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Advanced Organic Chemistry: Mass spectrum of ethanoic acid

The mass 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 mass spectra of ethanoic acid

mass spectrum of ethanoic acid fragmentation pattern of m/z m/e ions for analysis and identification of ethanoic acid image diagram doc brown's advanced organic chemistry revision notes 

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 mass spectrum of ethanoic acid

[M]+ is the molecular ion peak (M) with an m/z of 60 corresponding to [C2H4O2]+, the original ethanoic acid molecule minus an electron, [CH3COOH]+.

The small M+1 peak at m/z 61, corresponds to an ionised ethanoic acid molecule with one 13C atom in it i.e. an ionised ethanoic acid molecule of formula 13C12CH4O2

Carbon consists of ~1% 13C atoms, ethanoic acid has 2 carbon atoms, so about 1 in 50 molecules or fragments may contain a carbon-13 atom.

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of ethanoic acid.

Parent molecular ion:  [CH3COOH]+

(m/e) m/z value of [fragment]+ 45 43 42 29 28 15
[molecular fragment]+ [COOH]+ [CH3CO]+ [CH2CO]+ [HCO]+ [CO]+ [CH3]+

Explaining the principal fragments of the mass spectrum of ethanoic acid

Atomic masses: H = 1;  C = 12;  O = 16

Bond enthalpies = kJ/mol: C-C = 348;  C-H = 412;  C-O = 360;  C=O  = 743;  O-H = 463

The formation of the two most abundant ions arises from a C-C or a C-O bond scission

Formation of m/z 45 ion:

[CH3COOH]+  ===>  [COOH]+  +  CH3

C-C bond scission, mass change 60 - 15 = 45.

Formation of m/z 43 ion:

[CH3COOH]+  ===>  [CH3CO]+  +  OH

The m/z 43 ion is the base peak, the most stable fragment.

Loss of hydrogen gives the m/z 42 ion [CH2CO]+.

The m/z 42 ion can split to give the m/z 28 ion [CO]+.

The m/z 28 ion could also be formed by scission of the [COOH]+ ion.

The m/z 60 [CH3COOH]+ and m/z 43 [CH3CO]+ ions can break down forming the m/z 15 [CH3]+ ion.


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. The mass spectrum of ethanoic acid has very prominent peaks for m/z ions 43 and 45, both of which arise from the splitting of ethanoic acid's parent molecular ion..

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).

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).


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