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Interpreting the 13C NMR spectrum of
2,3-dimethylpentane
[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 & AP honors chemistry courses: Molecular
spectroscopy - analysing
13C NMR spectrum of
2,3-dimethylpentane
[updated
October 31st 2025]
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brown
Re-edit 13C NMR spectrum of
CH3CH(CH3)CH(CH3)CH2CH3
This is a BIG
website, PLEASE take time to explore it
Links
associated with 2,3-dimethylpentane
C-13
NMR spectroscopy - spectra index
See also
comparing the
1H NMR and 13C NMR spectra of the nine alkane structural isomers of C7H16
Introductory note on the 13C NMR spectrum of 2,3-dimethylpentane
Students and teachers please note that my explanation of the
carbon-13 NMR spectrum of 2,3-dimethylpentane is designed for advanced, but
pre-university, chemistry courses.
The description does not involve
the chemical shift δ
spin-spin coupling effects for 2,3-dimethylpentane 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
2,3-dimethylpentane molecule.
The most common solvent used for investigating the C13 NMR
spectrum of compounds like 2,3-dimethylpentane, 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 - 2,3-dimethylpentane here.
2,3-dimethylpentane
C7H16
For more
see
The molecular structure,
classification and
naming of alkanes
Interpreting the C-13 NMR spectrum of 2,3-dimethylpentane
As you can see from the diagram above there are
7 different chemical shift lines in the C-13 NMR spectrum of
2,3-dimethylpentane
indicating 7 different chemical environments of the carbon atoms.
(CH3)2CHCH(CH3)CH2CH3
Chemical shifts (a) to (g) on the C-13 NMR
spectrum diagram for 2,3-dimethylpentane.
However, just looking at the formula, you might
only expect 6 different chemical environments because you would
tend to assume the carbon atoms
a
and
b
would be equivalent for these two methyl groups attached to the
same CH grouping (on the left of the molecule).
Apparently, this is not the case, so is this due
to the effect of the chiral carbon atom
d
giving rise to R/S isomers with different sets of chemical
shifts?
It would appear that the two end methyl group
carbons are not equivalent, but I'm not sure why?
This means
there are theoretically 7 different 13C NMR chemical shifts
.
I think this is a university level situation, so
I'm leaving it at that, as My advanced organic chemistry notes
are designed for pre-university students!
I would value other opinions on interpreting the 13C
NMR spectrum of 2,3-dimethylpentane.
Key
points about the 13C NMR spectrum of 2,3-dimethylpentane
The
¹³C NMR spectrum of 2,3-dimethylpentane is very complicated and, in high
resolution, can show up to 7 distinct carbon environments due to lack of
molecular symmetry, all appearing in the alkyl region (δ ~8–40 ppm).
C3 is
chiral and causes complications in interpreting the 13C NMR
spectrum of 2,3-dimethylpentane.
Overview: ¹³C NMR
of 2,3-dimethylpentane
2,3-Dimethylpentane (C7H16)
is a branched very unsymmetrical alkane, resulting in an increase in
unique carbon signals.
Its ¹³C NMR spectrum is a classic example
of how non-equivalent carbon environments increase the signal count.
Table: ¹³C NMR
Chemical Shifts and Assignments
for 2,3-dimethylpentane
A simplified interpretation, sorting the
chemical shifts into four groups
- but, in reality, it is too complicated for pre-university students, see the
13C NMR chemical shift diagram below the table.
| δ (ppm) |
Carbon Type |
Environment |
Notes |
| ~8–10 |
CH3 (terminal
methyls) |
C–CH3
(C1 and C5) |
Equivalent due to
symmetry |
| ~18–20 |
CH3
(gem-dimethyl) |
CH3–C(CH3)2
(C2 and C3 methyls) |
Four methyls give one
signal |
| ~27–30 |
CH (methine) |
Central CH on C2 and C3 |
Equivalent methine
carbons |
| ~36–38 |
CH2 |
C4 (between two CH
groups) |
Unique; flanked by
methine and methyl groups |
BUT, high resolution 13C NMR
chemical shift information for 2,3-dimethylpentane is shown on the (repeated)
diagram below.
The seven, instead of six chemical shifts
is due to the asymmetry of the 13C fields caused by the chiral carbon
C3.
The C2 methyl groups are not equivalent.
https://sdbs.db.aist.go.jp/
diagram 13C
δ ppm spectral database of
organic compounds
Common
Misconceptions in Exams
- Expecting seven signals for seven
carbons: Symmetry reduces the
number of unique environments—only four signals appear.
