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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), C4H8Oaldehydes 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 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.

The parent molecular m/z 72 ion is [CH3CH2CH2CHO]+.

m/z value of [fragment]+ 71 57 54 44 43
[molecular fragment]+ [CH3CH2CH2C=O]+ [CH2CH2CHO]+ [C4H6]+ [CH2=CH-OH]+ [CH3CH2CH2]+
m/z value of [fragment]+ 41 39 29 29 28 28 27 15
[molecular fragment]+ [C3H5]+ [C3H3]+ [CH3CH2]+ [HC=O]+ [C2H4]+ [CO]+ [C2H3]+ [CH3]+

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

Atomic masses: H = 1; C = 12; O = 16; 

Bond enthalpies kJ/mol: C-C = 348;  C-H = 412;  C-O = 360; C=O = 743

Possible equations to explain some of the most abundant ion peaks of butanal

Formation of m/z 71 ion:

[CH3CH2CH2CHO]+  ===>  [CH3CH2CH2C=O]+  +  H

C-H bond scission in parent molecular ion, hydrogen atom loss, mass change 72 - 1 = 71.

Formation of m/z 57 ion:

[CH3CH2CH2CHO]+  ===>  [CH2CH2CHO]+  +  CH3

C-C bond scission in the parent molecular ion, loss of end methyl group, mass change 72 - 15 = 57.

Formation of m/z 54 ion from water loss:

[CH3CH2CH2CHO]+  ===>  [C4H6]+  +  H2O

Elimination of water, mass change 72 - 18 = 54.

(Note there is a tiny peak for an m/z 18 ion, evidence of water present, so there is a small chance the water molecule becomes ionised and the organic fragment is electrically neutral.

Formation of m/z 44 ion:

[CH3CH2CH2CHO]+  ===>  [CH2=CH-OH]+  +  CH3CH

This ion is formed by a rearrangement, followed by a C-C bond fission, mass change 72 - 28 = 44.

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

The m/z 45 ion could be the same ion with a 13C carbon atom in it i.e. [13C12CH4O]+.

Formation of m/z 43 ion:

[CH3CH2CH2CHO]+  ===>  [CH3CH2CH2HC=O]+  +  HC=O

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

Both involve C-C bond scission of the parent molecular ion, mass change 72 - 29 = 43.

Formation of m/z 29 ion:

[CH3CH2CH2CHO]+  ===>  [HC=O]+  +  CH3CH2CH2

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

Both involve C-C bond scission of the parent molecular ion, mass change 72 - 43 = 29.

Hydrogen loss from the [CH3CH2]+ gives m/z ions from 28 down to 26.

Formation of m/z 28 ion:

[CH3CH2CH2CHO]+  ===>  [C2H4]+  +  CH2=CH-OH

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

The m/z 28 ion could also be [CO]+ as well as [CH2=CH2]+.

Formation of m/z 15 ion from water loss:

[CH3CH2CH2CHO]+  ===>  [CH3]+  +  CH2CH2CHO

C-C bond scission of parent molecular ion, mass change 72 - 57 = 15.

The m/z 15 ion can be formed from smaller fragments by the same process e.g. from m/z 71, 57 or 43 ions.


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Stick diagram of the relative abundance of ionised fragments in the fingerprint pattern of the mass spectrum of butanal. Table of the m/e m/z values and formula of the ionised fragments in the mass spectrum of butanal. The m/e m/z value of the molecular ion peak in the mass spectrum of butanal.  The m/e m/z value of the base ion peak in the mass spectrum of butanal. Possible examples of equations showing the formation of the ionised fragments in butanal. Revision notes on the mass spectrum of butanal. Matching and deducing the structure of the butanal molecule from its mass spectrum.


Links associated with butanal

The chemistry of ALDEHYDES and KETONES revision notes INDEX

The infrared spectrum of Butanal

The H-1 NMR spectrum of Butanal

The C-13 NMR spectrum of Butanal

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