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Interpreting the
13C NMR spectrum of
2-methylbut-2-ene (2-methyl-2-butene)
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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-2-ene
[spectra
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analysis infrared spectrum of
(CH3)2C=CHCH3
The
chemistry of alkenes
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C-13
NMR spectroscopy - spectra index
Introductory note on the 13C NMR spectrum of 2-methylbut-2-ene
Students and teachers please note that my explanation of the
carbon-13 NMR spectrum of 2-methylbut-2-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-2-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-2-ene molecule.
The most common solvent used for investigating the C13 NMR
spectrum of compounds like 2-methylbut-2-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-2-ene (2-methyl-2-butene) here.
2-methylbut-2-ene C5H10
(CH3)2C=CHCH3
 ,
The molecular structure and naming of alkenes
Interpreting the C-13 NMR spectrum of 2-methylbut-2-ene (2-methyl-2-butene)
As you can see from the diagram above there are
5 different chemical shift lines in the C-13 NMR spectrum of
2-methylbut-2-ene (2-methyl-2-butene)
indicating 5 different chemical environments of the carbon atoms.
CH3C(CH3)=CHCH3
(Note the 5 colours indicating the
5 different chemical environments of the carbon atoms in
2-methylbut-2-ene (2-methyl-2-butene).
The carbon-13 NMR spectra a provides direct evidence of
5 different
carbon atom environments in the 2-methylbut-2-ene (2-methyl-2-butene) molecule from
5 different chemical
shifts (ppm).
Note that 2-methylbut-2-ene does NOT exhibit E/Z
isomerism (cis/tans) because two of the groups on one of the carbon
atoms of the double bond are identical.
See
STEREOISOMERISM general definition, E/Z (cis/trans) isomerism
CH3C(CH3)=CHCH3
However, despite this, there are small differences in
the field experienced by the carbon atoms of the two methyl groups
attached to the 'purple' carbon of the C=C bond, because there is a
difference in their 13C chemical shifts (a is 17.3 and
b is 25.7 ppm).
This must be due to the asymmetry of the groups (H
and methyl) attached to the 'right-hand' carbon atom of the C=C
alkene double bond.
The carbon atoms of
the C=C alkene double give the largest C-13 NMR chemical shifts of
2-methyl-2-butene.
Beware on the
basis of the resolution of the spectra presented:
(a) and (b) maybe quoted as the same chemical shift for
the two end methyl group carbons.
BUT, the two end methyl groups experience slightly
different chemical environments.
(b) and (e)
maybe quoted as the same chemical shift for the two carbons of the
alkene group.
It is often quoted there are only four chemical shifts
for the C-13 NMR spectrum of 2-methylbut-2-ene, but there are actually
five!
BUT, again, the two C=C carbon atoms experience slightly
different chemical environments.
In both cases this is because 2-methylbut-2-ene is NOT a
symmetrical alkene and rotation is restricted by the >C=C< alkene double
bond.
Summary of key points for the C-13 NMR spectrum of 2-methylbut-2-ene plus
extra exam revision comments
A structured breakdown of the ¹³C NMR spectrum of
2-methylbut-2-ene (C5H10), tailored for
advanced A-level chemistry revision. This includes chemical shifts, carbon
environments, misconceptions, and exam tips aligned with major exam boards.
Key
Structural Features for
the C-13 NMR spectrum of 2-methylbut-2-ene
- Alkene: Internal C=C double bond
- Methyl groups: Two attached to the double bond
- Alkyl chain: One methyl and one methylene group on the
opposite side
- Symmetry: May reduce the number of distinct carbon
environments (take care here)
The NMR
Chemical Shifts and Carbon Environments for
the C-13 NMR spectrum of 2-methylbut-2-ene
| Chemical Shift
(δ, ppm) |
Carbon Type |
Environment Description |
| ~130–135, 118.7 ppm
and 132.1 ppm |
Alkene C (C=C) |
Two non-equivalent sp² carbons in the
double bond, C1 & C2 |
| ~25–30, 17.3 ppm
and 25.7 ppm |
Allylic CH3 |
Two non-equivalent methyl groups
attached to C1 of the C=C group |
| ~10–15, 13.4 ppm |
Terminal CH3 |
C4 of the methyl group at end of alkyl
chain |
Total signals: 4 distinct peaks due to symmetry
Reference: Tetramethylsilane (TMS) at 0 ppm
CH3C(CH3)=CHCH3
There is a subtle difference between the two terminal methyl groups
of the C=C group and it’s easy to overlook in introductory contexts
where symmetry is assumed a bit too liberally.
