[Author © Dr Phil Brown GRIC, PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses & US K12 grade 11, grade 12 and AP honors chemistry courses: Molecular spectroscopy analysis of ethanoic acid [spectra page updated RE-EDIT]

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Ethanoic acid, C₂H₄O₂, , , , aliphatic carboxylic acid

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]⁺

Unless otherwise stated, C means a ¹²C atom, if not, the isotopic carbon atom ¹³C will be indicated.

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.

Carbon-13 only accounts for ~1% of all carbon atoms (¹²C ~99%), but the more carbon atoms in the molecule, the greater the probability of observing this ¹³C M+1 peak.

Ethanoic acid has 2 carbon atoms, so on average, ~1 in 50 molecules will contain a ¹³C atom.

The most abundant ion of the molecule under mass spectrometry investigation (ethanoic acid) is usually given an arbitrary abundance value of 100, called the base ion peak, and all other abundances ('intensities') are measured against it.

The base peak ion for ethanoic acid is the m/z 43 ion [CH₃CO]⁺

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

Parent molecular ion m/z 60:  [CH₃COOH]⁺

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

Suggested equations explaining the principal fragments of the mass spectrum of ethanoic acid

PLEASE NOTE I have found it difficult to find 'authentic' equations to explain mass spectra fragmentation patterns and it is complex chemistry! I've identified the formulae of the ionised fragments on the mass spectrum diagram, but the equations are from the internet or my conjecture as to how the ions might be formed - please take care in using the information, especially for assignments at university or pre-university level.

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of ethanoic acid

Atomic masses: H = 1;  C = 12 (1 in ~100 is 13);  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 of the parent molecular ion

mass change 60 - 15 = 45 (M-15 ion peak)

Formation of m/z 28, 42 and 43 ions:

(i) [CH₃COOH]⁺  ===>  [CH₃CO]⁺  +  OH

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

C-O bond fission of the parent molecular ion

mass change 60 - 17 = 43 (M-17 ion peak)

(ii) Loss of water from the parent molecular ion gives the m/z 42 ion [CH₂CO]⁺.

[CH₃COOH]⁺  ===>  [CH₂CO]⁺  +  H₂O

mass change 60 - 18 = 42 (M-18 ion peak)

(iii) The m/z 42 ion can split to give the m/z 28 ion [CO]⁺ and CH₂

[CH₂CO]⁺  ===>  [CO]⁺  +  CH₂

mass change 42 - 14 = 28

(iv) The m/z 28 ion could also be formed by scission of the m/z 45 ion [COOH]⁺ ion.

[COOH]⁺  ===>  [CO]⁺  +  OH

mass change 45 - 17 = 28

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

 (v) m/z 15 ion, the [CH₃]⁺ ion.

 [CH₃COOH]⁺  ===>  [CH₃]⁺  +  COOH

mass change 60 - 45 = 15 (M-45 ion peak)

 [CH₃CO]⁺  ===>  [CH₃]⁺  +  CO

mass change 43 - 28 = 15

 

Exam practise questions based on the mass spectrum of ethanoic acid

Key points about the mass spectrum of ethanoic acid and exam practise questions

Ethanoic acid’s mass spectrum shows a molecular ion peak at m/z = 60 (CH₃COOH⁺),

with key fragment peaks at m/z = 45 (COOH⁺), m/z = 43 (CH₃CO⁺), and m/z = 15 (CH₃⁺).

Students often confuse fragment origins or forget that only positively charged ions are detected.

Key Mass Spectrum Features of Ethanoic Acid

Common Misconceptions

• Neutral fragments are detected → False. Only positive ions appear in the spectrum.
• Base peak = molecular ion → Not always; base peak is the most intense, often a fragment.
• Exact m/z values always match theory → Isotopes (¹³C, ²H) can cause minor peaks.
• All acids fragment the same way → Different acids show different dominant fragments.

Exam Revision Tips

• Always identify molecular ion peak (M⁺) first to confirm molecular mass.
• Learn common fragment ions (CH₃⁺, CH₃CO⁺, COOH⁺).
• Compare isomers: e.g., methyl methanoate (ester) has M⁺ = 60 but different fragmentation.
• Remember base peak ≠ molecular ion.
• Practice distinguishing acid vs. ester vs. alcohol spectra.

Practise Multiple Choice Questions based on the mass spectrum of ethanoic acid

Q1. What is the molecular ion peak of ethanoic acid?

1. m/z 43
2. m/z 60
3. m/z 15
4. m/z 45

Q2. Which fragment corresponds to m/z 43?

1. CH₃⁺
2. CH₃CO⁺
3. COOH⁺
4. CH₃COOH⁺

Q3. Why are neutral fragments not detected?

1. They are too small.
2. They are unstable.
3. They escape the detector.
4. They lack charge.

Q4. Which fragment corresponds to m/z 15?

1. CH₃⁺
2. CH₃CO⁺
3. COOH⁺
4. OH⁺

Q5. Which fragment corresponds to m/z 45?

1. CH₃⁺
2. CH₃CO⁺
3. COOH⁺
4. CH₃COOH⁺

Q6. Which peak can clearly distinguish ethanoic acid from its isomer methyl methanoate?

1. m/z 60
2. m/z 43
3. m/z 45
4. m/z 15

Q7. Which statement is true about the base peak?

1. Always molecular ion.
2. Always smallest fragment.
3. Most intense peak.
4. Always COOH⁺.

Q8. Which fragment is absent in ethanol compared to ethanoic acid?

1. CH₃⁺
2. CH₃CO⁺
3. OH⁺
4. COOH⁺

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. SEE also More on the isomers of C2H4O2
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 methyl methanoate 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 acid and methyl methanoate are similar, both molecules give two C-13 NMR spectral lines (meaning 2 different 13C chemical environments).