mass spectrum of 2-methylpropanal C4H8O (CH3)2CHCHO fragmentation pattern of m/z m/e ions for analysis and identification of isobutyraldehyde image diagram doc brown's advanced organic chemistry revision notes

Advanced Organic Chemistry: Mass spectrum of 2-methylpropanal (CH₃)₂CHCHO

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Interpreting the mass spectrum of 2-methylpropanal (iso-butyraldehyde)

[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 2-methylpropanal (mass spectra) [spectra page updated April 3rd 2026 *]

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Introductory note on the mass spectrum of 2-methylpropanal

Students and teachers please note my explanation of the mass spectrum of 2-methylpropanal is designed for advanced, but pre-university, chemistry courses.

If M represents the 2-methylpropanal 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 2-methylpropanal.

I've included a stick diagram and table of m/z ions for the mass spectrum of 2-methylpropanal 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 2-methylpropanal.

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 2-methylpropanal. 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 2-methylpropanal, but the mass spectrometer software does!

mass spectrum of 2-methylpropanal C4H8O (CH3)2CHCHO fragmentation pattern of m/z m/e ions for analysis and identification of isobutyraldehyde image diagram doc brown's advanced organic chemistry revision notes

2-methylpropanal C₄H₈O

The molecular structure and naming of aldehydes and ketones

Interpreting the fragmentation pattern of the mass spectrum of 2-methylpropanal

[M]⁺ is the molecular ion peak (M) with an m/z of 72 corresponding to [C₄H₈O]⁺, the original 2-methylpropanal molecule minus an electron, [(CH₃)₂CHCHO]⁺

The small M+1 peak at m/z 73 ion, corresponds to an ionised 2-methylpropanal molecule with one ¹³C atom in it i.e. an ionised 2-methylpropanal 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.

2-methylpropanal 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 (2-methylpropanal) 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 peak ion for the mass spectrum of 2-methylpropanal is the m/z 43 [C₃H₇]⁺

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

Unless otherwise indicated, assume the carbon atoms in 2-methylpropanal are the ¹²C isotope.

Some of the possible positive ions, [molecular fragment]⁺, formed in the mass spectrometry of 2-methylpropanal.

The parent molecular ion peak, m/z 72 is for the [C₄H₈O]⁺ or [(CH₃)₂CHCHO]⁺. ion

m/z value of [fragment]⁺	71	57	55	44	43	42	41	40
[molecular fragment]⁺	[C₄H₇O]⁺	[C₃H₅O]⁺	[C₃H₃O]⁺	[¹³C¹²C₂H₇]⁺	[C₃H₇]⁺	[C₃H₆]⁺	[C₃H₅]⁺	[C₃H₄]⁺

m/z value of [fragment]⁺	40	39	38	37	29	28	28	27	26	15
[molecular fragment]⁺	[C₃H₄]⁺	[C₃H₃]⁺	[C₃H₂]⁺	[C₃H]⁺	[CHO]⁺	[CO]⁺	[C₂H₄]⁺	[C₂H₃]⁺	[C₂H₂]⁺	[CH₃]⁺

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

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 the most abundant ion peaks of 2-methylpropanal (tabulated above)

Formation of m/z 71 ion:

[(CH₃)₂CHCHO]⁺ ===> [C₄H₇O]⁺ + H

Scission of C-H bond in the aldehyde group?,

mass change 72 - 1 = 71 (M-1 ion peak)

Formation of m/z 57 ion:

[(CH₃)₂CHCHO]⁺ ===> [C₃H₅O]⁺ + CH₃

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

mass change 72 - 15 = 57 (M-15 ion peak)

Structure of ion?: [CH₃CH₂C=O]⁺ or [CH₃CHCHO]⁺ ?

Formation of m/z 55 ion:

[C₃H₅O]⁺ ===> [C₃H₃O]⁺ + H₂

Loss of hydrogen from the m/z 57 ion.

Formation of m/z 43 and 44 ions:

[(CH₃)₂CHCHO]⁺ ===> [(CH₃)₂CH]⁺ + CHO

Scission of C-C bond in the parent molecular ion, mass change 72 - 29 = 43, secondary carbocation formed.

The CHO can also be ionised to give the m/z 29 ion (see below).

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

Could the m/z 43 ion also be due to [CH₃C=O]⁺ ?

The m/z 43 ion can lose hydrogen atoms/molecule to give the m/z ions 42 down to 37.

The m/z 44 ion could be [¹³C¹²C₂H₇]⁺ ion, in a ratio of 1:25 compared to the m/z 43 ion - see earlier note on cabon-13 abundance in the molecules of 2-methylpropanal.

The m/z 43 ion can lose protons to give the m/z 38 to 41 ions.

Formation of m/z 39 ion:

[?]⁺ ===> [C₃H₃]⁺ + ?

Formation of m/z 29 ion:

[(CH₃)₂CHCHO]⁺ ===> [CHO]⁺ + (CH₃)₂CH

Scission of C-C bond in the parent molecular ion,

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

Theoretically the m/z 29 ion could be the [C₂H₅]⁺ ion.

Note that an accurate mass spectrometer can sort them out, it can measure relative fragment ion masses to four decimal places e.g. using very accurate relative isotopic masses,

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

For m/z 29: [CHO]⁺ = 29.0027 and [C₂H₅]⁺ = 29.0390, a difference of 0.0363 in relative ion mass

Formation of m/z 28 ion:

The m/z 28 ion could be [CO]⁺ or less likely [C₂H₄]⁺

[CHO]⁺ ===> [CO]⁺ + CH

[C₃H₅O]⁺ ===> [CO]⁺ + C₂H₅

Formation? Proton loss from m/z 29 ion or 'ethyl' loss from the m/z 57 ion?

Note that an accurate mass spectrometer can sort them out, it can measure relative fragment ion masses to four decimal places e.g. using very accurate relative isotopic masses,

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

For m/z 28: [CO]⁺ = 27.9949 and [C₂H₄]⁺ = 28.0312, a difference of 0.0363 in ion relative mass

Formation of m/z 15 ion:

[(CH₃)₂CHCHO]⁺ ===> [CH₃]⁺ + (CH₃)₂CH

Scission of C-C bond in the parent molecular ion,

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

Lower probability of this fragment being ionised.

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The infrared spectrum of 2-methylpropanal (methylpropanal, iso-butyraldehyde)

The H-1 NMR spectrum of 2-methylpropanal (methylpropanal, iso-butyraldehyde)

The C-13 NMR spectrum of 2-methylpropanal (methylpropanal, iso-butyraldehyde)

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