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

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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,

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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