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Advanced Organic Chemistry: Mass spectrum of chlorobenzene

The mass spectrum of chlorobenzene

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 chlorobenzene

C6H5Cl mass spectrum of chlorobenzene fragmentation pattern of m/z m/e ions for analysis and identification of  pairs of isotopic peaks due to chlorine isotopes image diagram doc brown's advanced organic chemistry revision notes 

chlorobenzene, C6H5Cl, (c) doc b 

Interpreting the fragmentation pattern of the mass spectrum of chlorobenzene

[M]+ are the molecular ion peaks with an m/z of 112 and 114 corresponding to [C6H535Cl]+ AND [C6H535Cl]+ the original chlorobenzene molecule minus an electron, [C6H5Cl]+.

There are two molecular ion peaks because of the two most abundant isotopes of chlorine in the chlorobenzene molecule.

Chlorine is composed of ~75% chlorine-35, 35Cl AND ~25% of chlorine 37, 37Cl (ratio 3:1).

Therefore you expect a ratio of ~3:1 in the twin peaks for any molecular ion or fragment ion that contains chlorine, and you can see this in the mass spectrum diagram of chlorobenzene above.

The small M+1 peaks at m/z 113 and 115, corresponds to an ionised chlorobenzene molecule with one 13C atom in it i.e. an ionised chlorobenzene molecule of formula 13C12C5H5Cl.

You can also see they are also in a ratio of 3:1.

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

m/z value of [fragment]+ 77 56 51 50 38
[molecular fragment]+ [C6H5]+ [C4H3]+ [C4H3]+ [C4H2]+ [C3H2]+

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

Equations to explain the most abundant ion peaks of chlorobenzene

Unusually, the m/z 112 ion, the molecular ion [C6H535Cl]+, is the base peak ion, the most abundant and 'stable' ion fragment.

Formation of m/z 77 ion: [C6H5Cl]+  ===>  [C6H5]+  +  Cl

The phenyl cation is a characteristic fragment in the mass spectra of benzene compounds due to the stability of the benzene ring.


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