mass spectrum of 3-hydroxybutanone C4H8O2 CH3COCH(OH)CH3 fragmentation pattern of m/z m/e ions for analysis and identification of acetoin image diagram doc brown's advanced organic chemistry revision notes

Advanced pre-university Organic Chemistry: Mass spectrum of 3-hydroxybutanone

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Interpreting and explaining the mass spectrum of 3-hydroxybutanone (acetoin)

[Author © Dr Phil Brown 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 spectrometry - analysing the mass spectra of 3-hydroxybutanone [spectra page updated April 3rd 2026 *]

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Introductory note on the mass spectrum of 3-hydroxybutanone

Students and teachers please note my explanation of the mass spectrum of 3-hydroxybutanone is designed for advanced, but pre-university, chemistry courses.

If M represents the 3-hydroxybutanone molecule, the initial ionisation to give the molecular ion is:

M_(g) + high KE e^- ==> [M•]⁺_(g) + 2e^- and 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 3-hydroxybutanone and only the formation of singly charged positive are considered for the mass spectrum of 3-hydroxybutanone.

I've included a stick diagram and table of m/z ions for the mass spectrum of 3-hydroxybutanone 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 3-hydroxybutanone.

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 and compared the accurate ion masses if appropriate for 3-hydroxybutan-2-one (acetoin). 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 3-hydroxybutan-2-one (acetoin), but the mass spectrometer software does!

mass spectrum of 3-hydroxybutanone C4H8O2 CH3COCH(OH)CH3 fragmentation pattern of m/z m/e ions for analysis and identification of acetoin image diagram doc brown's advanced organic chemistry revision notes

C4H8O2, 3-hydroxybutanone 3-hydroxybutan-2-one acetoin CH3COCH(OH)CH3 C₄H₈O₂, 3-hydroxybutanone, 3-hydroxybutan-2-one, 'acetoin', CH₃COCH(OH)CH₃

The molecular structure and naming of aliphatic alcohols

The molecular structure and naming of aldehydes and ketones

Interpreting the fragmentation pattern of the mass spectrum of 3-hydroxybutanone

[M]⁺ is the molecular ion peak (M) with an m/z of 88 corresponding to [C₄H₈O₂]⁺, the original 3-hydroxybutanone molecule minus an electron, [CH₃COCH(OH)CH₃]⁺

The small M+1 peak at m/z 89, corresponds to an ionised 3-hydroxybutanone molecule with one ¹³C atom in it i.e. an ionised 3-hydroxybutanone molecule of formula [¹³C¹²C₃H₈O₂]⁺

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.

3-hydroxybutanone has 4 carbon atoms, so on average, ~1 in 4 molecules will contain a ¹³C atom.

The most abundant ion of the molecule under mass spectrometry investigation (3-hydroxybutanone) 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 3-hydroxybutanone is m/z 43 [C₂H₃O]⁺ which may have the structures [CH₃C=O]⁺

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of 3-hydroxybutanone.

Unless otherwise indicated, assume the carbon atoms in 3-hydroxybutanone are the ¹²C isotope.

Some of the possible positive ions, [molecular fragment]⁺, formed in the mass spectrometry of 3-hydroxybutanone.

The parent molecular m/z 88 is [C₄H₈O₂]⁺ or [CH₃COCH(OH)CH₃]⁺

m/z value of [fragment]⁺	87	73	72	71	57	46	45 C₂H₅O
[molecular fragment]⁺	[C₄H₇O₂]⁺	[C₃H₅O₂]⁺	[C₃H₄O₂]⁺	[C₃H₇C=O]⁺	[C₃H₅O]⁺	[¹³C¹²CH₅O]⁺	[CH₃CHOH]⁺

m/z value of [fragment]⁺	44	43	42	29	27	18	17	15
[molecular fragment]⁺	[¹³C¹²CH₃O]⁺	[C₂H₃O]⁺	[C₂H₂O]⁺	[C₂H₅]⁺	[C₂H₃]⁺	[H₂O]⁺	[OH]⁺	[CH₃]⁺

In some mass spectrum diagrams of 3-hydroxybutanone, the m/z 45 ion is shown as the base peak ion, obviously both are the most abundant relatively stable ions.

