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Advanced Organic Chemistry: Mass spectrum of propan-2-ol

The mass spectrum of propan-2-ol  (2-propanol)

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 propan-2-ol

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

mass spectrum of propan-2-ol fragmentation pattern of m/z m/e ions for analysis and identification of 2-propanol image diagram doc brown's advanced organic chemistry revision notes 

Propan-2-ol C3H8O, alcohols and ether structure and naming (c) doc b , alcohols and ether structure and naming (c) doc balcohols and ether structure and naming (c) doc b , alcohols and ether structure and naming (c) doc b

Interpreting the fragmentation pattern of the mass spectrum of propan-2-ol  (2-propanol, isopropyl alcohol)

[M]+ is the molecular ion peak (M) with an m/z of 60 corresponding to [C3H8O]+, the original propan-2-ol molecule minus an electron, [CH3CH(OH)CH3]+.

However, this is a highly unstable molecular ion, only a trace is detected.

The small M+1 peak at m/z 61, corresponds to an ionised propan-2-ol molecule with one 13C atom in it i.e. an ionised propan-2-ol molecule of formula 13C12C2H8O

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of propan-2-ol.

m/z value of [fragment]+ 59 45 43 ? 43 ? 41
[molecular fragment]+ [CH3CHOCH3]+ [CH3CHOH]+ [CH3CO]+ [C3H7]+ [C3H5]+
m/z value of [fragment]+ 39 31 29 ? 29 27 19 15
[molecular fragment]+ [C3H3]+ [CH2OH]+ [CHO]+ [C2H5]+ [C2H3]+ [?]+ [CH3]+

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of propan-2-ol

Examples of equations to explain some of the most abundant ion peaks of the mass spectrum of propan-2-ol

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

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

Formation of m/z 59 ion:

[CH3CH(OH)CH3]+  ===>  [CH3CHOCH3]+  +  H

C-H bond scission and proton loss from parent molecular ion m/z 60 of propan-2-ol, mass change 60 - 1 = 59..

Formation of m/z 45 ion:

[CH3CH(OH)CH3]+  ===>  [CH3CHOH]+  +  CH3

C-C bond scission of the parent molecular ion, mass change 60 - 15 = 45.

The m/z 45 ion is the base peak, the most abundant stable ion fragment for propan-2-ol.

Formation of m/z 27 ion:

[CH3CHOH]+  ===>  [CH3CHOH]+  +  H2O

Elimination of water from the m/z 45 ion, mass change 45 - 18 = 27.

Formation of m/z 15 ion:

[CH3CH(OH)CH3]+  ===>  [CH3]+  +  CH3CHOH

C-C bond scission of the parent molecular ion, mass change 60 - 45 = 15, but the m/z 45 ion is more likely to be formed from this fragmentation reaction.

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. 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. The molecular ion peak height for propan-2-ol is relative small compared to the other two spectra suggesting it is much the less stable m/z 56 ion.

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: 2-propanol isopropyl alcohol image diagram on how to interpret and explain the mass spectrum of propan-2-ol m/z m/e base peaks, image and diagram of the mass spectrum of propan-2-ol, details of the mass spectroscopy of propan-2-ol,  low and high resolution mass spectrum of propan-2-ol, prominent m/z peaks in the mass spectrum of propan-2-ol, comparative mass spectra of propan-2-ol, the molecular ion peak in the mass spectrum of propan-2-ol, analysing and understanding the fragmentation pattern of the mass spectrum of propan-2-ol, characteristic pattern of peaks in the mass spectrum of propan-2-ol, relative abundance of mass ion peaks in the mass spectrum of propan-2-ol, revising the mass spectrum of propan-2-ol, revision of mass spectroscopy of propan-2-ol, most abundant ions in the mass spectrum of propan-2-ol, how to construct the mass spectrum diagram for abundance of fragmentation ions in the mass spectrum of propan-2-ol, how to analyse the mass spectrum of propan-2-ol, how to describe explain the formation of fragmented ions in the mass spectra of propan-2-ol equations for explaining the formation of the positive ions in the fragmentation of the ionised molecule of propan-2-ol recognising the base ion peak of propan-2-ol 2-propanol isopropyl 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-2-ol. Table of the m/e m/z values and formula of the ionised fragments in the mass spectrum of propan-2-ol. The m/e m/z value of the molecular ion peak in the mass spectrum of propan-2-ol.  The m/e m/z value of the base ion peak in the mass spectrum of propan-2-ol. Possible examples of equations showing the formation of the ionised fragments in propan-2-ol. Revision notes on the mass spectrum of propan-2-ol. Matching and deducing the structure of the propan-2-ol molecule from its mass spectrum. Mass spectroscopy of aliphatic alcohols, mass spectra of propan-2-ol, an isomer of molecular formula C3H8O 2-propanol


Links associated with propan-2-ol

The chemistry of ALCOHOLS revision notes INDEX

The infrared spectrum of Propan-2-ol (2-propanol, isopropyl alcohol)

The H-1 NMR spectrum of Propan-2-ol (2-propanol, isopropyl alcohol)

The C-13 NMR spectrum of Propan-2-ol (2-propanol, isopropyl alcohol)

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