- Assuming all methyls are distinct:
The four gem-dimethyl methyls are chemically equivalent.
- Confusing CH2
and CH signals: CH2
carbons appear slightly downfield compared to CH due to electron density
differences.
- Overinterpreting chemical shift
values: In saturated alkanes,
shifts are tightly clustered and not diagnostic of position without context.
Exam Revision Tips
(AQA, Edexcel, OCR, WJEC, CCEA, CIE, IB, AP)
- Count environments, not atoms:
Use symmetry to predict the number of signals.
- Know typical alkane shifts:
CH3
~10–20 ppm, CH2
~20–40 ppm, CH ~25–50 ppm.
- Comparing C7H16 isomers:
Practice distinguishing 2,3-dimethylpentane from 2,4-dimethylpentane or
n-heptane by signal count and integration, note the differences in
the number of 13C signals due to equivalence or
non-equivalence of methyl groups (see below).
- Practice drawing and labeling environments: Helps
visualize equivalence or non-equivalence.
- Link to ¹H NMR and mass spectrometry: Use all spectra together
for full structural elucidation.
- Interpretation: Be ready
to deduce structure from spectra without labels—practice assigning
peaks from scratch.
Tips for spotting
equivalent methyl group carbons in 13C NMR
e.g. 4 methyl groups on C2 of 2,2-dimethypentane
- Check for identical attachments:
If two or more methyl groups are bonded to the same carbon and that carbon is not
chiral, they are usually equivalent.
- Look for symmetry: Even
partial symmetry (like in
2,2-dimethylpentane) 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
2,2-dimethylbutttane with
3-methylpentane, where methyl
carbon 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.
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Comparing the
1H NMR and 13C NMR spectra of the nine alkane structural isomers of C7H16
You can distinguish all 9 isomers from a data combination of their number of
1H NMR
chemical shifts,
and their resulting integrated 1H proton ratios, plus, their number of
13C
chemical shifts. |
|
Name of the alkane structural isomer of molecular
formula C7H16 |
Abbreviated structural formulae
of the nine isomers of molecular formula C7H16 (interpretation complications with 3-methylhexane and
2,3-dimethylpentane because they exhibit R/S isomerism due to a
chiral carbon) |
Skeletal formula of the
nine
alkane isomers of
molecular formula C7H16 |
Number of 1H NMR chemical shifts (δ) and
proton ratio (links
to spectrum) |
Number of 13C chemical shifts (δ)
(links
to spectrum) |
|
heptane |
 |
 |
4 δ: proton ratio: 3:2:2:1 (6:4:4:2 in the molecule) |
4 δ shifts |
|
2-methylhexane |
 |
 |
6 δ: proton ratio :
6:3:2:2:2:1 |
6 δ shifts |
|
3-methylhexane |
 |
 |
7 δ: proton ratio:
3:3:3:2:2:2:1 (simplification) ! |
7
δ shifts |
|
3-ethylpentane |
 |
 |
3 δ: proton ratio:
9:6:1 |
3 δ
shifts |
|
2,2-dimethylpentane |
 |
 |
4 δ: proton ratio:
9:3:2:2 |
5 δ shifts |
|
2,3-dimethylpentane |
|
|
6 δ: proton ratio:
6:3:3:2:1:1 (simplification) ! |
6 δ
shifts (simplification) !!! |
|
2,4-dimethylpentane |
 |
 |
3 δ: proton ratio:
12:2:2 |
3 δ
shifts |
|
3,3-dimethylpentane |
 |
 |
3 δ: proton ratio:
3:3:2 (6:4:4 in the molecule) |
4 δ
shifts |
|
2,2,3-trimethylbutane |
 |
 |
3 δ: proton ratio:
9:6:1 |
4 δ shifts |
Key words & phrases:
C7H16 Interpreting the C-13 NMR spectra of
2,3-dimethylpentane, C-13 nmr spectrum of 2,3-dimethylpentane, understanding the
carbon-13 nmr spectrum of 2,3-dimethylpentane, explaining the line pattern in the high
resolution C-13 nmr spectra of 2,3-dimethylpentane, revising the C-13 nmr spectrum of
2,3-dimethylpentane, ppm
chemical shifts of the C-13 nmr spectrum of 2,3-dimethylpentane, how to construct the diagram of
the C-13 nmr spectrum of 2,3-dimethylpentane, how to analyse the chemical shifts in the
carbon-13 NMR spectrum of 2,3-dimethylpentane deducing the chemical environment of all the
carbon atoms in 2,3-dimethylpentane examining the c13 nmr spectrum of
2,3-dimethylpentane analysing the
13-c nmr spectrum of 2,3-dimethylpentane how do you sketch and interpret the C-13 NMR spectrum
of 2,3-dimethylpentane interpreting interpretation of the C-13 NMR spectrum of
2,3-dimethylpentane
13C NMR spectrum of 2,3-dimethylpentane
(CH3)2CHCH(CH3)CH2CH3 Molecular structure diagram of the
carbon-13 NMR diagram for the 13C NMR spectrum of 2,3-dimethylpentane. Deducing the number
of different chemical environments of the carbon atoms in the
2,3-dimethylpentane molecule
from the 13C chemical shifts in the carbon-13 NMR spectrum of
2,3-dimethylpentane. Revision
notes on the carbon-13 NMR spectrum of 2,3-dimethylpentane. Matching and deducing the
structure of the 2,3-dimethylpentane molecule from its 13C NMR spectrum.
