[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 butan-1ene [spectra page updated April 3rd 2026 *]

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 Mass spectrometry - spectra index

Introductory note on the mass spectrum of but-1-ene

Students and teachers please note my explanation of the mass spectrum of but-1-ene is designed for advanced, but pre-university, chemistry courses.

If M represents the but-1-ene 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 but-1-ene and only the formation of singly charged positive are considered for the mass spectrum of but-1-ene.

I've included a stick diagram and table of m/z ions for the mass spectrum of but-1-ene 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 but-1-ene.

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 but-1-ene. 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 but-1-ene, but the mass spectrometer software does!

But-1-ene, , ,

Interpreting the fragmentation pattern of the mass spectrum of but-1-ene

[M]⁺ is the parent molecular ion peak (M) with an m/z of 56 corresponding to [C₄H₈]⁺, the original but-1-ene molecule minus an electron, [H₂C=CHCH₂CH₃]⁺.

The small M+1 peak at m/z 57, corresponds to an ionised but-1-ene molecule with one ¹³C atom in it i.e. an ionised but-1-ene molecule of formula [¹³C¹²C₃H₈]⁺

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.

But-1-ene 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 (but-1-ene) is usually given an arbitrary abundance value of 100, called the base ion peak, and all other abundances ('intensities') are measured against it.

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of but-1-ene.

Unless otherwise indicated, assume the carbon atoms in but-1-ene are the ¹²C isotope.

Some of the possible positive ions, [molecular fragment]⁺, formed in the mass spectrometry of but-1-ene.

The parent molecular ion peak is from the m/z 56 ion: [H₂C=CHCH₂CH₃]⁺.

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of but-1-ene

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

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

Possible equations to explain the most abundant ion peaks of but-1-ene (tabulated above)

Formation of m/z 55 to 50 ions:

[H₂C=CHCH₂CH₃]⁺  ===>  [C₄H₇]⁺  +  H

[H₂C=CHCH₂CH₃]⁺  ===>  [C₄H₆]⁺  +  H₂

C-H bond scission in the parent molecular ion, loss of hydrogen atom or elimination of a hydrogen molecule, mass changes 56 - 1 = 55 or 56 - 2 = 54.

Further loss of hydrogen atom/molecule from these fragments can give m/z ions from 53 down to 49.

They could also undergo C-C bond scission to give smaller ionised fragments containing 1-3 carbon atoms.

Formation of m/z 41 ion:

[H₂C=CHCH₂CH₃]⁺  ===>  [C₃H₅]⁺  +  CH₃

C-C bond scission in the parent molecular ion, loss methyl group, mass change 56 - 15 = 41.

The m/z 41 ion is the base peak ion, the most abundant and 'stable' ion fragment.

Further loss of hydrogen atom/molecule can give m/z ions of 40 down to 36.

The m/z 42 ion could be formed in the same way, but containing a carbon-13 isotope, with the structure [¹³C¹²C₂H₅]⁺ or it could be the [C₃H₆]⁺ 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 42: [C₃H₆]⁺  = 42.0468  and  [¹³C¹²C₂H₅]⁺ =  42.0424, a difference of 0.0044 in relative ion mass.

Formation of m/z 29 ion:

[H₂C=CHCH₂CH₃]⁺  ===>  [CH₃CH₂]⁺  +  C₂H₃

C-C bond scission in the parent molecular ion, mass change 56 - 27 = 29.

Further hydrogen loss gives m/z ions from 28 down to 25 e.g.

Formation of m/z 27 ion:

[C₂H₅]⁺  ===>  [C₂H₃]⁺  +  H₂

Elimination of a hydrogen molecule from the m/z 29 ion, mass change 29 - 2 = 27.

It could also be formed by hydrogen atom loss from the m/z 28 ion.

Formation of m/z 15 ion:

[H₂C=CHCH₂CH₃]⁺  ===>  [CH₃]⁺  +  H₂C=CHCH₂

C-C bond scission, mass change 56 - 41 = 15, but this time the methyl group carries the positive charge.

The chemistry of ALKENES revision notes INDEX

The infrared spectrum of but-1-ene

The H-1 NMR spectrum of but-1-ene  (Please read 8 points at the top of the ¹H NMR index page)

The C-13 NMR spectrum of but-1-ene

Mass spectrometry index

ALL SPECTROSCOPY INDEXES

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