image diagram mass spectrum of benzene fragmentation pattern of ions for analysis and identification of benzene doc brown's advanced organic chemistry revision notes

Advanced Organic Chemistry: Mass spectrum of benzene C₆H₆

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Interpreting the mass spectrum of benzene C₆H₆ (re-edit)

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US K12 grade 11 grade 12 organic chemistry courses involving molecular spectroscopy analysing mass spectra of benzene

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Mass spectroscopy - spectra index

image diagram mass spectrum of benzene fragmentation pattern of ions for analysis and identification of benzene doc brown's advanced organic chemistry revision notes

aromatic benzene C₆H₆, skeletal formula , structural/displayed formula

Interpreting the mass spectrum of benzene C₆H₆

[M]⁺ is the molecular ion peak (M) with an m/z of 78 corresponding to [C₆H₆]⁺, the original molecule minus an electron.

The small M+1 peak at m/z 79, corresponds to an ionised benzene molecule with one ¹³C atom in it i.e. an ionised molecule of formula [¹³C¹²C₅H₆]⁺

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.

Benzene has 6 carbon atoms, so on average, ~1 in 17 molecules will contain a ¹³C atom.

The most abundant ion of the molecule under mass spectrometry investigation (benzene) is usually given an arbitrary abundance value of 100, called the base ion peak, and all other abundances ('intensities') are measured against it.

In this case the base peak ion is also the molecular ion of benzene itself [C₆H₆]⁺

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

Unless otherwise indicated, assume the carbon atoms in benzene are the ¹²C isotope.

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

The parent molecular ion of benzene m/z 78: [C₆H₁₀]⁺ which, unusually, is also the base ion peak.

Data table of some of the ions formed in the fragmentation pattern of the mass spectrum of benzene

m/z value of [fragment]⁺	78	77	76	74	52	51	50	39
[molecular fragment]⁺	[C₆H₆]⁺	[C₆H₅]⁺	[C₆H₄]⁺	[C₆H₂]⁺	[C₄H₄]⁺	[C₄H₃]⁺	[C₄H₂]⁺	[C₃H₃]⁺

Explaining the principal fragments

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

Atomic masses: H = 1; C = 12 (13 for ~1 in 100)

Bond enthalpies = kJ/mol: = 518 (benzene ring); C-H = 412

The 'phenyl' fragment m/z 77 is formed by the loss a proton from the molecular ion.

[C₆H₆]⁺ ===> [C₆H₅]⁺ + H

Mass change 78 - 1 = 77

This 'phenyl' fragment is very indicative of the presence of a benzene ring in the molecule.

The m/z 77 ion can further lose a hydrogen atom/molecule to give m/z ions of 76, 75, 74 and 73.

The molecular m/z 78 ion itself could lose a hydrogen molecule to give the m/z 76 ion.

The m/z ions 50 to 52

These involve the loss of an unionised two carbon fragment to give a C₄ fragment ion e.g.

[C₆H₆]⁺ ===> [C₄H₄]⁺ + C₂H₂

The m/z 39 ion

This is a common fragment in many organic spectra (I think its a triangle with a delocalised system stabilising the positive charge).

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