Carbon-13 NMR spectroscopy of aliphatic alkanes,
13C NMR spectra of 2,3-dimethylpentane, a structural isomer of molecular formula C7H16
How do you interpret the chemical shifts of the C-13 NMR spectrum
of 2,3-dimethylpentane How to interpret the C-13 NMR spectrum of
2,3-dimethylpentane Explanatory diagram of the 13C C-13 carbon-13 NMR spectrum of the number of different carbon atom
environments in the 2,3-dimethylpentane molecule from its carbon-13 NMR spectrum to
help work out the molecular structure of the
2,3-dimethylpentane molecule? The uses
and distinctive features of the carbon-13 NMR spectrum of the
2,3-dimethylpentane
molecule explained. What do the number and values of the chemical
shifts from the c-13 carbon-13 NMR spectrum tell us about the
2,3-dimethylpentane
molecule? explaining the decoupled carbon-13 NMR spectrum of
2,3-dimethylpentane
with a detailed interpretation diagram of all the C-13 chemical shifts and
intensities
Links associated
with
2,3-dimethylpentane
The infrared spectrum of
2,3-dimethylpentane
The mass spectrum of
2,3-dimethylpentane
The H-1 NMR spectrum of
2,3-dimethylpentane
The chemistry of ALKANES
revision notes INDEX
C-13
NMR spectroscopy index
ALL SPECTROSCOPY INDEXES
All Advanced Organic
Chemistry Notes
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Infrared spectra of the isomers of C7H16
The infrared
spectrum of heptane
The
infrared spectrum of 2-methylhexane
The
infrared spectrum of 3-methylhexane
The
infrared spectrum of 3-ethylpentane
The infrared spectrum of
2,2-dimethylpentane
The infrared spectrum of
2,3-dimethylpentane
The infrared spectrum of
2,4-dimethylpentane
The infrared spectrum of
3,3-dimethylpentane
The infrared spectrum
of 2,2,3-trimethylbutane
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Mass spectra of the isomers of C7H16
The mass
spectrum of heptane
The mass
spectrum of 2-methylhexane
The mass
spectrum of 3-methylhexane
The
mass spectrum of 3-ethylpentane
The mass spectrum of
2,2-dimethylpentane
The mass spectrum of
2,3-dimethylpentane
The mass spectrum of
2,4-dimethylpentane
The mass spectrum of
3,3-dimethylpentane
The mass
spectrum of 2,2,3-trimethylbutane
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1H NMR spectra of the isomers of C7H16
The H-1 NMR
spectrum of heptane
The
H-1 NMR spectrum of 2-methylhexane
The
H-1 NMR spectrum of 3-methylhexane
The
H-1 NMR spectrum of 3-ethylpentane
The H-1 NMR spectrum of
2,2-dimethylpentane
The H-1 NMR spectrum of
2,3-dimethylpentane
The H-1 NMR spectrum of
2,4-dimethylpentane
The H-1 NMR spectrum of
3,3-dimethylpentane
The H-1
NMR spectrum of 2,2,3-trimethylbutane
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13C NMR spectra of the isomers of C7H16
The C-13 NMR
spectrum of heptane
The
C-13 NMR spectrum of 2-methylhexane
The
C-13 NMR spectrum of 3-methylhexane
The
C-13 NMR spectrum of 3-ethylpentane
The C-13 NMR spectrum of
2,2-dimethylpentane
The C-13 NMR spectrum of
2,3-dimethylpentane
The C-13 NMR spectrum of
2,4-dimethylpentane
The C-13 NMR spectrum of
3,3-dimethylpentane
The
C-13 NMR spectrum of 2,2,3-trimethylbutane
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