[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 cumene [spectra page updated Mar 22nd 2026 *]

 email doc brown  Re-edit mass spectrum of cumene C₆H₅CH(CH₃)₂

 Mass spectrometry - spectra index

Introductory note on the mass spectrum of cumene

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

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

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

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 cumene, but the mass spectrometer software does!

 (1-methylethyl)benzene, 2-phenylpropane, C₉H₁₂ , C₆H₅CH(CH₃)₂ , , (cumene)

Interpreting the fragmentation pattern of the mass spectrum of (1-methylethyl)benzene (cumene)

[M]⁺ is the molecular ion peak (M) with an m/z of 120 corresponding to [C₉H₁₂]⁺, the original (1-methylethyl)benzene (cumene) molecule minus an electron, [C₆H₅CH(CH₃)₂]⁺.

The small M+1 peak at m/z 121, corresponds to an ionised (1-methylethyl)benzene (cumene) molecule with one ¹³C atom in it i.e. an ionised (1-methylethyl)benzene (cumene) molecule of formula [¹³C¹²C₈H₁₂]⁺

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.

Cumene has 9 carbon atoms, so on average, ~1 in 11 parent molecule or fragment ions will contain a ¹³C atom.

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of (1-methylethyl)benzene (cumene).

The base ion peak is for the m/z 105 ion, [C₈H₉]⁺, which, by mass spectrum convention, is arbitrarily given the height intensity of 100.

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of (1-methylethyl)benzene (cumene)

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);  Cl = 35 or 37 (~ratio 3:1)

Bond enthalpies kJ/mol:   = 518; C-C = 348;  C-H = 412

Suggested equations to explain the most abundant ion peaks of (1-methylethyl)benzene (cumene)

Formation of m/z 105 and 15 ions:

[C₆H₃CH(CH₃)₂]⁺  ===>  [C₈H₉]⁺  +  CH₃

C-C bond scission of the [C₉H₁₂]⁺ ion to break off a methyl group from the alkyl side-chain.

mass change 120 - 15 = 105 (M-15 ion peak)

The m/z 105 ion is the base peak ion, the most abundant and 'stable' ion fragment, formed by the loss of a methyl group from the parent molecular ion (120 - 15 = 105).

The methyl group can also be ionised too i.e. to give the m/z 15 ion

[C₆H₃CH(CH₃)₂]⁺  ===>  [CH₃]⁺  +  C₈H₉

mass change 120 - 105 = 15, though of much less probability - much smaller intensity peak compared to m/z 10 peak.

The m/z 106 ion will be produced in the same way, but containing a ¹³C atom i.e. the fragment ion [¹³C¹²C₇H₉]⁺ rather than the [C₈H₁₀]⁺ 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, from which you can calculate (predict) that the accurate relative ion masses:

For m/z 106: [C₈H₁₀]⁺ = 106.0780  * [¹³C¹²C₇H₉]⁺ =  106.0736, relative ion mass difference of 0.0044

The formation of the m/z 91 ion:

[C₉H₁₂]⁺  ===>  [C₇H₇]⁺  +  C₂H₅

mass change 120 - 29 = 91 (M-29 peak)

[C₈H₉]⁺  ===>  [C₇H₇]⁺  +  CH₂

mass change 105 - 14 = 91

e.g. C-C bond scission of the m/z 120 or 105 ions and proton shifts.

The formation of the m/z 77 ion:

[C₉H₁₂]⁺  ===>  [C₆H₅]⁺  +  C₃H₇

C-C bond scission as the alkyl group breaks away from the parent molecular ion.

mass change 120 - 43 = 77 (M-43 ion peak)

This is a phenyl cation and very characteristic in the mass spectra of aromatic benzene compounds like cumene.

The formation of the m/z 51 ion:

[C₆H₅]⁺  ===>  [C₄H₃]⁺  +  C₂H₂

mass change 77 - 26 = 51

The formation of the m/z 39 and 41 ion:

[?]⁺  ===>  [C₃H₅]⁺  +  ?

[?]⁺  ===>  [C₃H₃]⁺  +  ?

The m/z 39 or 41 ion could be formed from any fragment with at least three carbon atoms and at least three hydrogen atoms.

The m/z 41 ion can lose hydrogen to give the m/z 39 ion.

The infrared spectrum of (1-methylethyl)benzene  or  2-phenylpropane (Cumene)

The H-1 NMR spectrum of (1-methylethyl)benzene  or  2-phenylpropane (Cumene)

The C-13 NMR spectrum of (1-methylethyl)benzene  or  2-phenylpropane (Cumene)

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