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Advanced Organic Chemistry: 1H NMR spectrum of methyl ethanoate

Interpreting the H-1 (proton) NMR spectrum of methyl ethanoate (methyl acetate)

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 ethanoate

1H proton nmr spectrum of methyl ethanoate low/high resolution diagrams C3H6O2 CH3COOCH3 analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 H-1 nmr for methyl acetate 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 ethanoate here.

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

methyl ethanoate (methyl acetate), (c) doc b,(c) doc b , (c) doc b, (c) doc b

Interpreting the H-1 NMR spectrum of methyl ethanoate

In terms of spin-spin coupling from the possible proton magnetic orientations, for methyl ethanoate I have only considered the interactions of non-equivalent protons on adjacent carbon atoms BUT this does not apply to methyl ethanoate

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

CH3COOCH3

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

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

Although there are 6 hydrogen atoms in the molecule, there are only 2 possible different chemical environments for the hydrogen atoms in methyl ethanoate molecule.

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

The high resolution 1H NMR spectrum of methyl ethanoate

All low and high resolution spectra of methyl ethanoate show 2 groups of proton resonances and in the 1:1 ratio expected from the structural formula of methyl ethanoate.

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

Because there no protons on adjacent carbon atoms, the n+1 rule does not apply to methyl ethanoate.

The two 1H NMR resonances are therefore NOT split by spin - spin coupling and both appear on the spectrum as singlets in the proton ratio 1:1 (because there are to groups of CH3 protons, 3:3 = 1:1).

(a) 1H Chemical shift 2.05 ppm single for methyl group protons: CH3COOCH3

(b) 1H Chemical shift 3.66 ppm singlet: for methyl group protons: CH3COOCH3


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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