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Advanced Organic Chemistry: 1H NMR spectrum of 2-methoxypropane

Interpreting the H-1 hydrogen-1 (proton) NMR spectrum of 2-methoxypropane

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-methoxypropane

low/high resolution 1H proton nmr spectrum of 2-methoxypropane C4H10O CH3CH(OCH3)CH3 analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 H-1 nmr for methyl isopropyl ether explaining spin-spin coupling for line splitting 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 resonances, called chemical shifts, are measured with respect to the TMS, and depend on the individual (electronic) chemical environment of the hydrogen atoms in an organic molecule - 2-methoxypropane here.

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

2-methoxypropane    C4H10    alcohols and ether structure and naming (c) doc b    alcohols and ether structure and naming (c) doc b    alcohols and ether structure and naming (c) doc b

Interpreting the H-1 NMR spectrum of 2-methoxypropane

In terms of spin-spin coupling from the possible proton magnetic orientations, for 2-methoxypropane I have only considered the interactions of non-equivalent protons on adjacent carbon atoms e.g. -CH2-CH3, -CH2-CH2- protons etc.

For relatively simple molecules, the low resolution H-1 NMR spectrum of 2-methoxypropane is a good starting point (low resolution diagram above, proton ratio  of 1:3:6).

The hydrogen atoms (protons) of 2-methoxypropane occupy 3 different chemical environments so that the low resolution NMR spectra should show 3 principal peaks of different H-1 NMR chemical shifts (diagram above for 2-methoxypropane).

CH3CH(OCH3)CH3  or  (CH3)2CHOCH3

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

Chemical shifts (a) to (c) on the H-1 NMR spectrum diagram for 2-methoxypropane.

Although there are 10 hydrogen atoms in the molecule, there are only 3 possible different chemical environments for the hydrogen atoms in 2-methoxypropane molecule.

The integrated signal proton ratio 6:1:3 observed in the high resolution H-1 NMR spectrum, corresponds with the structural formula of 2-methoxypropane.

The high resolution 1H NMR spectrum of 2-methoxypropane

All low and high resolution spectra of 2-methoxypropane show 3 groups of proton resonances and in the 6:1:3 ratio expected from the formula of 2-methoxypropane.

The 6 protons of the two methyl groups directly attached to the CH group are all equivalent to each other due to the symmetry of 2-methoxypropane.

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

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

(a) 1H Chemical shift 1.20 ppm for protons of two equivalent methyl groups: (CH3)2CHOCH3

This 1H resonance is split into a doublet by the CH group proton (n+1 = 2).

Evidence for the presence of a CH3-C-CH3 grouping in the molecule of 2-methoxypropane

(b) 1H Chemical shift 3.70 ppm for the CH group proton: (CH3)2CHOCH3

This 1H resonance is split into septet by six methyl protons (n+6 = 7).

Evidence for the presence of a ? group in the molecule of 2-methoxypropane

(c) 1H Chemical shift 3.27 ppm for the methoxy group protons: (CH3)2CHOCH3

This 1H resonance does not show line splitting because there is no proton on the adjacent oxygen atom.

Evidence for the presence of an 'isolated' CH3 group in the molecule of 2-methoxypropane.

Note the decreasing effect on the 1H chemical shift as the proton is further from the more electronegative oxygen atom in 2-methoxypropane.


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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