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Interpreting the
13C NMR spectrum of
3-methylbut-1-ene
(3-methyl-1-butene)
[Author
©
Dr WP 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
3-methylbut-1-ene
[updated
October 25th 2025]
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C-13
NMR spectroscopy - spectra index
Introductory note on the 13C NMR spectrum of 3-methylbut-1-ene
Students and teachers please note that my explanation of the
carbon-13 NMR spectrum of 3-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 3-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
3-methylbut-1-ene molecule.
The most common solvent used for investigating the C13 NMR
spectrum of compounds like 3-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 - 3-methylbut-1-ene (3-methyl-1-butene) here.
3-methylbut-1-ene C5H10,
 ,
 ,
The molecular structure and naming of alkenes
Interpreting the C-13 NMR spectrum of 3-methylbut-1-ene (3-methyl-1-butene)
As you can see from the diagram above there are
4 different chemical shift lines in the C-13 NMR spectrum of
3-methylbut-1-ene (3-methyl-1-butene)
indicating 4 different chemical environments of the carbon atoms.
H2C=CHCH(CH3)2
(Note the 4 colours indicating the
4 different chemical environments of the carbon atoms in
3-methylbut-1-ene (3-methyl-1-butene).
Chemical shifts
(a) to (d) on the C-13 NMR
spectrum diagram for 3-methylbut-1-ene.
The carbon atoms
of the methyl groups in 3-methylbut-1-ene are equivalent to each
other, so, being in identical chemical environments, give the
same C-13 NMR chemical shift
d (22.3
ppm).
All the other
carbon atoms in 3-methylbut-1-ene are in different chemical
environments with different C-13 NMR chemical shifts
a,
b
and c
(114, 145.9 and 32.7 ppm).
Unsaturated carbon
atoms (>C=C<)tend to have higher C-13 NMR chemical shifts.
The carbon-13 NMR spectra a provides direct evidence of
4 different
carbon atom environments in the 3-methylbut-1-ene (3-methyl-1-butene) molecule from
4 different chemical
shifts (ppm).
Key points about the 13C NMR
spectrum of 3-methylbut-1-ene
Molecular Overview: 3-Methylbut-1-ene
Molecular formula:
C5H10
Structure:
This is a branched terminal alkene with two equivalent methyl groups on the same
carbon.
Number of unique ¹³C environments:
4
Key
¹³C NMR Chemical Shifts
|
δ (ppm) |
Carbon
Type |
Environment |
Assignment |
|
~140–145, 145.9 ppm |
sp² (C=C, quaternary) |
Internal alkene carbon
(=CH–) |
C-2 (connected to CH
and =CH2) |
|
~110–115, 114.4 ppm |
sp² (C=C, terminal CH2) |
Terminal vinyl carbon
(CH2=) |
C-1 |
|
~25–35, 32.7 ppm |
sp³ (tertiary C) |
CH bonded to two CH3
and CH2=CH– |
C-3 (methine carbon) |
|
~15–20, 22.3 ppm |
sp³ (methyl, CH3) |
Two equivalent methyl
groups on C-3 |
C-4 and C-5
(equivalent) |
Note: Exact chemical shifts
may vary slightly depending on solvent and instrument, but these ranges are
typical and exam-relevant.
https://sdbs.db.aist.go.jp/
diagram δ ppm spectral database of organic
compounds
Common
Misconceptions
- Expecting five signals for five
carbons
- The two methyl groups are
equivalent, giving one signal, not two.
- Misidentifying alkene carbons
- The quaternary alkene carbon
(no H) appears further downfield (~140 ppm) than the vinyl CH2
(~110 ppm).
- Assuming methyls always appear at
~10 ppm
- Methyl shifts vary with environment;
here, ~15–20 ppm is typical due to proximity to a branched carbon.
- Overinterpreting peak intensity
- In ¹³C NMR, peak height ≠
number of carbons — integration is not quantitative like in ¹H
NMR.
Exam Revision Tips
- Count unique environments:
Use symmetry and bonding to determine how many distinct carbon signals to
expect.
- Use δ values to assign:
- >100 ppm
→ sp² (alkene/aromatic)
- ~10–50 ppm
→ sp³ (alkyl/methyl)
- Alkene clue:
Two signals >100 ppm suggest a C=C bond.
- Check for methyl equivalence:
Methyls on the same carbon are often equivalent — one signal.
- Link to ¹H NMR:
Cross-reference proton environments to support carbon assignments.
Tips for Spotting
Equivalent Methyl Groups in 13C NMR
e.g. the 2 methyl groups on C3 of 3-methylbut-1-ene
- Check for identical attachments:
If two methyl groups are bonded to the same carbon and that carbon is not
chiral, they’re usually equivalent.
- Look for symmetry: Even
partial symmetry (like in 3-methylbut-1-ene) can lead to equivalence.
- Use integration clues:
If two methyl groups give a single peak with integration of 6H, that’s a
strong hint they’re equivalent.
- Compare with isomers:
Try contrasting with 2-methylbut-2-ene or pent-2-ene, where methyl
environments differ more clearly.
- Counting methyls as separate
signals: Leads to
overestimating the number of peaks in ¹³C NMR spectra.
- Assuming all methyls are
equivalent: Not true in
asymmetric or chiral environments.
- Ignoring branching effects:
Branching can create or remove equivalence depending on the substitution
pattern.
Key words & phrases:
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resolution C-13 nmr spectra of 3-methylbut-1-ene (3-methyl-1-butene), revising the C-13 nmr spectrum of
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carbon-13 NMR spectrum of 3-methylbut-1-ene (3-methyl-1-butene) deducing the chemical environment of all the
carbon atoms in 3-methylbut-1-ene (3-methyl-1-butene) examining the c13 nmr spectrum of
3-methylbut-1-ene (3-methyl-1-butene) analysing the
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of 3-methylbut-1-ene (3-methyl-1-butene) How to interpret the C-13 NMR spectrum of
3-methylbut-1-ene (3-methyl-1-butene) Explanatory diagram of the 13C C-13 carbon-13 NMR spectrum of the number of different carbon atom
environments in the 3-methylbut-1-ene (3-methyl-1-butene) molecule from its carbon-13 NMR spectrum to
help work out the molecular structure of the 3-methylbut-1-ene
(3-methyl-1-butene) molecule? The uses
and distinctive features of the carbon-13 NMR spectrum of the
3-methylbut-1-ene (3-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
3-methylbut-1-ene (3-methyl-1-butene)
molecule? explaining the decoupled carbon-13 NMR spectrum of
3-methylbut-1-ene (3-methyl-1-butene)
with a detailed diagram of all the C-13 chemical shifts and
intensities
Links associated
with
3-methylbut-1-ene (3-methyl-1-butene)
The infrared spectrum of 3-methylbut-1-ene
The mass spectrum of 3-methylbut-1-ene
The H-1 NMR spectrum of 3-methylbut-1-ene
The chemistry of ALKENES
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NMR spectroscopy index
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