionised fragments in the
mass spectrum of 1-bromobutane. The m/e m/z value of the molecular ion peak in the
mass spectrum of 1-bromobutane. The m/e m/z value of the base ion peak in the
mass spectrum of 1-bromobutane. Possible examples of equations showing the formation
of the ionised fragments in 1-bromobutane. Revision notes on the mass spectrum of
1-bromobutane.
Matching and deducing the structure of the 1-bromobutane molecule from its mass
spectrum. Mass spectroscopy of
aliphatic halogenoalkanes bromoalkanes alkyl bromides,
mass spectra of 1-bromobutane, an isomer of molecular formula
C4H9Br
How do you interpret the H-1 NMR spectrum of
1-bromobutane How to interpret
the H-1 NMR spectrum of 1-bromobutane Explanatory diagram of the chemical
shifts of the 1H H-1 proton NMR spectrum of the 1-bromobutane
molecule in terms of its molecular structure. Listing data of all the chemical shift peaks in ppm in the
proton NMR spectrum of 1-bromobutane. How to explain the H-1 NMR spectrum of
1-bromobutane. The chemical shifts and integrated values of the proton ratios in the 1-H NMR
spectrum of the 1-bromobutane molecule. How to work out the molecular
structure of the 1-bromobutane molecule from its proton NMR spectrum. The uses
and distinctive features of the proton NMR spectrum of the
1-bromobutane
molecule explained. What does the H-1 proton NMR spectrum chemical
shifts tell us about the
structure and properties of the 1-bromobutane
molecule? explaining the spin-spin proton coupling effects in the 1H
NMR spectrum of 1-bromobutane. 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 1-bromobutane
Links associated
with
1-bromobutane
The chemistry of HALOGENOALKANES (haloalkanes)
revision notes INDEX
The infrared spectrum of
1-bromobutane (n-butyl bromide)
The mass spectrum of 1-bromobutane
(n-butyl bromide)
The C-13 NMR spectrum of
1-bromobutane (n-butyl bromide)
H-1 proton NMR spectroscopy index
(Please
read 8 points at the top of the 1H NMR index page)
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ANSWERS
Advanced A-level chemistry - practise exam questions on
the 1H NMR spectrum of 1-bromobutane
If you think there are
any errors, please email me asap at
chem55555@hotmail.com
I don't mind if students/teachers do a selected printout
of these questions and answers.
Question 1
The structure of 1‑bromobutane is:
CH3–CH2–CH2–CH2–Br
In its ¹H NMR spectrum, how
many distinct proton signals are observed?
A. 2 B. 3
C. 4 D. 5
Correct answer: C
Explanation: There are four
different proton environments:
-
CH3– (methyl at
the end)
-
CH2–CH2–
(first CH2
next to CH2)
-
–CH
2–CH2–
(middle CH2)
-
–CH
2–Br
(CH2
directly bonded to Br)
Each gives a separate signal → 4 signals.
Common misconception: Thinking
the two internal CH₂ groups are equivalent— they
are not, because one is closer to Br than the
other.
Question 2 –
integration pattern
Which integration pattern best
matches the ¹H NMR of 1‑bromobutane?
A. 3H : 2H : 2H : 2H
B. 2H : 2H : 2H : 2H
C. 3H : 3H : 2H : 2H
D. 1H : 2H : 3H : 2H
Correct answer: A
Explanation: Total protons:
-
CH3 (3H)
-
three CH2 groups (each 2H)
So the integration must be 3H : 2H : 2H
: 2H.
Common misconception: Assuming
integration numbers must add to a “round” total
like 10 or 12—what matters is the ratio,
not the absolute numbers.
Question 3
Which group in
1‑bromobutane gives the most downfield
(highest ppm) ¹H NMR signal?
A.
CH3–
B.
CH3–CH2–
C.
–CH2–CH2–
(middle CH2) D.
–CH2–Br
Correct answer: D
Explanation: The CH2
directly attached to bromine is
strongly deshielded by the electronegative Br
atom, so it appears furthest downfield
(largest ppm, typically ~3–4 ppm).
Common misconception: Thinking
the CH3 group is always most
downfield—actually, CH3 is usually most
upfield in simple alkyl chains.
Question 4
In 1‑bromobutane,
what is the expected splitting pattern
for the terminal
CH3–
group in the ¹H NMR spectrum?
A. Singlet B. Doublet
C. Triplet D. Quartet
Correct answer: C
Explanation: The CH3
group is adjacent to a CH2
group (2 equivalent neighbouring protons). Using
the n + 1 rule:
n=2⇒
2+1=3
→ triplet.
Common misconception:
Forgetting that only equivalent
neighbouring protons count in n+1; the
CH3 is not directly coupled to all
protons in the chain.
Question 5 –
splitting of the CH2–Br group
What is the expected splitting pattern
for the
–CH2–Br
group in 1‑bromobutane?
A. Triplet B.
Quartet C. Doublet
D. Multiplet with no clear pattern
Correct answer:
A
Explanation:
–CH2–Br
is adjacent to a CH2
group (2 equivalent protons). Using n+1:
n=2⇒3
→ triplet.
Common misconception: Thinking
bromine causes splitting—halogens like Br and Cl
do not normally cause
observable proton splitting in ¹H NMR.
Question 6
You are given two
spectra: one for 1‑bromobutane and one for
2‑bromobutane. Which feature would most
clearly indicate the spectrum is
1‑bromobutane?
A. A single ¹H NMR signal integrating to 10H
B. A downfield triplet (2H) around 3–4 ppm from
CH2–Br
C. A singlet (1H) around 2 ppm from a CH–Br
proton
D. No signals above 1 ppm
Correct answer: B
Explanation: 1‑bromobutane has
a CH2–Br group
giving a downfield triplet (2H)
around 3–4 ppm. 2‑bromobutane has a
CH–Br (secondary carbon), giving a
1H signal with a different
splitting pattern (often multiplet, sextet), not
a 2H triplet.
Common misconception: Thinking
any bromine‑containing alkane will show the same
CH₂–Br pattern—position of Br (primary versus
secondary) changes both integration
and splitting.
If you think there are
any errors, please email me asap at
chem55555@hotmail.com
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