Common
Misconceptions about
the C-13 NMR spectrum of 2-methylbut-2-ene
|
Misconception |
Clarification |
| All methyl
carbons appear at the same shift |
Their
position relative to C=C affects shielding and δ value |
| Peak height
reflects number of carbons |
In ¹³C NMR,
peak intensity is not proportional to carbon count |
| Splitting
patterns are present |
¹³C NMR
typically shows singlets due to proton decoupling |
| Integration
is used like in ¹H NMR |
Integration
is not standard in ¹³C NMR — focus on number and position of peaks |
Exam
Revision Tips for questions that may involve
the C-13 NMR spectrum of 2-methylbut-2-ene
(AQA, Edexcel, OCR,
WJEC, CCEA, CIE, IB)
What to Focus On:
- Number of peaks: Reflects distinct carbon environments
— symmetry reduces peak count
- Chemical shift ranges: Use data sheet to match δ values
to functional groups
- Alkene carbons: Typically appear between 110–160 ppm
- Alkyl carbons: Methyl and methylene groups appear below
50 ppm
Exam-Style Strategy:
- Step 1: Count peaks — expect 4 for 2-methylbut-2-ene
- Step 2: Match δ values to carbon types using data sheet
- Step 3: Use symmetry to explain equivalent environments
- Step 4: Rule out functional groups not present (e.g.
C=O, aromatic)
Key words & phrases:
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(2-methyl-2-butene), understanding the
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resolution C-13 nmr spectra of 2-methylbut-2-ene (2-methyl-2-butene), revising the C-13 nmr spectrum of
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chemical shifts of the C-13 nmr spectrum of 2-methylbut-2-ene
(2-methyl-2-butene), how to construct the diagram of
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carbon atoms in 2-methylbut-2-ene (2-methyl-2-butene) examining the c13 nmr spectrum of
2-methylbut-2-ene (2-methyl-2-butene) analysing the
13-c nmr spectrum of 2-methylbut-2-ene (2-methyl-2-butene) how do you sketch and interpret the C-13 NMR spectrum
of 2-methylbut-2-ene (2-methyl-2-butene) interpreting interpretation of the C-13
NMR spectrum of 2-methylbut-2-ene (2-methyl-2-butene) C5H10
How do you interpret the chemical shifts of the C-13 NMR spectrum
of 2-methylbut-2-ene (2-methyl-2-butene) How to interpret the C-13 NMR spectrum of
2-methylbut-2-ene (2-methyl-2-butene) Explanatory diagram of the 13C C-13 carbon-13 NMR spectrum of the number of different carbon atom
environments in the 2-methylbut-2-ene (2-methyl-2-butene) molecule from its carbon-13 NMR spectrum to
help work out the molecular structure of the 2-methylbut-2-ene
(2-methyl-2-butene) molecule? The uses
and distinctive features of the carbon-13 NMR spectrum of the
2-methylbut-2-ene (2-methyl-2-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-2-ene (2-methyl-2-butene)
molecule? explaining the decoupled carbon-13 NMR spectrum of
2-methylbut-2-ene (2-methyl-2-butene) with a detailed diagram of all the
uncoupled C-13 chemical shifts and intensities
Links associated
with
2-methylbut-2-ene (2-methyl-2-butene)
The infrared spectrum of
2-methylbut-2-ene
The mass spectrum of
2-methylbut-2-ene
The H-1 NMR spectrum of
2-methybut-2-ene
The chemistry of ALKENES
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