[Author ©  Dr Phil Brown GRIC, PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses & US K12 grade 11-12 & AP honors chemistry courses: Molecular spectrometry of benzaldehyde [spectrum page updated RE-EDIT]

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Introductory note on the mass spectrum of benzaldehyde

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

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

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

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 benzaldehyde, but the mass spectrometer software does!

 benzaldehyde C₇H₆O, C₆H₅CHO , ,  

The molecular structure and naming of aromatic compounds

Interpreting the fragmentation pattern of the mass spectrum of benzaldehyde

[M]⁺ is the molecular ion peak (M) with an m/z of 106 corresponding to [C₇H₆O]⁺, the original benzaldehyde molecule minus an electron, [C₆H₅CHO]⁺

Unusually, the molecular ion peak is also the base ion peak.

Unless otherwise stated, C means a ¹²C atom, if not, the isotopic carbon atom ¹³C will be indicated.

The small M+1 peak at m/z 107 corresponds to an ionised benzaldehyde molecule with one ¹³C atom in it i.e. an ionised benzaldehyde molecule of formula ¹³C¹²C₆H₆O

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.

Benzaldehyde has 7 carbon atoms, so on average, ~1 in 14 molecules will contain a ¹³C atom.

You can see this in the intensity ratio of the m/z 106 : 107 peaks of ~14 : 1 for the two possible parent molecular ions [C₇H₆O]⁺  and  [¹³C¹²C₆H₆O]⁺, the latter containing one ¹³C atom.

The most abundant ion of the molecule under mass spectrometry investigation (benzaldehyde) 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 m/z 106, is also the molecular ion of benzaldehyde itself [C₇H₆O]⁺

BUT, other data sources give the phenyl cation [C₆H₅]⁺ m/z 77 as the base peak ion for the mass spectrum of benzaldehyde.

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of benzaldehyde. Unless otherwise stated, C means a ¹²C atom.

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of benzaldehyde

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 (13 for ~1 in 100)

Bond enthalpies = kJ/mol: = 518 (benzene ring); C-C = 348.  C-H = 412,  C=O = 743

Equations to explain the most abundant ion peaks of benzaldehyde

Formation of m/z 105 ion:

[C₆H₅CHO]⁺  ===>  [C₆H₅CO]⁺  +  H

Proton loss from the aldehyde group of the parent molecular ion, and the most abundant (most stable) fragment ion,

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

It is relatively unusual for the base ion m/z 106, [C₆H₅CHO]⁺) peak to be the same as the molecular ion peak.

Formation of m/z 78 ion:

[C₆H₅CHO]⁺  ===>  [C₆H₆]⁺  +  CO

This is produced by a proton migration-addition to the phenyl group and expulsion of a carbon monoxide molecule.

mass change 106 - 28 = 78 (M-28 ion peak)

Formation of m/z 77 ion:

[C₆H₅CHO]⁺  ===>  [C₆H₅]⁺  +  CHO

mass change 106  - 29 = 77 (M-29 ion peak)

or via:

[C₆H₅CO]⁺  ===>  [C₆H₅]⁺  +  CO

mass change 105 - 28 = 77

Both involve a C-C bond scission of the benzene ring and aldehyde group carbon.

This is quite a typical ion from monosubstituted benzene derivatives.

The m/z 77 and 78 ions can further lose a hydrogen atom/molecule to give m/z ions of 76, 75, 74 and 73.

Formation of m/z 51 ion:

mass change 77 - 26 = 51

[C₆H₅]⁺  ===>  [C₄H₃]⁺  +  C₂H₂

mass change 77 - 26 = 51

Formation of m/z 39 ion:

[?]⁺  ===>  [C₃H₃]⁺  +  ?

From any fragment ion from [C₆H₅]⁺ C-C bond breaking?

Formation of m/z 29 ion:

[C₆H₅CHO]⁺  ===>  [CHO]⁺  +  C₆H₅

mass change 106 - 77 = 29 (M-77 ion peak)

Highly unlikely to be the [C₂H₅]⁺ 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   ¹⁶O = 15.9949, from which you can calculate (predict) that the accurate relative ion masses are:

For m/z 29: [CHO]⁺ = 29.0027  and [C₂H₅]⁺ = 29.0390, a difference of 0.0363 in relative ion mass.