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Advanced Organic Chemistry: 1H NMR spectrum of methyl propanoate

Interpreting the proton NMR spectrum of methyl propanoate (methyl propionate)

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 methyl propanoate

1H proton nmr spectrum of methyl propanoate low/high resolution diagrams C4H8O2 CH3CH2COOCH3 analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 H-1 nmr for methyl propionate 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 - methyl propanoate here.

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

Methyl propanoate   (methyl propionate)   (c) doc b   (c) doc b   (c) doc b  an ester

Interpreting the H-1 NMR spectrum of methyl propanoate

In terms of spin-spin coupling from the possible proton magnetic orientations, for methyl propanoate I have only considered the interactions of non-equivalent protons on adjacent carbon atoms e.g. -CH2-CH3, -CH2-CH2-, protons etc. (problems with -OH N-H proton resonance)

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

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

CH3CH2COOCH3

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

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

Although there are 8 hydrogen atoms in the molecule, there are only 3 possible different chemical environments for the 8 hydrogen atoms in methyl propanoate molecule.

The integrated signal proton ratio CH3CH2COOCH3 observed in the high resolution H-1 NMR spectrum, corresponds with the structural formula of methyl propanoate.

The high resolution 1H NMR spectrum of methyl propanoate

All low and high resolution spectra of methyl propanoate show 3 groups of proton resonances and in the 3:2:3 ratio expected from the structural formula of methyl propanoate.

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

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

(a) 1H Chemical shift 1.15 ppm, methyl protons : CH3CH2COOCH3

This resonance is split into a 1:2:1 triplet by the CH2 protons (n+1 = 3).

Evidence for the presence of a CH2 group in the molecule of methyl propanoate

(b) 1H Chemical shift 2.32 ppm, CH2 protons : CH3CH2COOCH3

This resonance is split into a 1:3:3:1 quartet by the CH3 protons (n+1 = 4).

Evidence for the presence of a CH3 group in the molecule of methyl propanoate

(c) 1H Chemical shift 3.67 ppm, methyl protons: CH3CH2COOCH3

This resonance is recorded as a singlet, because there are no protons on an adjacent atom.

Evidence from the proton ratio suggests the presence of a 2nd methyl group in the molecule of methyl propanoate

Note the decreasing effect on the 1H chemical shift as the proton is further from the more electronegative oxygen atoms in methyl propanoate.


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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