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Advanced Organic Chemistry: Mass spectrum of ethanal

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The mass spectrum of ethanal (acetaldehyde)

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 ethanal

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Mass spectroscopy - spectra index

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

Ethanal (acetaldehyde), C2H4O, aldehydes and ketones nomenclature (c) doc b , aldehydes and ketones nomenclature (c) doc b , aldehydes and ketones nomenclature (c) doc b , aldehydes and ketones nomenclature (c) doc b

Interpreting the fragmentation pattern of the mass spectrum of ethanal

[M]+ is the molecular ion peak (M) with an m/z of 44 corresponding to [C2H4O]+, the original ethanal molecule minus an electron, [CH3CHO]+.

The small M+1 peak at m/z 45, corresponds to an ionised ethanal molecule with one 13C atom in it i.e. an ionised ethanal molecule of formula 13C12CH4O

BUT, the m/z peak of 45 seems to big to be just due to a 13C molecule, only ~1% of carbon atoms are the 13C isotope, so is something else formed?

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

m/z value of [fragment]+ 43 29 15
[molecular fragment]+ [CH3C=O]+ [CHO]+ [CH3]+

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

Equations to explain the most abundant ion peaks of ethanal

Formation of m/z 43 ion:  [CH3CHO]+  ===>  [CH3C=O]+  +  H

Proton loss from the parent molecular ion

Formation of m/z 29 ion: [CH3CHO]+  ===>  [CHO]+  +  CH3

The m/z 29 ion is the base peak, the most abundant 'stable' ion fragment formed by C-C bond fission.

Formation of m/z 15 ion: [CH3CHO]+  ===>  [CH3]+  +  CHO

Less likely alternative ion from C-C bond fission.


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Links associated with ethanal

The infrared spectrum of ethanal (acetaldehyde)

The H-1 NMR spectrum of ethanal (acetaldehyde)

The C-13 NMR spectrum of ethanal (acetaldehyde)

The chemistry of ALDEHYDES and KETONES revision notes INDEX

Mass spectroscopy index

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