Both the m/z 43 and 45 ions are formed via the same bond scission of the parent molecular ion.

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 3-hydroxybutanone

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; O = 16

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

Some suggested possible equations to explain the most abundant ion peaks of 3-hydroxybutanone (tabulated above)

Formation of m/z 87 ion:

[CH₃COCH(OH)CH₃]⁺ ===> [C₄H₇O₂]⁺ + H

C-H bond scission, strongest single bond in the parent molecular ion, loss of hydrogen atom,

mass change 88 - 1 = 87 (M-1 ion peak), small peak - low probability.

Formation of m/z 73 ion:

[CH₃COCH(OH)CH₃]⁺ ===> [C₃H₅O₂]⁺ + CH₃

CC bond scission, loss of methyl group from either end of the parent molecular ion.

mass change 88 - 15 = 73 (M-15 ion peak), small peak - low probability.

Formation of m/z 72 ion:

[C₄H₇O₂]⁺ ===> [C₃H₄O₂]⁺ + CH₃

CC bond scission, loss of methyl group from the m/z 87 ion.

mass change 88 - 15 = 73 (M-15 ion peak), small peak - low probability.

Formation of m/z 71 ion:

[CH₃COCH(OH)CH₃]⁺ ===> [CH₃COCHCH₃]⁺ + OH

C-O bond scission, loss of hydroxyl group from the parent molecular ion,

mass change 88 - 17 = 71 (M-17 ion peak)

Formation of m/z 57 ion:

[?]⁺ ===> [C₃H₅O]⁺ + ?

?, but of low probability, very small peak.

Formation of m/z 45 ion:

[CH₃COCH(OH)CH₃]⁺ ===> [CH₃CHOH]⁺ + CH₃CO

C-C bond scission of the parent molecular ion, the 2nd biggest peak, nearly as intense as the base ion m/z 43 peak.

mass change 88 - 43 = 45 (M-43 ion peak)

The m/z 46 ion could be formed in the same way, but containing a ^carbon-13 atom i.e. [¹³C¹²CH₅O]⁺ rather than [C₂H₆O]⁺ or [CH₂O₂]⁺

Formation of m/z 43 ion:

[CH₃COCH(OH)CH₃]⁺ ===> [CH₃CO]⁺ + CH₃CHOH

C-C bond scission of the parent molecular ion to give the [C₂H₃O]⁺ ion.

mass change 88 - 45 = 43 (M-43 ion peak)

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

The m/z 44 ion could be formed in the same way, but the ion contains one ¹³C isotope atom i.e. [¹³C¹²CH₃O]⁺ rather than the [C₂H₄O]⁺ ion.

An accurate mass spectrometer sorts this out, measuring relative fragment ion masses to four decimal places e.g. using very accurate relative isotopic masses,

¹²C = 12.0000 ¹³C = 13.0034, ¹H = 1.0078, ¹⁶O = 15.9949, you can then calculate (predict) that the accurate relative ion masses are:

For m/z 44: [C₂H₄O]⁺ = 44.0261 and [¹³C¹²CH₃O]⁺ = 44.0217, a difference of 0.0044 in relative ion mass.

Formation of m/z 18 ion:

[CH₃COCH(OH)CH₃]⁺ ===> [H₂O]⁺ + C₄H₆O

Elimination of water from the parent molecular ion or a fragment ion

Formation of m/z 17 ion:

[CH₃COCH(OH)CH₃]⁺ ===> [OH]⁺ + CH₃COCHCH₃

C-O bond scission in the parent molecular ion, very low probability

mass change: 88 - 71 = 17

Formation of m/z 15 ion:

[CH₃COCH(OH)CH₃]⁺ ===> [CH₃]⁺ + C₃H₅O₂

C-C bond scission of the parent molecular ion, mass change 88 - 55 = 43, or from C-C bond scission in one of the larger fragments e.g. the m/z 43 or 45 ions.

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Links associated with 3-hydroxybutanone

The infrared spectrum of 3-hydroxybutanone (3-hydroxybutan-2-one, 'acetoin')

The H-1 spectrum of 3-hydroxybutanone (3-hydroxybutan-2-one, 'acetoin')

The C-13 spectrum of 3-hydroxybutanone (3-hydroxybutan-2-one, 'acetoin')

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