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Advanced Organic Chemistry: H-1 NMR spectrum of hexane

The H-1 hydrogen-1 (proton) NMR spectrum of hexane

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US K12 grade 11 grade 12 organic chemistry courses involving molecular spectroscopy analysing H-1 NMR spectra of hexane

See also comparing infrared, mass, 1H NMR & 13C NMR spectra of the structural alkane isomers of C6H14

low and high resolution H-1 proton nmr spectrum of hexane analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 1-H nmr for hexane doc brown's advanced organic chemistry revision notes

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 - hexane here.

The chemical shifts quoted in ppm on the diagram of the H-1 NMR spectrum of hexane 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 hexane molecule.

Hexane C6H14, alkanes structure and naming (c) doc b , alkanes structure and naming (c) doc b , alkanes structure and naming (c) doc b

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

Interpreting the H-1 NMR spectrum of hexane

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

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

CH3CH2CH2CH2CH2CH3

Note the ratio 3:2:2 of the three colours of the protons in the three chemically different environments

Although there are 14 hydrogen atoms in the molecule, there only 3 possible chemical environments for the hydrogen atoms in hexane molecule.

The integrated signal proton ratio 6:4:4 = 3:2:2 observed, corresponds with the structural formula of hexane shown above.

The high resolution H-1 NMR spectrum of hexane

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

e.g. -CH2-CH3, -CH-CH2- protons etc.

All low and high resolution spectra of hexane show 3 groups of protons and in the ratio expected from the formula of hexane.

The ppm quoted on the diagram represent the peak of resonance intensity for a particular proton group in the molecule of hexane - since the peak' is at the apex of a band of H-1 NMR resonances due to spin - spin coupling field splitting effects - see high resolution notes on hexane below.

So, using the chemical shifts and applying the n+1 rule to hexane

δ (a) 1H Chemical shift 0.89 ppm for the 2 x CH3 protons  CH3CH2CH2CH2CH2CH3

The CH3 proton resonance is split into a 1:2:1 triplet by the adjacent CH2 protons, (n+1 = 3).

Evidence for the presence of a CH2 group in the molecule of hexane

δ (b) 1H Chemical shift 1.29 ppm for the 2 x 'outer' CH2 protons  CH3CH2CH2CH2CH2CH3

This CH2 proton resonance is split into a 1:5:10:10:5:1 sextet by the CH3 and CH2 protons on either side (n+1 = 6)

Evidence for the presence of a CH3CH2CH2 group in the molecule of hexane

δ (c) 1H Chemical shift 1.27 ppm for the 2 x 'central' CH2 protons  CH3CH2CH2CH2CH2CH3

At first sight you would think this CH2 proton resonance is split into a 1:4:6:4:1 quintet by the CH2 and CH2 protons on either side (n+1 = 5).

However, theoretically, these particular pair of CH2 protons are equivalent to each other due to their central symmetry in the symmetrical hexane molecule and should not split each other's resonance.

Therefore, theoretically, this resonance is just split by one the adjacent groups of CH2 protons into a 1:2:1 triplet (n+1 = 3).

I would appreciate a second opinion on this interpretation.

Very high resolution is needed to sort out the two CH2 resonances because the chemical shifts are so close together.

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.
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

Key words & phrases: Interpreting the proton H-1 NMR spectra of hexane, low resolution & high resolution proton nmr spectra of hexane, H-1 nmr spectrum of hexane, understanding the hydrogen-1 nmr spectrum of hexane, explaining the line splitting patterns in the high resolution H-1 nmr spectra of hexane, revising the H-1 nmr spectrum of hexane, proton nmr of hexane, ppm chemical shifts of the H-1 nmr spectrum of hexane, explaining and analyzing spin spin line splitting in the H-1 nmr spectrum, how to construct the diagram of the H-1 nmr spectrum of hexane, how to work out the number of chemically different protons in the structure of the hexane organic molecule, how to analyse the chemical shifts in the hydrogen-1 H-1 proton NMR spectrum of hexane using the n+1 rule to explain the spin - spin coupling ine splitting in the proton nmr spectrum of hexane deducing the nature of the protons from the chemical shifts ppm in the H-1 nmr spectrum of hexane examining the 1H nmr spectrum of  hexane analysing the 1-H nmr spectrum of hexane how do you sketch and interpret the H-1 NMR spectrum of hexane interpreting interpretation of the H-1 proton NMR spectrum of hexane

Molecular structure diagram of the proton NMR diagram for the 1H NMR spectrum of hexane. The proton ratio in the 1H NMR spectrum of hexane. Deducing the number of different chemical environments of the protons in the hexane molecule from the 1H chemical shifts in the hydrogen-1 NMR spectrum of hexane. Analysing the high resolution 1H NMR spectrum of hexane. Analysing the low resolution 1H NMR spectrum of hexane. You may need to know the relative molecular mass of hexane to deduce the molecular formula from the proton ratio of the 1H NMR spectrum of hexane. Revision notes on the proton NMR spectrum of hexane. Matching and deducing the structure of the hexane molecule from its hydrogen-1 NMR spectrum. Proton NMR spectroscopy of  aliphatic alkanes, 1H NMR spectra of hexane, an isomer of molecular formula C6H14


Links associated with hexane

The chemistry of ALKANES revision notes INDEX

The infrared spectrum of hexane

The mass spectrum of hexane

The H-1 NMR spectrum of hexane

The C-13 NMR spectrum of hexane

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

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