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Advanced Organic Chemistry: Mass spectrum of propanone

The mass spectrum of propanone (acetone)

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 propanone

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

propanone (acetone, dimethyl ketone), C3H6O,  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 propanone  (acetone)

[M]+ is the molecular ion peak (M) with an m/z of 58 corresponding to [C3H6O]+, the original propanone molecule minus an electron, [CH3COCH3]+.

The small M+1 peak at m/z 59, corresponds to an ionised propanone molecule with one 13C atom in it i.e. an ionised propanone molecule of formula 13C12C2H6O

Carbon-13 only accounts for ~1% of all carbon atoms (12C ~99%), but the more carbon atoms in the molecule, the greater the probability of observing this 13C M+1 peak.

Propanone has 3 carbon atoms, so on average, ~1 in 33 molecules will contain a 13C atom and you can actually see the M + 1 peak is much smaller than the M peak (ratio ~33 : 1)

This sort of argument also applies to fragment ions from the parent molecular ion of propanone - though the ratio will be greater.

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

m/z value of [fragment]+ 43 42 39 27 15
[molecular fragment]+ [CH3CO]+ [CH2CO]+ [C3H3]+ [C2H3]+ [CH3]+

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

Atomic masses: H = 1;  C = 12 (13 for ~1 in 100);  O = 16

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

Equations to explain the most abundant ion peaks of propanone

Formation of m/z 43 ion: [CH3COCH3]+  ===>  [CH3CO]+  +  CH3

C-C bond scission in the parent molecular ion, mass change 58 - 15 = 43.

The m/z 43 ion is the base peak ion, the most abundant ion fragment.

This can lose a proton to give [CH2CO]+. with an m/z of 42

Formation of m/z 15 ion: [CH3COCH3]+  ===>  [CH3]+  +  CH3CO

C-C bond scission of the parent molecular ion.

The m/z 15 ion could also be formed by C-C bond scission of the m/z 43 ion with the elimination of a carbon monoxide molecule.

[CH3CO]+  ===>  [CH3]+  +  CO


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Links associated with propanone

The infrared spectrum of Propanone (acetone)

The H-1 NMR spectrum of Propanone (acetone)

The C-13 NMR spectrum of Propanone (acetone)

The chemistry of ALDEHYDES and KETONES revision notes INDEX

Mass spectroscopy index

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