Introductory note on the 1H NMR spectra of 3-methylpentane

Students and teachers please note my explanation of the proton NMR spectrum of 3-methylpentane is designed for advanced, but pre-university, chemistry courses.

The chemical shift δ splitting pattern effects for 3-methylpentane are confined to a proton spin-spin coupling effects analysed using the n+1 rule for adjacent non-equivalent proton fields (n is the number of neighbouring protons in a non-equivalent different chemical environment for the 3-methylpentane molecule).

It is assumed that the integrated intensities of the ¹H NMR δ chemical shifts give the ratio of the protons in the different non-equivalent chemical environments of the 3-methylpentane molecule.

The most common solvent used for investigating the 1H NMR spectrum of compounds like 3-methylpentane, is CDCl₃ and other deuterated solvents to avoid confusion with a ¹H NMR signal, ²D (²H) has a different NMR chemical shift.

TMS is the acronym for tetramethylsilane, formula Si(CH₃)₄, whose protons are arbitrarily given a chemical shift of 0.0 ppm. This is the 'standard' in ¹H NMR spectroscopy and all other proton shifts, called chemical shifts, depend on the individual (electronic) chemical environment of the hydrogen atoms in an organic molecule - 3-methylpentane here.

The chemical shifts quoted in ppm on the diagram of the H-1 NMR spectrum of 3-methylpentane represent the peaks of the intensity of the chemical shifts of (which are often groups of split lines at high resolution) AND the relative integrated areas under the peaks gives you the ratio of protons in the different chemical environments of the 3-methylpentane molecule.

3-methylpentane C₆H₁₄,  ,  ,

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

Interpreting the H-1 NMR spectrum of 3-methylpentane

For relatively simple molecules, the low resolution H-1 NMR spectrum of 3-methylpentane is a good starting point (low resolution diagram above).

The hydrogen atoms (protons) of 3-methylpentane occupy 4 different chemical environments so that the low resolution NMR spectra should show 4 peaks of different H-1 NMR chemical shifts (diagram above for 3-methylpentane).

CH₃CH₂CH(CH₃)CH₂CH₃ or (CH₃CH₂)₂CHCH₃  

Note the proton ratio 6:4:1:3 of the four colours of the protons in the four chemically different environments

Although there are 14 hydrogen atoms in the molecule, there only 4 possible different chemical environments for the hydrogen atoms in the symmetrical 3-methylpentane molecule.

Unfortunately I couldn't get very high resolution H-1 NMR data for 3-methylpentane.

The high resolution H-1 NMR spectrum of 3-methylpentane

In terms of spin-spin coupling from the possible proton magnetic orientations, for 3-methylpentane I have only considered the interactions of non-equivalent protons on adjacent carbon atoms

e.g. -CH₂-CH₃, -CH-CH₂- protons etc.

You need very high resolution to sort out all four of the principal 1H NMR resonances of 3-methylpentane.

We can apply the n+1 rule to 3-methylpentane and make some theoretical predictions using some colour coding!

δ (a) CH₃CH₂CH(CH₃)CH₂CH₃:

The 'blue' CH₃ proton resonance will be split by the adjacent CH₂ protons into a 1:2:1 triplet (n+1 = 3).

Both sets of 'blue' methyl group protons are equivalent to each other - same ¹H chemical environment due to the symmetry of the 3-methylpentane molecule, giving the same 1-H NMR chemical shift.

δ (b) CH₃CH₂CH(CH₃)CH₂CH₃:

The 'purple' CH₂ proton resonance will be split by the adjacent sets of three CH₃ protons and the one CH proton into a 1:4:6:4:1 quintet (n+1 = 5)

Both sets of 'purple' -CH₂- group protons are equivalent to each other - same ¹H chemical environment due to the symmetry of the 3-methylpentane molecule.

δ (c) CH₃CH₂CH(CH₃)CH₂CH₃:

The 'green' CH proton resonance will be split into an octet by the adjacent 2 x CH₂ and 1 x CH₃ protons (n+1 = 8).

δ (d) CH₃CH₂CH(CH₃)CH₂CH₃:

The 'brown' CH₃ proton resonance will be split into a doublet by the adjacent 'green' CH proton.

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Key points about the 1H NMR spectrum of 3-methylpentane

The ¹H NMR spectrum of 3-methylpentane shows four distinct proton environments with chemical shifts between 0.9–1.4 ppm, all typical of saturated alkyl groups.

The integration reflects 14 protons in a 6:3:4:1 ratio.

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Key Proton Environments in 3-Methylpentane

3-Methylpentane (C₆H₁₄) is a branched alkane with no electronegative atoms or π systems, so all protons appear in the upfield region.

Here's a breakdown of the chemical shifts and integration:

Chemical Shift (δ, ppm)	Proton Type	Environment	Integration	Notes
~0.90, 0.69-1.01 ppm	CH3	Terminal methyl (2 x C–CH3)	6H	Triplet or doublet depending on coupling
~1.00, 0.69-1.01 ppm	CH3	Branched methyl (C–CH(CH3)–)	3H	Doublet due to adjacent CH
~1.20, 1.01-1.57 ppm	CH2	Methylene (–CH2–)	4H	Multiplet
~1.40 1.01-1.57 ppm	CH	Methine (–CH–) adjacent to CH3	1H	Multiplet
Total			14H	Matches molecular formula C6H14

Sources: ChemicalBook spectrum, Heriot-Watt NMR handout.

