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Advanced Organic Chemistry: 1H NMR spectrum of 2,2-dimethylpentane

The H-1 hydrogen-1 (proton) NMR spectrum of 2,2-dimethylpentane

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 2,2-dimethylpentane

C7H16 low and high resolution 1H proton nmr spectrum of 2,2-dimethylpentane analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 H-1 nmr for 2,2-dimethylpentane 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 - 2,2-dimethylpentane here.

The chemical shifts quoted in ppm on the diagram of the H-1 NMR spectrum of 2,2-dimethylpentane 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 2,2-dimethylpentane molecule.

2,2-dimethylpentane C7H16 alkanes structure and naming (c) doc b alkanes structure and naming (c) doc b alkanes structure and naming (c) doc b

Interpreting the H-1 NMR spectrum of 2,2-dimethylpentane

For relatively simple molecules, the low resolution H-1 NMR spectrum of 2,2-dimethylpentane is a good, but limited starting point.

Theoretically, the 16 hydrogen atoms (protons) of 2,2-dimethylpentane occupy 4 different chemical environments so that the low resolution NMR spectra should show 4 principal peaks of different H-1 NMR chemical shifts (diagram above for 2,2-dimethylpentane).

BUT, as you can see, you get only 2 principal peaks in the integrated proton ratio observed of 3:1 for the CH3:CH2 proton ratio.

(CH3)3CCH2CH2CH3

Note the 'real' proton ratio 9:2:2:3 of the 4 colours of the protons in the 4 chemically different environments

Chemical shifts (a) to (d) on the H-1 NMR spectrum diagram for 2,2-dimethylpentane.

Although there are 16 hydrogen atoms in the molecule, there are only 4 possible different chemical environments for the hydrogen atoms in 2,2-dimethylpentane molecule.

The high resolution 1H NMR spectrum of 2,2-dimethylpentane

The ppm quoted on the diagram represent the peak of resonance intensity for a particular proton group in the molecule of 2,2-dimethylpentane - 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 2,2-dimethylpentane below.

So, using the chemical shifts and applying the n+1 rule to 2,2-dimethylpentane and make some predictions using some colour coding! (In problem solving you work the other way round!)

(a) 1H Chemical shift 0.86 ppm for the -C(CH3)3 protons: (CH3)3CCH2CH2CH3

Resonance (a) would not be readily split, since no protons on the adjacent carbon atom on the right.

(b) and (c) 1H Chemical shift 1.18 ppm for the -CH2- protons: (CH3)3CCH2CH2CH3

These are almost identical, but theoretically different - you would need very high resolution spectra to sort them out.

(b) is split into a 1:2:1 triplet by the neighbouring (c) CH2 protons (n+1 = 3).

Evidence of a CH2 group in the molecule.

(c) is split into a sextet by the neighbouring CH2 and CH3 protons (n+1 =6).

Evidence of a CH2-C-CH3 grouping in the molecule.

(d) 1H Chemical shift 0.88 ppm for the 'lone' CH3 group: (CH3)3CCH2CH2CH3

(d) is split into a 1:2:1 triplet by the neighbouring (c) CH2 protons (n+1 = 3).

Evidence of a 2nd CH2 group in the molecule.

Very similar to 1H resonance (a) and would need very high resolution spectrum to resolve (a) and (d).


Number of directly adjacent protons 1H causing splitting Splitting pattern produced from the n+1 rule on spin-spin coupling 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

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