Advanced Organic Chemistry: The mass spectrum of propan-1-ol CH3CH2CH2OH

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Interpreting and explaining the mass spectrum of propan-1-ol CH3CH2CH2OH

[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 propan-1-ol [spectra page updated RE-EDIT]

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 Mass spectrometry - spectra index

See also comparing the infrared, mass, 1H NMR and 13C NMR spectra of the 3 isomers of C3H8O

and Isomers of molecular formula C3H8O (with selected spectra data)

Introductory note on the mass spectrum of propan-1-ol (1-propanol)

Students and teachers please note my explanation of the mass spectrum of propan-1-ol is designed for advanced, but pre-university, chemistry courses.

If M represents the propan-1-ol molecule, the initial ionisation to give the molecular ion is:

M(g) + high KE e-  ==> [M•]+(g) + 2e- and for fragmentation equations assume [M]+ is the start of the processes and all species are in a gaseous state.

I've not usually shown an unpaired electron on e.g. an ion or a non-ionised alkyl radical R e.g.

[M•]+ ==> [X]+  +  R•, but you should be aware this is a more accurate depiction of some processes.

I've used simplified equations to show how some of the ions that might be formed in the fragmentation pattern for the mass spectrum of propan-1-ol and only the formation of singly charged positive are considered for the mass spectrum of propan-1-ol.

I've included a stick diagram and table of m/z ions for the mass spectrum of propan-1-ol and doing the mass spectrum analysis under standard conditions, databases can be compiled based on complex fingerprint patterns, often involving the relative intensities of many fragment ions, and used to identify compounds including propan-1-ol.

In selected cases, where two different fragment ions have the same integer m/z value, I've pointed out that modern mass spectrometers can measure relative ion mass to four decimal places. So, using accurate isotopic masses, I've calculated the accurate ion masses, BUT strictly speaking, 0.0005 should be deducted for singly charged ions to account for the loss of the electron in their formation. I have NOT done this for propan-1-ol, but the mass spectrometer software does!

mass spectrum of propan-1-ol fragmentation pattern of m/z m/e ions for analysis and identification of 1-propanol image diagram doc brown's advanced organic chemistry revision notes 

Propan-1-ol C3H8O, alcohols and ether structure and naming (c) doc b , alcohols and ether structure and naming (c) doc b , alcohols and ether structure and naming (c) doc b , alcohols and ether structure and naming (c) doc b

primary alcohol  The molecular structure and naming of aliphatic alcohols and ethers

Interpreting the fragmentation pattern of the mass spectrum of propan-1-ol

[M]+ is the molecular ion peak (M) with an m/z of 60 corresponding to [C3H8O]+, the original propan-1-ol molecule minus an electron, [CH3CH2CH2OH]+

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 propan-1-ol molecule with one 13C atom in it i.e. an ionised propan-1-ol molecule of formula 13C12C2H8O

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.

Propan-1-ol has 3 carbon atoms, so on average, ~1 in 33 molecules will contain a 13C atom.

The most abundant ion of the molecule under mass spectrometry investigation (propan-1-ol) 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 ion peak for the mass spectrum of pentane is the m/z 31 ion [CH2OH]+  or [CH3O]+

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of propan-1-ol. Unless otherwise stated, assume all the carbon atoms are the isotope 12C.

The parent molecular ion for propan-1-ol is the m/z ion 60  [CH3CH2CH2OH]+  =  [C3H8O]+
m/z value of [fragment]+ 59 [C3H7O]+ 57 45 [C2H5O]+ 43 42 41
[molecular fragment]+ [CH3CH2CH2O]+ [C3H5O]+ [CH2CH2OH]+ [C3H7]+ [C3H6]+ [C3H5]+
m/z value of [fragment+ 39 32 ? 31 [CH3O]+ 29 28 27 26 15
[molecular fragment]+ [C3H3]+ [13CH3O]+ [CH2OH]+ [C2H5]+ [C2H4]+ [C2H3]+ [C2H2]+ [CH3]+

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of propan-1-ol

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.

Atomic masses: H = 1; C = 12 (~1% 13); O = 16

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

Examples of suggested possible equations to explain some of the most abundant ion peaks of the mass spectrum of propan-1-ol

Formation of m/z 59 and 57 ion:

[CH3CH2CH2OH]+  ===>  [CH3CH2CH2O]+  +  H

O-H or C-H bond scission and proton loss from the parent molecular ion,

mass loss 60 - 1 =59 (M-1 ion peak)

[CH3CH2CHOH]+ other possible structure of m/z 59 ion?

