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

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, , , ,
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

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