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Advanced Organic Chemistry: H-1 NMR spectrum of 3-methylpentane

The H-1 hydrogen-1 (proton) NMR spectrum of 3-methylpentane

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 3-methylpentane

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 3-methylpentane analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 1-H nmr for 3-methylpentane 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 - 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, 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 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 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 chemical environments for the hydrogen atoms in 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.

We can apply the n+1 rule to 3-methylpentane and make some 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.

δ (b) CH3CH2CH(CH3)CH2CH3:

The 'purple' CH2 proton resonance will be split by the adjacent 2 sets of adjacent CH2 protons and the 1 CH proton into a 1:3:3:1 quartet (n+1 = 4)

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 CH proton.

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 3-methylpentane, low resolution & high resolution proton nmr spectra of 3-methylpentane, H-1 nmr spectrum of 3-methylpentane, understanding the hydrogen-1 nmr spectrum of 3-methylpentane, explaining the line splitting patterns in the high resolution H-1 nmr spectra of 3-methylpentane, revising the H-1 nmr spectrum of 3-methylpentane, proton nmr of 3-methylpentane, ppm chemical shifts of the H-1 nmr spectrum of 3-methylpentane, 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 3-methylpentane, how to work out the number of chemically different protons in the structure of the 3-methylpentane organic molecule, how to analyse the chemical shifts in the hydrogen-1 H-1 proton NMR spectrum of 3-methylpentane using the n+1 rule to explain the spin - spin coupling ine splitting in the proton nmr spectrum of 3-methylpentane deducing the nature of the protons from the chemical shifts ppm in the H-1 nmr spectrum of 3-methylpentane examining the 1H nmr spectrum of  3-methylpentane analysing the 1-H nmr spectrum of 3-methylpentane how do you sketch and interpret the H-1 NMR spectrum of 3-methylpentane interpreting interpretation of the H-1 proton NMR spectrum of 3-methylpentane

Molecular structure diagram of the proton NMR diagram for the 1H NMR spectrum of 3-methylpentane. The proton ratio in the 1H NMR spectrum of 3-methylpentane. Deducing the number of different chemical environments of the protons in the 3-methylpentane molecule from the 1H chemical shifts in the hydrogen-1 NMR spectrum of 3-methylpentane. Analysing the high resolution 1H NMR spectrum of 3-methylpentane. Analysing the low resolution 1H NMR spectrum of 3-methylpentane. You may need to know the relative molecular mass of 3-methylpentane to deduce the molecular formula from the proton ratio of the 1H NMR spectrum of 3-methylpentane. Revision notes on the proton NMR spectrum of 3-methylpentane. Matching and deducing the structure of the 3-methylpentane molecule from its hydrogen-1 NMR spectrum. Proton NMR spectroscopy of  aliphatic alkanes, 1H NMR spectra of 3-methylpentane, a structural isomer of molecular formula C6H14


Links associated with 3-methylpentane

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)

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