[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 - analysing the mass spectra of methylpropene, 2-methylpropene [spectra page updated April 4th 2026 *]

 email doc brown Re-edit the mass spectrum of H₂C=C(CH₃)₂

 Mass spectrometry - spectra index

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

If M represents the 2-methylpropene 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-methylpropene and only the formation of singly charged positive are considered for the mass spectrum of 2-methylpropene.

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

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 methylpropene. 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 methylpropene, but the mass spectrometer software does!

The molecular structure and naming of alkenes

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

[M]⁺ is the parent molecular ion peak (M) with an m/z of 56 corresponding to [C₄H₈]⁺, the original 2-methylpropene molecule minus an electron, [(CH₃)₂C=CH₂]⁺

The small M+1 peak at m/z 57, corresponds to an ionised 2-methylpropene molecule with one ¹³C atom in it i.e. an ionised 2-methylpropene 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.

2-methylpropene 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-methylpropene) 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 2-methylpropene is m/z 41 ion [C₃H₅]⁺

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

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

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

The parent molecular ion of 2-methylpropene m/z 56: [C₄H₈]⁺  or  [(CH₃)₂C=CH₂]⁺

Data table of some of the ions formed in the fragmentation pattern of the mass spectrum of 2-methylpropene

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

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)

Bond enthalpies = kJ/mol: C-C = 348;  C-H = 412;  C=C = 612

Possible equations to explain some of the most abundant ion peaks in the mass spectrum of 2-methylpropene (tabulated above)

Formation of m/z 55 ion:

[(CH₃)₂C=CH₂]⁺  ===>  [C₄H₇]⁺  +  H

C-H bond scission of the parent molecular ion with loss of a hydrogen atom,

mass change 56 - 1 = 55 (M-1 ion peak)

Further loss of hydrogen atom/molecule creates m/z ions from 54 down to 50 (see table of ions).

Formation of m/z 41 and 42 ions:

[(CH₃)₂C=CH₂]⁺  ===>  [C₃H₅]⁺  +  CH₃

C-H bond scission of the parent molecular ion with loss of a methyl group,

mass change 56 - 15 = 41 (M-15 ion peak)

The m/z 41 ion gives the base ion peak, the most abundant 'stable' ion formed from 2-methylpropene.

Further loss of hydrogen atom/molecule creates m/z ions from 40 down to 37 (see table of ions).

The m/z 42 ion is probably formed in the same way but contains a 13C carbon isotope atom i.e. [¹³C¹²C₂H₅]⁺ rather than the [C₃H₆]⁺ 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

e.g. using accurate relative isotopic masses:

¹H = 1.0078   ¹²C = 12.0000   ¹³C = 13.0034, 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:

[C₄H₇]⁺  ===>  [C₂H₅]⁺  +  C₂H₂

e.g. C-C bond scission in a larger fragment and proton rearrangement,

mass change 55 - 26 = 29.

Further loss of hydrogen atom/molecule creates m/z ions of 28 down to 25 (see table of ions).

Formation of m/z 15 ion:

[(CH₃)₂C=CH₂]⁺  ===>  [CH₃]⁺  +  C₃H₅

C-H bond scission of the parent molecular ion with loss of a C₃H₅ group,

mass change 56 - 41 = 15 (M-41 ion peak)

but much less likely than the formation of the m/z 41 ion.

The m/z 15 ion can also be formed from other fragments with at least two carbon atoms.

The chemistry of ALKENES revision notes INDEX

The infrared spectrum of 2-methylpropene (2-methylprop-1-ene)

The H-1 NMR spectrum of 2-methylpropene (2-methylprop-1-ene)

The C-13 NMR spectrum of 2-methylpropene (2-methylprop-1-ene)

Mass spectrometry index

ALL SPECTROSCOPY INDEXES

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