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Interpreting
and explaining the
13C 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 (13C NMR)
[spectra page updated
RE-EDIT]
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brown
Re-edit 13C NMR spectrum of CH3CH2CH2CH2Br
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C-13
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 13C NMR
spectrum of 1-bromobutane
Introductory note on the 13C NMR spectrum of 1-bromobutane
Students and teachers please note that my explanation of the
carbon-13 NMR spectrum of 1-bromobutane is designed for advanced, but
pre-university, chemistry courses.
The description does not involve
the chemical shift δ
spin-spin coupling effects for 1-bromobutane and the relative size of the carbon-13
NMR shifts does not give the ratio of the carbon atoms in the
different non-equivalent chemical environments of the 1-bromobutane molecule.
The most common solvent used for investigating the
13C
NMR
spectrum of compounds like 1-bromobutane, is CDCl3 and
other deuterated solvents.
TMS is the acronym for tetramethylsilane, formula Si(CH3)4,
whose 13C atoms are arbitrarily given a chemical shift of 0.0
ppm. This is the 'standard' in 13C NMR spectroscopy and all other
13C resonances, called chemical shifts, are measured with
respect to the TMS, and depend on the
individual (electronic) chemical environment of the 13C atoms in
an organic molecule - 1-bromobutane here.
1-bromobutane,
C4H9Br,
CH3CH2CH2CH2Br,
CH3-CH2-CH2-CH2-Br
The molecular structure and naming of haloalkanes
Interpreting the C-13 NMR spectrum of 1-bromobutane
As you can see from the diagram above there are
4 different chemical shift lines in the C-13 NMR spectrum of
1-bromobutane
indicating 4 different chemical environments of the 4 carbon
atoms of 1-bromobutane.
CH3CH2CH2CH2Br
(Note the 4 different colours indicating the
4 different chemical environments of the 4 carbon atoms in
1-bromobutane).
13C chemical shifts (a) to (d) on the C-13 NMR
spectrum diagram for 1-bromobutane.
(a) CH3-CH2-CH2-CH2-Br:
The 13C NMR chemical shift of 13.2 ppm for the methyl
group carbon atom 4.
(b) CH3-CH2-CH2-CH2-Br:
The 13C NMR chemical shift of 21.4 ppm for carbon
atom 3 of the CH2 group.
(a) CH3-CH2-CH2-CH2-Br:
The 13C NMR chemical shift of 35.0 ppm for carbon
atom 2 of the CH2 group.
(b) CH3-CH2-CH2-CH2-Br:
The 13C NMR chemical shift of 33.4 ppm for carbon
atom 1 joined to the bromine atom.
The carbon-13 NMR spectra provides direct evidence of
4 different carbon atom environments for the 4 carbon atoms in the
1-bromobutane molecule,
deduced from the presence of 4 different 13C NMR chemical
shifts (ppm).
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QUESTIONS
Advanced A-level chemistry - practise exam questions on
the 13C 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.
Q1
How many distinct
¹³C NMR signals appear in the
spectrum of 1‑bromobutane?
A. 5 B. 3
C. 4 D. 2
Q2
What is the relative integration pattern
in a standard broadband‑decoupled ¹³C NMR
spectrum of 1‑bromobutane?
A. 3 : 2 : 2 : 2
B. All signals appear with
similar intensity
C. 1 : 1 : 1 : 1
D. 4 : 3 : 2
: 1
Q3
Which carbon appears furthest downfield
(highest ppm) in the ¹³C NMR of 1‑bromobutane?
A. CH 2–
B. CH3–CH2–
C. –CH2–CH2–
D. –CH2–Br
Q5
How many 13C NMR signals does
2‑bromobutane (CH3–CHBr–CH2–CH3)
show?
A. 2 B. 3
C. 4 D. 5
Q6
Which statement
correctly distinguishes the two isomers using
¹³C NMR?
