[Author © Dr Phil Brown PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses, IB chemistry & US K12 grade 11, grade 12 and AP honors chemistry courses: Molecular spectroscopy analysis of propanal [spectra page updated Mar 26th 2026 *]

 Mass spectroscopy - spectra index

Introductory note on the mass spectrum of propanal

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

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

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

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 propanal, but the mass spectrometer software does!

An aldehyde The molecular structure and naming of aldehydes and ketones

Interpreting the fragmentation pattern of the mass spectrum of propanal  (propionaldehyde)

[M]⁺ is the molecular ion peak (M) with an m/z of 58 corresponding to [C₃H₆O]⁺, the original propanal molecule minus an electron, [CH₃CH₂CHO]⁺.

The small M+1 peak at m/z 59, corresponds to an ionised propanal molecule with one ¹³C atom in it i.e. an ionised propanal molecule of formula ¹³C¹²C₂H₆O

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.

Propanal 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 (propanal) 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 propanal is the m/z 29 ion [C₂H₅]⁺  or  [CHO]⁺

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

The parent molecular ion for propanal is the m/z 58 ion [CH₃CH₂CHO]⁺  or   [C₃H₆O]⁺

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

Atomic masses: H = 1; C = 12 (13 for ~1 in 100; O = 16; 

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

Examples of suggested possible equations to explain some of the most abundant ion peaks of the mass spectrum of propanal

There are many fragments and lots of possible fragmentation process, some are more favoured than others, but this becomes university level analysis!

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.

Formation of m/z 57 ion:

[CH₃CH₂CHO]⁺  ===>  [CH₃CH₂C=O]⁺  +  H

C-H bond scission in the parent molecular ion,

mass change 58 - 1 = 57 (M-1 ion peak)

The m/z 57 ion may undergo C-C bond scission to give the m/z 28 ion or m/z 29 ion

[CH₃CH₂C=O]⁺  ===>  [C=O]⁺  or  [C₂H₅]⁺

Formation of m/z 39 ion

[C₃H₅O]⁺  ===>  [C₃H₃]⁺  +  H₂O

Elimination of water from the m/z 57  ion gives the m/z 39 ion of [C₃H₃]⁺

mass change 57 - 18 = 39

Other fragmentation reactions can also produce the triangular m/z 39 ion

Formation of m/z 30 and 31 ions:

(i) [?]⁺  ===>  [H₂C=O]⁺  +  ?

mass change = ? - ? = 30

(i) [?]⁺  ===>  [H₃CO]⁺]⁺  +  ?

mass change = ? - ? = 31

Formation of m/z 28 and 29 ions:

Two possibilities

(i) [CH₃CH₂CHO]⁺  ===>  [CHO]⁺  +  CH₃CH₂

(ii) [CH₃CH₂CHO]⁺  ===>  [CH₃CH₂]⁺  +  CHO

C-C bond fission of the parent molecular ion, mass change 58 - 29 = 29.

The m/z 29 ion is the base peak ion, the most abundant ion fragment from two sources, both formed from alternative ionisation of fragments from a C-C bond fission in the parent molecular ion of propanal.

Hydrogen atom loss from either gives the m/z 28 ion [C₂H₄]⁺  or  [CO]⁺ and further proton loss from the m/z 28 ion gives the m/z 26 and 27 ions.

The m/z 26 ion can also be formed by elimination of ethyne (C₂H₂) from larger ions

I'm afraid there are lots of possible ions that can be formed.

I'm not sure which fragment species predominates for these pairs with the same integer m/z value, which I'm sure can be formed by other fragmentation reactions, BUT an accurate mass spectrometer can sort them out - can measure relative fragment ion masses to four decimal places.

e.g. using accurate relative isotopic masses:

¹H = 1.0078  ¹²C = 12.0000 ¹⁶O = 15.9949

Accurate relative ion masses: [CHO]⁺ = 29.0027  and [C₂H₅]⁺ = 29.0390, difference of 0.0363 in relative ion mass

Accurate relative ion masses: [CO]⁺ = 27.9949  and [C₂H₄]⁺ = 28.0312, difference of 0.0363 in ion relative mass

The calculations ignore the relative mass of 0.0005 for the electron lost on ionisation, but this doesn't affect the relative mass difference.

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:

Formed by C-C bond scission in the parent molecular ion or the m/z 29 ion.

[CH₃CH₂CHO]⁺  ===>  [CH₃]⁺  +  C₂H₃O

[CH₃CH₂]⁺  ===>  [CH₃]⁺  +  CH₂

The isomers of molecular formula C₃H₆O (with some selected spectra data)

The chemistry of ALDEHYDES and KETONES revision notes INDEX

The infrared spectrum of Propanal (propionaldehyde)

The H-1 NMR spectrum of Propanal (propionaldehyde)

The C-13 NMR spectrum of Propanal (propionaldehyde)

Mass spectroscopy index

ALL SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

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