Advanced Organic Chemistry: Mass spectrum of benzaldehyde C6H5CHO

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Interpreting the mass spectrum of benzaldehyde C6H5CHO

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 benzaldehyde

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Mass spectroscopy - spectra index

C7H6O C6H5CHO mass spectrum of benzaldehyde fragmentation pattern of m/z m/e ions for analysis and identification of benzaldehyde image diagram doc brown's advanced organic chemistry revision notes 

 benzaldehyde C7H6O, C6H5CHO , (c) doc b , (c) doc b 

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]+.

This molecular ion peak is also the base ion peak.

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

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

m/z value of [fragment]+ 105 78 77 52 51 50 39 29
[molecular fragment]+ [C6H5CO]+ [C6H6]+ [C6H5]+ [C4H4]+ [C4H3]+ [C4H2]+ [C3H3]+ [CHO]+

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

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.

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.

Formation of m/z 77 ion:

[C6H5CHO]+  ===>  [C6H5]+  +  CHO

or via: [C6H5CO]+  ===>  [C6H5]+  +  CO

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

Formation of m/z 29 ion:

[C6H5CHO]+  ===>  [CHO]+  +  C6H5


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