Advanced Organic Chemistry: Mass spectrum of 1,2-dimethylbenzene C6H4(CH3)2

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Interpreting the mass spectrum of 1,2-dimethylbenzene

[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: Mass spectrometry - analysing the mass spectrum of 1,2-dimethylbenzene [spectra page updated Mar 16th 2026 *]

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Introductory note on the mass spectrum of 1,2-dimethylbenzene

Students and teachers please note my explanation of the mass spectrum of 1,2-dimethylbenzene is designed for advanced, but pre-university, chemistry courses.

If M represents the 1,2-dimethylbenzene 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 1,2-dimethylbenzene and only the formation of singly charged positive are considered for the mass spectrum of 1,2-dimethylbenzene.

I've included a stick diagram and table of m/z ions for the mass spectrum of 1,2-dimethylbenzene 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 1,2-dimethylbenzene.

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 1,2-dimethylbenzene, but the mass spectrometer software does!

C8H10 mass spectrum of 1,2-dimethylbenzene fragmentation pattern of m/z m/e ions for analysis and identification of o-xylene image diagram doc brown's advanced organic chemistry revision notes 

1,2-dimethylbenzene , C8H10 , C6H4(CH3)2 , (c) doc b , (c) doc b

The molecular structure and naming of aromatic compounds

Interpreting the fragmentation pattern of the mass spectrum of 1,2-dimethylbenzene

[M]+ is the molecular ion peak (M) with an m/z of 106 corresponding to [C8H10]+, the original 1,2-dimethylbenzene molecule minus an electron, [C6H4(CH3)2]+

The small M+1 peak at m/z 107, corresponds to an ionised 1,3-dimethylbenzene molecule with one 13C atom in it i.e. an ionised 1,2-dimethylbenzene molecule of formula [13C12C7H10]+

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.

1,2-dimethylbenzene has 8 carbon atoms, so on average, ~1 in 13 molecules of will contain a 13C atom.

The same argument applies to the molecular and fragment ions e.g. check out the pair of ions at m/z 91/92 and 106/107 where you would expect a ratio of ~13/1 in intensities (peak heights) in the mass spectrum of 1,2-dimethylbenzene.

m/z 91 [C7H7]+, m/z 92 [13C12C6H7]+, m/z 106 [C8H10]+  m/z 107 [13C12C7H10]+

Note: Unless otherwise stated, assume all carbon atoms are the 12C isotope.

The most abundant ion of the molecule under mass spectrometry investigation 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 1,2-dimethylbenzene is the m/z 91 ion [C7H7]+

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of 1,2-dimethylbenzene.

m/z ion of 92 for 1,2-dimethybenzene could be [C7H8]+ or more likely  [13C12C6H7]+  (see notes above and below data table of ions)
m/z value of [fragment]+ 105 91 79 78 77 65 63 51 39
[molecular fragment]+ [C8H9]+ [C7H7]+ [C6H7]+ [C6H6]+ [C6H5]+ [C5H5]+ [C5H3]+ [C4H3]+ [C3H3]+

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 1,2-dimethylbenzene

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.

Suggested equations to explain the most abundant ion peaks of the mass spectrum of 1,2-dimethylbenzene

Formation of m/z 105 ion

[C6H4(CH3)2]+  ===>  [C8H9]+  +  H

Quite an abundant ion, formed by proton loss from the parent molecular ion.

mass change 106 - 1 = 105 (M-1 ion peak)

Formation of m/z 91 ion

[C6H4(CH3)2]+  ===>  [C7H7]+  +  CH3

C-C bond fission to cleave away a methyl group.

The m/z 91 ion is the base peak ion, the most abundant and 'stable' ion fragment, formed by the loss of a methyl group.

mass change 106 - 15 = 91 (M-15 ion peak)

The m/z 92 ion is probably formed by the same fragmentation process, but containing one carbon-13 isotope atom i.e.  [13C12C6H7]+ and not [C7H8]+

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 are:

m/z 92: [C7H8]+ = 92.0264 or  [13C12C6H7]+ = 92.0580, a relative ion mass difference of 0.0316.

Formation of the m/z 77 ion

[C7H7]+  ===>  [C6H5]+  +  CH2

C-C bond fission and proton rearrangement in the m/z 91 ion to produce a phenyl cation and very characteristic in the mass spectrum of aromatic benzene compounds like 1,4-dimethylbenzene.

mass change 91 - 14 = 77

Formation of the m/z 65 ion

[C7H7]+  ===>  [C5H5]+  +  C2H2

C-C bond fission: mass change 91 - 26 = 65

Formation of the m/z 63 ion

[C6H5]+  ===>  [C5H3]+  +  CH2

C-C bond fission.  mass change 77 - 14 = 63

Formation of the m/z 51 ion

[C6H5]+  ===>  [C4H3]+  +  C2H2

C-C bond fission.  mass change 77 - 26 = 51

Formation of the m/z 39 ion

[C5H5]+  ===>  [C3H3]+  +  C2H2

C-C bond fission.  mass change 65 - 26 = 39

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Links associated with 1,2-dimethylbenzene

The infrared spectrum of 1,2-dimethylbenzene

The infrared spectrum of 1,3-dimethylbenzene

The infrared spectrum of 1,4-dimethylbenzene

The mass spectrum of 1,3-dimethylbenzene

The mass spectrum of 1,4-dimethylbenzene

The H-1 NMR spectrum of 1,2-dimethylbenzene

The H-1 NMR spectrum of 1,3-dimethylbenzene

The H-1 NMR spectrum of 1,4-dimethylbenzene

The C-13 NMR spectrum of 1,2-dimethylbenzene

The C-13 NMR spectrum of 1,3-dimethylbenzene

The C-13 NMR spectrum of 1,4-dimethylbenzene

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