The m/z 59 ion can eliminate hydrogen to give the m/z 57 ion

[C3H7O]+  ===>  [C3H5O]+  +  H2

Formation of m/z 45 ion:

[CH3CH2CH2OH]+  ===>  [CH2CH2OH]+  +  CH3

C-C bond scission in the parent molecular ion of propan-1-ol, loss of methyl group,

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

Formation of m/z 43 ion:

[CH3CH2CH2OH]+  ===>  [C3H7]+  +  OH

C-O bond scission in the parent molecular ion of propan-1-ol, loss of hydroxyl group,

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

Loss of hydrogen atom/molecule from the m/z 43 ion gives rise to m/z ions of 42 down to 38.

Formation of m/z 42 ion:

[CH3CH2CH2OH]+  ===>  [C3H6]+  +  H2O

Loss of water (mass 18) in an elimination reaction,

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

Loss of hydrogen atom/molecule from the m/z 42 ion gives rise to m/z ions of 41 down to 38.

Formation of m/z 31 ion:

[CH3CH2CH2OH]+  ===>  [CH2=OH]+  +  CH3CH2

C-C bond scission of the parent molecular ion, loss of ethyl group,

mass change 60 - 29 = 31 (M-29 ion peak)

The m/z 31 ion is the base peak ion, the most abundant ion fragment for propan-1-ol.

Formation of m/z 29 ion:

[CH3CH2CH2OH]+  ===>  [CH3CH2]+  +  CH2=OH

C-C bond scission of the parent molecular ion, loss of ethyl group,

mass change 60 - 31 = 29 (M-31 ion peak)

The less likely, but alternative ionisation to the formation of the m/z 31 ion.

Loss of hydrogen atoms from the m/z 29 ion gives rise to ions of m/z 28, 27 and 26.

Formation of m/z 15 ion:

C-C bond scission in the parent molecular ion or any fragment with a methyl group.

[CH3CH2CH2OH]+  ===>  [CH3CH2]+  +  CH2=OH

Key points about the mass spectrum of propan-1-ol and practice questions

Key Mass Spectrum Features of Propan-1-ol (C3H8O, Mᵣ = 60)

m/z value Ion fragment Origin Notes
60 Molecular ion (C3H8O⁺) Whole molecule ionised Often weak due to fragmentation
59 [M–H]⁺ Loss of H radical Sometimes visible
45 CH2OH⁺ Loss of CH3 radical Strong peak
43 C3H7⁺ (propyl cation) Loss of OH radical Often base peak, not here?
31 CH3OH⁺ Hydroxymethyl fragment Diagnostic of alcohols, base peak ion
29 C2H5⁺ Ethyl cation Common alkyl fragment
15 CH3⁺ Methyl cation Very common in organic spectra

Spectra data source https://sdbs.db.aist.go.jp/Disclaimer.aspx for m/z ions

Common Misconceptions

  • Thinking the molecular ion is always the base peak → In alcohols, fragmentation is strong, so M⁺ is often weak.
  • Assuming neutral fragments are detected → Only positive ions are detected.
  • Forgetting isomer differences → Propan-2-ol has same M⁺ (60) but different fragmentation pattern (stronger m/z 45 peak).

Exam Revision Tips

  • Always identify molecular ion peak (M⁺) first to confirm molecular mass.
  • Learn diagnostic fragments: CH2OH⁺ (31) confirms alcohol.
  • Compare isomers: propan-1-ol versus propan-2-ol vs. propanal.
  • Remember base peak ≠ molecular ion.
  • Practice distinguishing alcohol vs. aldehyde vs. ketone spectra.

Practice Multiple Choice Questions based on the mass spectrum of propan-1-ol

Each question has A–D options, model answer, and feedback explaining distractors.

If you think there are any errors email doc b asap

Jot down your responses and check out the ANSWERS!

Q1. What is the molecular ion peak of propan-1-ol?

  1. m/z 31
  2. m/z 60
  3. m/z 43
  4. m/z 15

Q2. Which fragment corresponds to m/z 31?

  1. CH3⁺
  2. CH2OH⁺
  3. C2H5⁺
  4. C3H7⁺

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. CH2OH⁺
  3. C2H3⁺
  4. OH⁺

Q5. Which is an isomer of propan-1-ol also has M⁺ = 60 but different fragmentation?

  1. Propan-2-ol
  2. Propanal
  3. Propanoic acid
  4. Propanone

Q6. Which statement is true about the base peak?

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

Q7. Which feature distinguishes alcohols from ketones in mass spectra?

