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

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Advanced Organic Chemistry: Mass spectrum of butanal (butyraldehyde)

The mass spectrum of butanal

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 butanal

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

butanal (butyraldehyde), C₄H₈O, , ,

Interpreting the fragmentation pattern of the mass spectrum of butanal

[M]⁺ is the molecular ion peak (M) with an m/z of 72 corresponding to [C₄H₈O]⁺, the original butanal molecule minus an electron, [CH₃CH₂CH₂CHO]⁺.

The small M+1 peak at m/z 73, corresponds to an ionised butanal molecule with one ¹³C atom in it i.e. an ionised butanal molecule of formula ¹³C¹²C₃H₈O

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

m/z value of [fragment]⁺	71	57	44	43	43
[molecular fragment]⁺	[CH₃CH₂CH₂C=O]⁺	[CH₂CH₂CHO]⁺	[CH₂=CH-OH]⁺	[CH₃CO]⁺	[CH₃CH₂CH₂]⁺

m/z value of [fragment]⁺	41	39	29	29	28	27	15
[molecular fragment]⁺	[C₃H₅]⁺	[C₃H₃]⁺	[CH₃CH₂]⁺	[HC=O]⁺	[C₂H₄]⁺	[C₂H₃]⁺	[CH₃]⁺

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

Equations to explain the most abundant ion peaks of butanal

Formation of m/z 71 ion: [CH₃CH₂CH₂CHO]⁺ ===> [CH₃CH₂CH₂C=O]⁺ + H

Proton loss from the molecular ion.

Formation of m/z 57 ion: [CH₃CH₂CH₂CHO]⁺ ===> [CH₂CH₂CHO]⁺ + CH₃

Formation of m/z 44 ion: [CH₃CH₂CH₂CHO]⁺ ===> [CH₂=CH-OH]⁺ + CH₃CH

The m/z 44 ion is the base peak, the most abundant 'stable' ion fragment.

This ion is formed by a rearrangement, followed by a C-C bond fission.

Formation of m/z 28 ion: [CH₃CH₂CH₂CHO]⁺ ===> [CH₃CH]⁺ + CH₂=CH-OH

An alternative to C-C bond fission forming m/z ion 44 above.

Formation of m/z 29 ion: [CH₃CH₂CH₂CHO]⁺ ===> [HC=O]⁺ + CH₃CH₂CH₂

or alternatively: [CH₃CH₂CH₂CHO]⁺ ===> [CH₃CH₂]⁺ + CH₂CHO

m/z 54 from water loss: [CH₃CH₂CH₂CHO]⁺ ===> [C₄H₆]+ + H₂O

m/z 28 ion could be [CO]⁺ or [CH₂=CH₂]+ or [CH₃CH]⁺

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