[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 spectroscopy analysis of butanal [updated Mar 20th 2026 *]

 Mass spectroscopy - spectra index

Introductory note on the mass spectrum of butanal

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

If M represents the butanal 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 butanal and only the formation of singly charged positive are considered for the mass spectrum of butanal.

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

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 butanal, but the mass spectrometer software does!

Butanal (butyraldehyde), C₄H₈O,  , , aldehyde

The molecular structure and naming of aldehydes and ketones

Interpreting the fragmentation pattern of the mass spectrum of butanal

[M]⁺ is the molecular ion peak (M) with an m/z of 72 corresponding to [C₄H₈O]⁺, the original butanal molecule minus an electron, [CH₃CH₂CH₂CHO]⁺.

The small M+1 peak at m/z 73, corresponds to an ionised butanal molecule with one ¹³C atom in it i.e. an ionised butanal molecule of formula ¹³C¹²C₃H₈O

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.

Butanal has 4 carbon atoms, so on average, ~1 in 25 molecules will contain a ¹³C atom.

The most abundant ion of the molecule under mass spectrometry investigation (butanal) 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 butanal is m/z 44 ion  [C₂H₄O]⁺  or  [CH₂=CH-OH]⁺

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

The parent molecular ion is the m/z 72 ion [CH₃CH₂CH₂CHO]⁺.

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

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; C=O = 743

Possible equations to explain some of the most abundant ion peaks of butanal

Formation of m/z 71 ion:

[CH₃CH₂CH₂CHO]⁺  ===>  [CH₃CH₂CH₂C=O]⁺  +  H

C-H bond scission in parent molecular ion, hydrogen atom loss, mass change 72 - 1 = 71 (M-1 ion peak)

Formation of m/z 57 ion:

[CH₃CH₂CH₂CHO]⁺  ===>  [CH₂CH₂CHO]⁺  +  CH₃

C-C bond scission in the parent molecular ion, loss of end methyl group, mass change 72 - 15 = 57 (M-15 ion peak)

Formation of m/z 54 ion from water loss:

[CH₃CH₂CH₂CHO]⁺  ===>  [C₄H₆]+  +  H₂O

Elimination of water, mass change 72 - 18 = 54 (M-18 ion peak)

(Note there is a tiny peak for an m/z 18 ion, evidence of water present, so there is a small chance the water molecule becomes ionised and the organic fragment is electrically neutral.

Formation of m/z 44 ion:

[CH₃CH₂CH₂CHO]⁺  ===>  [CH₂=CH-OH]⁺  +  C₂H₄

This ion is formed by a rearrangement, followed by a C-C bond fission,

mass change 72 - 28 = 44 (M-28 ion peak)

The m/z 44 ion is the base peak, the most abundant 'stable' ion fragment.

The m/z 45 ion could be formed in the same way, but ion has a ¹³C carbon atom in it i.e. [¹³C¹²CH₄O]⁺. rather than a [C₂H₅O]⁺ ion?

Note that an accurate mass spectrometer can sort out ions with the same integer m/z value because they can measure relative fragment ion masses to four decimal places.

e.g. using accurate relative isotopic masses:

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

For m/z 45: [C₂H₅O]⁺ = 45.0339  and  [¹³C¹²CH₄O]⁺ = 45.0295, relative ion mass difference of 0.0044.

Formation of m/z 43 ion:

[CH₃CH₂CH₂CHO]⁺  ===>  [CH₃CH₂CH₂]⁺  +  HC=O

C-C bond fission of the parent molecular ion, breaking off the aldehyde group.

Mass change 72 - 29 = 43 (M-31 ion peak)

or alternatively

[CH₃CH₂CH₂CHO]⁺  ===>  [CH₃CO]⁺  +  CH₃CH₂

Also a C-C bond scission of the parent molecular ion,

mass change 72 - 29 = 43 (M-29 ion peak).

Therefore we have two different ions with the same integer m/z value, but modern mass spectrometers can measure m/z values to four decimal places, so we can calculate ion masses using very accurate relative isotopic masses:

¹H = 1.0078    ¹²C = 12.0000   ¹⁶O = 15.9949, which can the used to calculate ...

relative mass of [CH₃CO]⁺ = 43.0183  and relative mass of [C₃H₇]⁺ = 43.0546

Mass difference 0.0363, no problem at all, and the mass spectrometer software can even subtract 0.0005 mass units for the loss of one electron!

So, the two different ions of the same integer m/z value can be separately identified and there separate abundances (peak intensity) can be measured.

The m/z 43 ion can lose hydrogen atoms to give the m/z ions 42 down to 39.

Formation of m/z 29 ion:

[CH₃CH₂CH₂CHO]⁺  ===>  [HC=O]⁺  +  CH₃CH₂CH₂

or alternatively: [CH₃CH₂CH₂CHO]⁺  ===>  [CH₃CH₂]⁺  +  CH₂CHO

Both involve C-C bond scission of the parent molecular ion, mass change 72 - 43 = 29.

Hydrogen loss from the [CH₃CH₂]⁺ gives m/z ions from 28 down to 26.

Formation of m/z 28 ion:

[CH₃CH₂CH₂CHO]⁺  ===>  [C₂H₄]⁺  +  CH₂=CH-OH

An alternative ionisation to C-C bond fission forming m/z ion 44 above.

The m/z 28 ion could also be [CO]⁺ as well as [CH₂=CH₂]⁺.

Formation of m/z 15 ion from water loss:

[CH₃CH₂CH₂CHO]⁺  ===>  [CH₃]⁺  +  CH₂CH₂CHO

C-C bond scission of parent molecular ion, mass change 72 - 57 = 15.

The m/z 15 ion can be formed from smaller fragments by the same process e.g. from m/z 71, 57 or 43 ions.

The chemistry of ALDEHYDES and KETONES revision notes INDEX

The infrared spectrum of Butanal

The H-1 NMR spectrum of Butanal

The C-13 NMR spectrum of Butanal

Mass spectroscopy index

ALL SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

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