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Interpreting the
13C NMR spectrum of
2-methylbut-1-ene
[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 - analysing the
C-13 NMR spectrum of
2-methylbut-1-ene
[spectra
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analysis C-13 spectrum of
CH3CH2C(CH3)=CH2
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The
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C-13
NMR spectroscopy - spectra index
Introductory note on the 13C NMR spectrum of 2-methylbut-1-ene
(2-methyl-1-butene)
Students and teachers please note that my explanation of the
carbon-13 NMR spectrum of 2-methylbut-1-ene is designed for advanced, but
pre-university, chemistry courses.
The description does not involve
the chemical shift δ
spin-spin coupling effects for 2-methylbut-1-ene 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
2-methylbut-1-ene molecule.
The most common solvent used for investigating the C13 NMR
spectrum of compounds like 2-methylbut-1-ene, 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 shifts, called chemical shifts, depend on the
individual (electronic) chemical environment of the 13C atoms in
an organic molecule - 2-methylbut-1-ene here.
2-methylbut-1-ene C5H10,
 ,
The molecular structure and naming of alkenes
Interpreting the C-13 NMR spectrum of 2-methylbut-1-ene
As you can see from the diagram above there are
5 different chemical shift lines in the C-13 NMR spectrum of
2-methylbut-1-ene
indicating 5 different chemical environments of the carbon atoms.
H2C=C(CH3)CH2CH3
Note the 5 colours indicating the
5 different chemical environments of the carbon atoms in
2-methylbut-1-ene for the five 13C NMR chemical shifts
(a) to
(e).
The carbon atoms of the
C=C alkene double bond give the largest C-13 NMR chemical shifts of
2-methylbut-1-ene.
The carbon-13 NMR spectra a provides direct evidence of
5 different
carbon atom environments in the 2-methylbut-1-ene molecule from 5 different chemical
shifts (ppm).
Summary of key points for the C-13 NMR spectrum of 2-methylbut-1-ene plus
extra exam revision comments
A structured breakdown of the
¹³C NMR spectrum of 2-methylbut-1-ene, tailored for
advanced A-level and IB Chemistry exam boards. This includes chemical
shifts, carbon environments, misconceptions, and revision strategies.
Molecular Overview
-
Structure:
CH2=C(CH3)CH2CH3
-
Molecular formula:
C5H10
-
Key features:
Terminal alkene CH2=, methyl branch attached to C=C bond, short alkyl chain
- ethyl chain
-
Number of unique
carbon environments:
5
Carbon
Environments & Chemical Shifts for
the C-13 NMR spectrum of 2-methylbut-1-ene
|
Label |
Carbon Type |
Environment |
δ (ppm) |
Notes |
|
b |
C=C(CH3)
(quaternary) |
Alkene carbon bonded to
CH3 |
~135–140, 147.7 ppm |
Deshielded due to π
electrons |
|
a |
CH2=C |
Terminal alkene CH2 |
~110–115, 108.6 ppm |
Vinylic carbon, highly
deshielded |
|
d |
–CH2– |
Allylic CH2
next to C=C |
~25–35, 30.8 ppm |
Slightly deshielded by
alkene |
|
c |
–CH3
(methyl branch) |
Attached to C=C |
~15–25, 22.4 ppm |
Allylic methyl |
|
e |
CH3–CH2 |
Ethyl group |
~10–15, 12.4 ppm |
Saturated alkyl carbon |
H2C=C(CH3)CH2CH3
These values are approximate
and may vary slightly depending on solvent and conditions (typically CDCl3).
Common
Misconceptions about
the C-13 NMR spectrum of 2-methylbut-1-ene
(see also below)
-
Assuming peak height =
number of carbons: In ¹³C NMR,
peak intensity is not proportional to the number of equivalent carbons.
-
Confusing alkene
shifts with aromatic: Alkene
carbons (~110–140 ppm) are a bit lower than aromatic (~120–160 ppm).
-
Overlooking symmetry:
Students may miscount environments if they don’t consider molecular
symmetry.
-
Expecting splitting:
¹³C spectra are typically decoupled, so all peaks appear as
singlets.
Exam Revision Tips
for questions involving the C-13 NMR spectrum of 2-methylbut-1-ene
(AQA, Edexcel, OCR, WJEC,
CCEA, CIE, IB)
(see also above)
Step-by-Step Strategy
-
Count peaks:
Each peak = unique carbon environment.
-
Match δ values:
Use data sheet to assign functional groups.
-
Sketch structure:
Label each carbon to confirm environments.
-
Compare isomers:
Practice with 2-methylbut-2-ene (internal alkene) for contrast.
Key Exam Prompts
-
“How many carbon environments
are present?”
-
“Suggest a structure
consistent with this spectrum.”
-
“Explain why two carbons give
the same signal.”
Integration with Other Techniques
-
Combine with ¹H NMR
for hydrogen environments.
-
Use IR to
confirm functional groups (e.g. C=C stretch ~1650 cm⁻¹).
-
Use mass spec
for molecular ion (m/z 70).
Key words & phrases: 2-methyl-1-butene Interpreting the C-13 NMR spectra of
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2-methylbut-1-ene, ppm
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2-methylbut-1-ene 2-methyl-1-butene
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of 2-methylbut-1-ene (2-methyl-1-butene) How to interpret the C-13 NMR spectrum of
2-methylbut-1-ene (2-methyl-1-butene) Explanatory diagram of the 13C C-13 carbon-13 NMR spectrum of the number of different carbon atom
environments in the 2-methylbut-1-ene (2-methyl-1-butene) molecule from its carbon-13 NMR spectrum to
help work out the molecular structure of the 2-methylbut-1-ene
(2-methyl-1-butene) molecule? The uses
and distinctive features of the carbon-13 NMR spectrum of the
2-methylbut-1-ene (2-methyl-1-butene)
molecule explained. What do the number and values of the chemical
shifts from the c-13 carbon-13 NMR spectrum tell us about the
2-methylbut-1-ene (2-methyl-1-butene)
molecule? explaining the decoupled carbon-13 NMR spectrum of
2-methylbut-1-ene (2-methyl-1-butene) with a detailed diagram of all the
uncoupled C-13 chemical shifts and intensities
Links associated
with
2-methylbut-1-ene
The Infrared spectrum of
2-methylbut-1ene
The mass spectrum of 2-methylbut-1ene
The H-1 NMR spectrum of
2-methylbut-1ene
The chemistry of ALKENES
revision notes INDEX
C-13
NMR spectroscopy index
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