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

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Common Misconceptions in ¹H NMR Interpretation

• Expecting downfield signals: No deshielding groups (like O, N, or aromatic rings) are present, so all signals are upfield (<2 ppm).

• Overinterpreting multiplicity: In branched alkanes, overlapping signals and complex coupling can obscure clean splitting patterns.

• Assuming symmetry: Branching breaks symmetry, so more environments appear than in straight-chain alkanes.

• Ignoring integration: Integration is crucial for matching proton counts to molecular formula.

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Exam Revision Tips for ¹H NMR Spectroscopy

These tips apply across AQA, Edexcel, OCR, WJEC, CCEA, CIE, IB HL/SL, and US AP Chemistry:

• Master chemical shift ranges: Alkyl (0.9–1.5 ppm), alcohol (3–4 ppm), aromatic (6–8 ppm), aldehyde (~9–10 ppm).

• Use integration to deduce structure: Match proton counts to molecular formula and identify equivalent groups.

• Practice with branched versus straight-chain alkanes: Compare 3-methylpentane to hexane and 2-methylpentane.

• Understand splitting patterns: Apply the n+1 rule, but be aware of overlapping multiplets in complex molecules.

• Use combined spectra: Integrate IR, MS, and NMR data to confirm identity—common in synoptic questions.

• Label protons on structure diagrams: Helps visualize environments and predict chemical shifts.

Comparing the infrared, mass, 1H NMR and 13C NMR spectra of the five structural alkane isomers of C6H14 NOTE: The images are linked to their original detailed spectral analysis pages AND can be doubled in size with touch screens to increase the definition to the original hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane and 2,3-dimethylbutane image sizes.  These five molecules are structural isomers of saturated alkanes of molecular formula C6H14 and exemplify the infrared, mass, 1H NMR and 13C NMR spectra of lower aliphatic alkanes (non-cyclic alkanes).
Infrared spectra below.
	INFRARED SPECTRA: Apart from the significant differences in the fingerprint region at wavenumbers 1500 to 400 cm-1, there are no other great striking differences, but each could be identified from its infrared spectrum. All the absorption bands are typical of molecules containing saturated alkyl structure and there are no characteristic infrared absorptions due to a specific functional group.
Infrared spectra above, mass spectra below.
	MASS SPECTRA: Base ion peaks plus m/z comments. Hexane: m/z 57, 42 and 56 prominent 2-methylpentane: m/z 43, 42 and 71 prominent 3-methylpentane: m/z 57, 41 and 56 prominent 2,2-dimethylbutane: m/z 43, 41, 57 and 71 prominent 2,3-dimethylbutane: m/z 43, 41, 42 and 71 prominent
Mass spectra above, 1H NMR spectra below.
	1H NMR SPECTRA: They can all be distinguished by their different integrated proton ratios - need very high resolution. Hexane: 3 1H δ shifts, H ratio 3:2:2 (6:4:4 in formula) 2-methylpentane: 5 1H δ shifts, H ratio 6:3:2:2:1 3-methylpentane: 4 1H δ shifts, H ratio 6:4:3:1 2,2-dimethylbutane: 3 1H δ shifts, H ratio 9:3:2 2,3-dimethylbutane: 2 1H δ shifts, H ratio 6:1 (12:2 in formula)
1H NMR spectra above, 13C NMR spectra below.
	13C NMR SPECTRA: From the number of shifts, you can't distinguish (iii) and (iv) but you can distinguish them from (i), (ii) and (v). (i) Hexane: 3 13C δ shifts (ii) 2-methylpentane: 5 13C δ shifts (iii) 3-methylpentane: 4 13C δ shifts (iv) 2,2-dimethylbutane: 4 13C δ shifts (v) 2,3-dimethylbutane: 2 13C δ shifts
13C NMR spectra above.

The splitting pattern from proton spin-spin coupling effects is analysed using the n+1 rule for adjacent non-equivalent proton fields (n is the number of neighbouring protons in a non-equivalent different chemical environment).

Number of protons 1H causing splitting	Splitting pattern produced from the n+1 rule and the theoretical ratio of line intensities
0 means no splitting							1
1 creates a doublet						1		1
2 creates a triplet					1		2		1
3 creates a quartet				1		3		3		1
4 creates a quintet			1		4		6		4		1
5 creates a sextet		1		5		10		10		5		1
6 creates a septet	1		6		15		20		15		6		1

The chemistry of ALKANES revision notes INDEX

The infrared spectrum of 3-methylpentane

The mass spectrum of 3-methylpentane

The C-13 NMR spectrum of 3-methylpentane

H-1 proton NMR spectroscopy index  (Please read 8 points at the top of the ¹H NMR index page)

ALL SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

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