C7H16 C-13 nmr spectrum of 2,4-dimethylpentane analysis of chemical shifts ppm interpretation of 13C chemical shifts ppm of 2,4-dimethylpentane C13 13-C nmr doc brown's advanced organic chemistry revision notes

Advanced Organic Chemistry: Carbon-13 NMR spectrum of 2,4-dimethylpentane (CH₃)₂CHCH₂CH(CH₃)₂

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Interpreting the ¹³C NMR spectrum of 2,4-dimethylpentane

[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 & AP honors chemistry courses: Molecular spectroscopy - analysing 13C NMR spectrum of 2,4-dimethylpentane [spectra updated Mar 19th 2026 *]

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Introductory note on the 13C NMR spectrum of 2,4-dimethylpentane

Students and teachers please note that my explanation of the carbon-13 NMR spectrum of 2,4-dimethylpentane is designed for advanced, but pre-university, chemistry courses.

The description does not involve the chemical shift δ spin-spin coupling effects for 2,4-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 2,4-dimethylpentane molecule.

The most common solvent used for investigating the C13 NMR spectrum of compounds like 2,4-dimethylpentane, is CDCl₃ and other deuterated solvents.

C7H16 C-13 nmr spectrum of 2,4-dimethylpentane analysis of chemical shifts ppm interpretation of 13C chemical shifts ppm of 2,4-dimethylpentane C13 13-C nmr doc brown's advanced organic chemistry revision notes

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 - 2,4-dimethylpentane here.

2,4-dimethylpentane C₇H₁₆

For more see The molecular structure, classification and naming of alkanes

Interpreting the C-13 NMR spectrum of 2,4-dimethylpentane

As you can see from the diagram above there are 3 different chemical shift lines in the C-13 NMR spectrum of 2,4-dimethylpentane indicating 3 different chemical environments of the carbon atoms.

(CH₃)₂CHCH₂CH(CH₃)₂

(Note the 3 different colours indicating the 3 different chemical environments of the 7 carbon atoms in 2,4-dimethylpentane).

Chemical shifts (a) to (c) on the C-13 NMR spectrum diagram for 2,4-dimethylpentane.

The symmetry of the 2,4-dimethylpentane molecule contributes to the reduction of the number of chemical environments compared to seven of the other C₇H₁₆ isomers (see comparison table below).

All the methyl group carbon atoms -CH₃ have the same chemical environment (¹³C NMR shift δ a) giving the same chemical shift, similarly for the -CH- carbon atoms (¹³C shift δ b) and only the central carbon atom of the -CH₂- group (¹³C shift δ c) of 2,4-dimethylpentane has its own unique chemical shift.

The carbon-13 NMR spectra a provides direct evidence of 3 different carbon atom environments for the 3 carbon atoms in the 2,4-dimethylpentane molecule, deduced from the presence of 3 different 13C chemical shifts (ppm).

Key points about the ¹³C NMR spectrum of for 2,4-dimethylpentane

Overview: ¹³C NMR of 2,4-Dimethylpentane

Despite having seven carbon atoms, 2,4-dimethylpentane exhibits only three distinct ¹³C NMR signals. This is due to:

Six equivalent CH₃ groups (methyls)
Two equivalent CH groups (methines)
One unique CH₂ group (methylene)

All signals appear in the upfield region (10–50 ppm), typical of saturated hydrocarbons.

¹³C Chemical Shifts and Assignments for 2,4-dimethylpentane

Chemical Shift (δ, ppm)	Carbon Type	Environment	Number of Equivalent Carbons
~11–15, 22.9 ppm	CH₃	Six equivalent methyl groups	6
~25–30, 25.6 ppm	CH	Two equivalent methine carbons	2
~32–35, 48.9 ppm	CH₂	Unique methylene carbon	1

Sources: SpectraBase and

https://sdbs.db.aist.go.jp/ diagram ¹³C δ ppm spectral database of organic compounds

Common Misconceptions

Misconception	Clarification
"Seven carbon atoms = seven signals."	Not if some are chemically equivalent — symmetry reduces signal count.
"Each CH₃ group gives a separate peak."	Only if they’re in different environments — here, all six CH₃ groups are equivalent.
"¹³C NMR shows integration."	It does not — peak intensity is not proportional to number of carbons.
"Multiplicity appears in ¹³C NMR."	Proton-decoupled spectra show singlets only — no splitting.

Exam Revision Tips (AQA, Edexcel, OCR, WJEC, CCEA, CIE, IB, AP)

Interpretation Strategy

Count signals: Use symmetry to predict environments.
Estimate shift ranges:
- CH₃: ~10–20 ppm
- CH: ~25–35 ppm
- CH₂: ~30–40 ppm
Don’t rely on intensity: Use chemical logic, not peak height.
Combine with ¹H NMR and IR: For full structure deduction.

Typical Board-Specific Tips

Focus on signal count, carbon type, and symmetry.
May ask for assignment of peaks to structure.
May integrate ¹³C NMR with other spectra.
Emphasizes pattern recognition and carbon environment deduction.

Practice Question

A compound with molecular formula C₇H₁₆ shows three singlets in its ¹³C NMR spectrum at δ = 12 ppm, 28 ppm, and 34 ppm. Suggest a structure and explain the signal count.

Model Answer

Suggested structure: 2,4-dimethylpentane

Justification:

C₇H₁₆ = saturated hydrocarbon.
Three signals = three unique carbon environments due to symmetry.
δ = 12 ppm = six equivalent CH₃ groups.
δ = 28 ppm = two equivalent CH groups.
δ = 34 ppm = one CH₂ group.
All shifts are consistent with alkane carbons.

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

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Links associated with 2,4-dimethylpentane

The infrared spectrum of 2,4-dimethylpentane

The mass spectrum of 2,4-dimethylpentane

The H-1 NMR spectrum of 2,4-dimethylpentane

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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

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

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

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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Key points about the 13C NMR spectrum of for 2,4-dimethylpentane