  1. Hydroxymethyl fragment at m/z 31
  2. Molecular ion mass
  3. Base peak intensity
  4. Fingerprint region

If you think there are any errors email doc b asap
Comparing the infrared, mass, 1H NMR and 13C NMR spectra of the 3 isomers of C3H8O

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 propan-1-ol, propan-2-ol and methoxyethane image sizes.
infrared spectrum of ethoxyethane wavenumbers cm-1 functional group detection fingerprint pattern identification of  diethyl ether doc brown's advanced organic chemistry revision notes I wasn't able to obtain an infrared spectrum for methoxyethane, so I've added the infrared spectrum of ethoxyethane to enable a few comparisons with two aliphatic alcohols

Comparing the infrared spectra of propan-1-ol, propan-2-ol and methoxyethane

Propan-1-ol, propan-2-ol and methoxyethane are structural isomers of molecular formula C3H8O

Propan-1-ol, propan-2-ol and methoxyethane exemplify infrared spectra of the lower members of the homologous series of aliphatic alcohols and ethers

INFRARED SPECTRA (above): There are, as expected, differences in the fingerprint region at wavenumbers 1500 to 400 cm-1, but most absorptions for all three molecules are the various C-O and the many C-H vibrational modes. However, there is one characteristic distinguishing absorption only present in the infrared spectra of alcohols, but not in ethers, that is the broad O-H stretching vibration peaking at ~3350 cm-1. There is also another broad absorption band (origin?) peaking at ~650 cm-1 in the alcohol spectra, but not in the ether spectra.
Comparing the mass spectra of propan-1-ol, propan-2-ol and methoxyethane

Propan-1-ol, propan-2-ol and methoxyethane are structural isomers of molecular formula C3H8O

Propan-1-ol, propan-2-ol and methoxyethane exemplify the mass spectra of the lower members of the homologous series of aliphatic alcohols and ethers

MASS SPECTRA (above): The base ion peaks are m/z 45 for propan-2-ol and methoxyethane, but that of propan-1-ol is m/z 31 which clearly distinguishes it from the other two mass spectra. Many of the fragmentation ions are common to all three spectra. The m/z 45 ion is peak is much smaller in the propan-1-ol spectrum compared to the other two.
Comparing the 1H proton NMR spectra of propan-1-ol, propan-2-ol and methoxyethane

Propan-1-ol, propan-2-ol and methoxyethane are structural isomers of molecular formula C3H8O

Propan-1-ol, propan-2-ol and methoxyethane exemplify the 1H proton NMR spectra of the lower members of the homologous series of aliphatic alcohols and ethers

1H NMR SPECTRA (above): The 1H NMR spectra of all three molecules give different integrated proton ratios for the different 1H chemical environments i.e. the proton ratios are as follows: propan-1-ol 3:2:2:1; propan-2-ol 6:1:1 and methoxyethane 3:2:3. Therefore, all three can be distinguished by their 1H NMR spectra.
Comparing the carbon-13 NMR spectra of propan-1-ol, propan-2-ol and methoxyethane

Propan-1-ol, propan-2-ol and methoxyethane are structural isomers of molecular formula C3H8O

Propan-1-ol, propan-2-ol and methoxyethane exemplify the carbon-13 NMR spectra of members of  the lower members of the homologous series of aliphatic alcohols and ethers

13C NMR SPECTRA (above): The 13C NMR spectra of propan-1-ol and methoxyethane show three different 13C NMR chemical shifts, but propan-2-ol can be distinguished from the other two by exhibiting only two chemical shift lines. You would need other spectral data to distinguish propan-1-ol and methoxyethane.