A. 1‑bromobutane has 3 signals; 2‑bromobutane has
4
B. 1‑bromobutane has 4 signals; 2‑bromobutane has
3
C. Both have 4 signals
D. Both have 3 signals
Q7
Which isomer of
C4H9Br
has the fewest ¹³C NMR signals?
A. 1‑bromobutane
B. 2‑bromobutane
C. 1‑bromo‑2‑methylpropane
D. 2‑bromo‑2‑methylpropane
Q8
For isomeric 1‑bromo‑2‑methylpropane (CH3–CH(CH3)–CH2Br),
how many ¹³C NMR signals are expected?
A. 2 B. 3
C. 4 D. 5
Q9 A compound with
formula C4H9Br shows only
2 signals in its 13C NMR spectrum.
Which structure is most consistent?
A. 1‑bromobutane
B. 2‑bromobutane
C. 1‑bromo‑2‑methylpropane
D. 2‑bromo‑2‑methylpropane
Q10
You have an unknown
C4H9Br
that shows 3 distinct ¹³C signals.
Which structure is most likely?
A. 1‑bromobutane
B. 2‑bromobutane
C. 1‑bromo‑2‑methylpropane
D. 2‑bromo‑2‑methylpropane
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.
|
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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 C-13 NMR spectra of
1-bromobutane, C-13 nmr spectrum of 1-bromobutane, understanding the
carbon-13 nmr spectrum of 1-bromobutane, explaining the line pattern in the high
resolution C-13 nmr spectra of 1-bromobutane, revising the C-13 nmr spectrum of
1-bromobutane, ppm
chemical shifts of the C-13 nmr spectrum of 1-bromobutane, how to construct the diagram of
the C-13 nmr spectrum of 1-bromobutane, how to analyse the chemical shifts in the
carbon-13 NMR spectrum of 1-bromobutane deducing the chemical environment of all the
carbon atoms in 1-bromobutane examining the c13 nmr spectrum of
1-bromobutane analysing the
13-c nmr spectrum of 1-bromobutane how do you sketch and interpret the C-13 NMR spectrum
of 1-bromobutane interpreting interpretation of the C-13 NMR spectrum of
1-bromobutane
assignment of chemical shifts in
the 13C NMR spectrum of 1-bromobutane type 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 chemical shifts of the C-13 NMR spectrum
of 1-bromobutane How to interpret the C-13 NMR spectrum of
1-bromobutane Explanatory diagram of the 13C C-13 carbon-13 NMR spectrum of the number of different carbon atom
environments in the 1-bromobutane molecule from its carbon-13 NMR spectrum to
help work out the molecular structure of the 1-bromobutane molecule? The uses
and distinctive features of the carbon-13 NMR spectrum of the
1-bromobutane
molecule explained. What do the number and values of the chemical
shifts from the c-13 carbon-13 NMR spectrum tell us about the
1-bromobutane
molecule? explaining the decoupled carbon-13 NMR spectrum of
1-bromobutane
with a detailed interpretation diagram of all the C-13 chemical shifts and
intensities
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 H-1 NMR spectrum of
1-bromobutane (n-butyl bromide)
C-13
NMR spectroscopy index
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1st year undergraduate students of chemistry.
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ANSWERS
Advanced A-level chemistry - practise exam questions on
the 13C 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.
Q1
How many distinct
¹³C NMR signals appear in the
spectrum of 1‑bromobutane?
A. 5 B. 3
C. 4 D. 2
Correct answer: C
Explanation: Four different
carbon environments:
-
CH3–
-
CH2–CH2–
-
–CH2–CH2–
-
–CH2–Br
Misconception: Students often
assume the two internal CH2 carbons
are equivalent. They are not: one is closer to
the bromine atom.
Q2
What is the
relative integration pattern in
a standard broadband‑decoupled ¹³C NMR spectrum
of 1‑bromobutane?
A. 3 : 2 : 2 : 2
B. All signals appear with similar intensity
C. 1 : 1 : 1 : 1
D. 4 : 3 : 2 : 1
Correct answer: B
Explanation: ¹³C NMR is
not quantitative at school level; each
distinct carbon gives one signal of broadly
similar intensity.
