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Advanced Organic Chemistry: H-1 NMR spectrum of ethanoic acid

The H-1 hydrogen-1 (proton) NMR spectrum of ethanoic acid (acetic acid)

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 ethanoic acid

low and high resolution H-1 proton nmr spectrum of ethanoic acid analysis interpretation of chemical shifts ppm spin spin line splitting diagram 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 - ethanoic acid here.

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

Ethanoic acid C2H4O2  (c) doc b , (c) doc b , (c) doc b , (c) doc b

For more see Structure and naming of CARBOXYLIC ACIDS and DERIVATIVES, including isomers

Interpreting the H-1 NMR spectrum of ethanoic acid

For relatively simple molecules, the low resolution H-1 NMR spectrum of ethanoic acid is a good starting point.

The hydrogen atoms (protons) of ethanoic acid occupy 2 different chemical environments so that the H-1 proton low resolution NMR spectra should show principal 2 peaks in the ratio 3 : 1 (diagram above).

CH3COOH

(note the 3: 1 ratio of the 2 colours of the protons indicating the 2 chemically different environments)

The proton ratio observed of 3:1, corresponds with the structural formula of ethanoic acid.

The two 1H chemical environments in the ethanoic acid molecule are the methyl group protons (CH3, 2.1 ppm chemical shift) and the hydroxyl proton (OH, 11.4 ppm chemical shift).

The latter is quite large due to the combined field effect of the two highly electronegative oxygen atoms.

There is no need to consider the very high resolution spectrum of ethanoic acid, as there are no other immediate protons adjacent to the H of the hydroxyl OH group, or the H's of the CH3 group so the spin-spin splitting lines would be to weak to be observed.


Comparing the infrared, mass, 1H NMR and 13C NMR spectra of the 2 isomers of C2H4O2

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 ethanoic acid (acetic acid) and methyl methanoate (methyl formate) image sizes.

INFRARED SPECTRA: Apart from the significant differences in the fingerprint region at wavenumbers 1500 to 400 cm-1, the most striking difference is the broad O-H stretching band ~3200 cm-1, found in the infrared spectrum of carboxylic acids, but absent in the infrared spectrum of esters.

MASS SPECTRA: Apart from the m/z of 60 for the parent molecular ion and m/z 15 ion [CH3]+, both ethanoic acid and methyl methanoate show few similarities in their mass spectra. Their base ion peaks are quite different - for ethanoic acid it is m/z 43 and for methyl methanoate it is m/z 31.

1H NMR SPECTRA: The 1H NMR spectra of ethanoic acid and methoxymethane are similar with two 'wide-apart' chemical shift singlet peaks in the integrated proton ratio of 3:1 (meaning 2 different 1H chemical environments). However, although no splitting is observed, the 1H chemical shifts for the two molecules are different, those of ethanoic acid are much further apart.

13C NMR SPECTRA: The 13C NMR spectra of ethanoic and methoxymethane are similar, both molecules give two C-13 NMR spectral lines (meaning 2 different 13C chemical environments).


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 ethanoic acid, low resolution & high resolution proton nmr spectra of ethanoic acid, H-1 nmr spectrum of ethanoic acid, understanding the hydrogen-1 nmr spectrum of ethanoic acid, explaining the line splitting patterns in the high resolution H-1 nmr spectra of ethanoic acid, revising the H-1 nmr spectrum of ethanoic acid, proton nmr of ethanoic acid, ppm chemical shifts of the H-1 nmr spectrum of ethanoic acid, 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 ethanoic acid, how to work out the number of chemically different protons in the structure of the ethanoic acid organic molecule, how to analyse the chemical shifts in the hydrogen-1 H-1 proton NMR spectrum of ethanoic acid


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