Key words & phrases: 1-propanol n-propyl alcohol image diagram on how to interpret and explain the mass spectrum of propan-1-ol m/z m/e base peaks, image and diagram of the mass spectrum of propan-1-ol, details of the mass spectroscopy of propan-1-ol,  low and high resolution mass spectrum of propan-1-ol, prominent m/z peaks in the mass spectrum of propan-1-ol, comparative mass spectra of propan-1-ol, the molecular ion peak in the mass spectrum of propan-1-ol, analysing and understanding the fragmentation pattern of the mass spectrum of propan-1-ol, characteristic pattern of peaks in the mass spectrum of propan-1-ol, relative abundance of mass ion peaks in the mass spectrum of propan-1-ol, revising the mass spectrum of propan-1-ol, revision of mass spectroscopy of propan-1-ol, most abundant ions in the mass spectrum of propan-1-ol, how to construct the mass spectrum diagram for abundance of fragmentation ions in the mass spectrum of propan-1-ol, how to analyse the mass spectrum of propan-1-ol, how to describe explain the formation of fragmented ions in the mass spectra of propan-1-ol equations for explaining the formation of the positive ions in the fragmentation of the ionised molecule of propan-1-ol recognising the base ion peak of propan-1-ol 1-propanol n-propyl alcohol isomer of molecular formula C3H8O Stick diagram of the relative abundance of ionised fragments in the fingerprint pattern of the mass spectrum of propan-1-ol. Table of the m/e m/z values and formula of the ionised fragments in the mass spectrum of propan-1-ol. The m/e m/z value of the molecular ion peak in the mass spectrum of propan-1-ol.  The m/e m/z value of the base ion peak in the mass spectrum of propan-1-ol. Possible examples of equations showing the formation of the ionised fragments in propan-1-ol. Revision notes on the mass spectrum of propan-1-ol. Matching and deducing the structure of the propan-1-ol molecule from its mass spectrum. Mass spectroscopy of aliphatic alcohols, mass spectra of propan-1-ol, an isomer of molecular formula C3H8O 1-propanal Explanatory diagram of the mass spectrum of the 2-propanol propan-2-ol molecule in terms of its molecular structure. Listing data of the prominent main peaks in the mass spectrum of 2-propanol propan-2-ol. How to explain the mass spectrum of 2-propanol propan-2-ol. The m/z value of the molecular ion peak in the mass spectrum of 2-propanol propan-2-ol. Identifying 2-propanol propan-2-ol from its mass spectrum pattern. The m/z m/e peak analysis of the mass spectrum of the 2-propanol propan-2-ol molecule. The uses of the mass spectrum of the 2-propanol propan-2-ol molecule. The distinctive features of the mass spectrum of the 2-propanol propan-2-ol molecule explained. explaining the fragmentation pattern of the mass spectrum of 2-propanol propan-2-ol equations showing the formation of the ionised fragments in the mass spectrum of 2-propanol propan-2-ol what does the mass spectrum tell you about the structure and properties of the 2-propanol propan-2-ol molecule? How do you interpret the mass spectrum of propan-1-ol How to interpret the mass spectrum of propan-1-ol Explanatory diagram of the mass spectrum of the propan-1-ol molecule in terms of its molecular structure. Table listing data of the m/z ion prominent main peaks in the mass spectrum of propan-1-ol. How to explain the mass spectrum of propan-1-ol. The m/z value of the molecular ion peak in the mass spectrum of propan-1-ol. Identifying propan-1-ol from its mass spectrum pattern. The m/z m/e peak analysis interpretation diagram of the mass spectrum of the propan-1-ol molecule. The uses of the mass spectrum of the propan-1-ol molecule.  The distinctive features of the mass spectrum of the propan-1-ol molecule explained. explaining the fragmentation pattern of the mass spectrum of propan-1-ol equations showing the formation of the ionised fragments in the mass spectrum of propan-1-ol  what does the mass spectrum tell you about the structure and properties of the propan-1-ol molecule? Data table of ionised fragments in the mass spectrum of propan-1-ol and equations for their formation in the fragmentation of the ionised propan-1-ol molecule.

ANSWERS to the Practice Multiple Choice Questions based on the mass spectrum of propan-1-ol

If you think there are any errors email doc b asap

Q1. What is the molecular ion peak of propan-1-ol?

  1. m/z 31

  2. m/z 60

  3. m/z 43

  4. m/z 15

Answer: B.

  • Correct: molecular ion at 60, equal to the molecular mass.

  • A = hydroxymethyl fragment.

  • C = propyl cation.

  • D = methyl cation.

Q2. Which fragment corresponds to m/z 31?

  1. CH3⁺

  2. CH2OH⁺

  3. C2H5⁺

  4. C3H7⁺

Answer: B.

  • Correct: hydroxymethyl cation.

  • A = 15.

  • C = 29.

  • D = 43.

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

Answer: D.

  • Correct: only ions are deflected/detected.

Q4. Which fragment corresponds to m/z 15?

  1. CH3⁺

  2. CH2OH⁺

  3. C2H3⁺

  4. OH⁺

Answer: A.

  • Correct: methyl cation.

Q5. Which is an isomer of propan-1-ol also has M⁺ = 60 but different fragmentation?

  1. Propan-2-ol

  2. Propanal

  3. Propanoic acid

  4. Propanone

Answer: A.

  • Correct: same molecular mass, different fragmentation pattern.

Q6. Which statement is true about the base peak?

  1. Always molecular ion

  2. Always smallest fragment

  3. Most intense peak

  4. Always hydroxymethyl

Answer: C.

  • Correct: base peak = most intense, not always molecular ion.

Q7. Which feature distinguishes alcohols from ketones in mass spectra?

  1. Hydroxymethyl fragment at m/z 31

  2. Molecular ion mass

  3. Base peak intensity

  4. Fingerprint region

Answer: A.

  • Correct: ketones lack CH2OH⁺ fragment.

If you think there are any errors email doc b asap

Links associated with propan-1-ol (1-propanol)

and Isomers of molecular formula C3H8O (with selected spectra data)

The chemistry of ALCOHOLS revision notes INDEX

The infrared spectrum of Propan-1-ol (1-propanol, n-propyl alcohol)

The H-1 NMR spectrum of Propan-1-ol (1-propanol, n-propyl alcohol)

The C-13 NMR spectrum of Propan-1-ol (1-propanol, n-propyl alcohol)

Mass spectrometry index

Isomers of molecular formula C3H8O (Mr = 60)

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