In the 13C NMR spectrum for 1-bromobutane I
obtained, the four intensities are similar, but
is NOT an integration rule which applies to 1H
NMR spectra.
Misconception: Transferring ¹H
NMR integration ideas (3H, 2H, etc.) directly to
¹³C NMR.
Q3
Which carbon
appears furthest downfield
(highest ppm) in the ¹³C NMR of 1‑bromobutane?
A. CH 2–
B. CH3–CH2–
C. –CH2–CH2–
D. –CH2–Br
Correct answer: D
Explanation: The carbon
directly bonded to bromine is most deshielded →
highest ppm.
Misconception: Assuming bromine
is “too large to affect NMR”. Electronegative
atoms still shift signals downfield.
Q5
How many 13C NMR signals does
2‑bromobutane (CH3–CHBr–CH2–CH3)
show?
A. 2 B. 3
C. 4 D. 5
Correct answer: C
Explanation: Carbons:
3
groups (ends) → 2 signals
CHBr (chiral centre) → 1 signal
one -CH2
→ 1 signal
Total = 4 signals.
Misconception: Not recognising
the two terminal CH3 groups are
different in 2‑bromobutane.
Q6
Which statement is
true about the two isomers using ¹³C NMR?
A. 1‑bromobutane has 3 signals; 2‑bromobutane has
4
B. 1‑bromobutane has 4 signals; 2‑bromobutane has
3
C. Both have 4 signals
D. Both have 3 signals
Answer: C
Explanation: 1‑bromobutane: 4
distinct carbons → 4 signals. 2‑bromobutane: 4
distinct carbons.
Misconceptions: D assumes the
two methyl groups are equivalent
Q7
Which isomer of
C4H9Br
has the fewest ¹³C NMR signals?
A. 1‑bromobutane
B. 2‑bromobutane
C. 1‑bromo‑2‑methylpropane
D. 2‑bromo‑2‑methylpropane
Correct answer: D
Explanation:
2‑bromo‑2‑methylpropane ((CH3)3CBr):
3
carbons → 1 signal
Central C–I carbon → 1 signal Total = 2
signals.
Misconception:
Thinking branching always increases the number
of signals; symmetry can reduce it - but the
latter works for D here..
Q8
For isomeric 1‑bromo‑2‑methylpropane (CH3–CH(CH3)–CH2Br),
how many ¹³C NMR signals are expected?
A. 2 B. 3
C. 4 D. 5
Correct answer: B
Explanation: Carbons:
-
2 x CH3 attached to CH
-
CH (attached to 2 x CH
3
and CH2Br)
CH2Br
All distinct → 3 signals.
Misconception:
Assuming the two CH2 groups are
equivalent; they are not.
Q9 A compound with
formula C4H9Br shows only
2 signals in its 13C NMR spectrum.
Which structure is most consistent?
A. 1‑bromobutane
B. 2‑bromobutane
C. 1‑bromo‑2‑methylpropane
D. 2‑bromo‑2‑methylpropane
Correct answer: D
Explanation: Only 2 distinct
carbons → highly symmetric →
2‑bromo‑2‑methylpropane.
Misconception: Thinking “2
signals = 2 carbons”; it actually means “2
types of carbon”.
Q10
You have an unknown
C4H9Br
that shows 3 distinct ¹³C signals.
Which structure is most likely?
A. 1‑bromobutane
B. 2‑bromobutane
C. 1‑bromo‑2‑methylpropane
D. 2‑bromo‑2‑methylpropane
Answer: C
Explanation: 1‑bromobutane
and 1‑bromo‑2‑methylpropane
have 4 signals; but 1‑bromo‑2‑methylpropane
gives 3 signals and highly symmetrical 2-bromo-2-methylpropane
gives only 2 signals.
Misconception: Ignoring
branching and symmetry when interpreting ¹³C
NMR; these strongly affect the number of
signals.
If you think there are
any errors, please email me asap at
chem55555@hotmail.com
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