[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-2-ol [spectra page updated RE-EDIT]

 This is a BIG chemistry website, PLEASE take time to explore it

 Mass spectrometry - spectra index  *  A couple of basic practise questions

The molecular structure and naming of aliphatic alcohols and ethers

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.

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

The even smaller 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

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-2-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-2-ol) is usually given an arbitrary abundance value of 100, called the base ion peak, and all other abundances ('intensities') are measured against it.

Base ion peak for the mass spectrum of pentane is the m/z 45 ion [CH₃CHOH]⁺ o  [C₂H₅O]⁺

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of propan-2-ol. Unless otherwise stated, assume all the carbon atoms are the isotope ¹²C.

The parent molecular ion for propan-2-ol is the m/z ion 60  [CH₃CH(OH)CH₃]⁺  =  [C₃H₈O]⁺

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

Suggested 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 (~1% 13); 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

[C₃H₈O]⁺  ===>  [C₃H₇O]⁺  +  H

(the structure of the fragment may not correspond with original molecule)

C-H or O-H bond scission and proton loss from parent molecular ion m/z 60 of propan-2-ol,

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

Formation of m/z 45 ion:

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

[C₃H₈O]⁺  ===>  [C₂H₅O]⁺  +  CH₃

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

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

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

Formation of m/z 43 ion:

[C₃H₈O]⁺  ===>  [C₃H₇]⁺  +  OH

C-O bond scission, loss of hydroxyl group

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

or from the m/z 45 ion?

[C₂H₅O]⁺  ===>  [C₂H₃O]⁺  +  H₂

Not sure whether [CH₃CO]⁺  or  [C₃H₇]⁺  (the latter the most likely?)

However, just thinking theoretically, accurate mass spectrometers can measure ion masses to four decimal places, and can distinguish between the two.

Very accurate relative isotopic masses: ¹H = 1.0078  ¹²C = 12.0000 ¹⁶O = 15.9949

Their accurate relative ion masses have a difference of 0.0363 in relative ion mass

[CH₃CO]⁺ = 43.0183  and [C₃H₇]⁺ = 43.0546

Formation of m/z 42 ion:

[CH₃CH(OH)CH₃]⁺  ===>  [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 39.

Formation of m/z 29 ion:

[CH₃CHOH]⁺  ===>  [C₂H₅]⁺  +  H₂O

Elimination of water from the m/z 45 ion,

mass change 45 - 18 = 27 (M-18 ion peak)

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 (M-45 ion peak)

but the m/z 45 ion is more likely to be formed from this fragmentation reaction.

Extra note on the m/z 19 ion

The m/z ion 19 is due to the formation of the oxonium (hydronium) ion [H₃O]⁺, characteristic of alcohol mass spectra.