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Advanced Organic Chemistry: Mass spectrum of ethylamine

Interpreting the mass spectrum of ethylamine (ethanamine)

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 ethylamine

mass spectrum of ethylamine C2H7N CH3CH2NH2 fragmentation pattern of m/z m/e ions for analysis and identification of ethanamine image diagram doc brown's advanced organic chemistry revision notes 

Ethylamine (aminoethane, ethanamine), C2H7N,  (c) doc b, (c) doc b, (c) doc b, (c) doc b

Interpreting the fragmentation pattern of the mass spectrum of ethylamine

[M]+ is the molecular ion peak with an m/z of 45 corresponding to [C2H7N]+, the original ethylamine molecule minus an electron, [CH3CH2NH2]+.

The tiny M+1 peak at m/z 46, corresponds to an ionised ethylamine molecule with one 13C atom in it i.e. an ionised ethylamine molecule of formula [13C12CH7N]+

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.

Ethylamine has 2 carbon atoms, so on average, ~1 in 50 molecules will contain a 13C atom.

The most abundant ion of the molecule under mass spectrometry investigation (ethylamine) is usually given an arbitrary abundance value of 100, called the base ion peak, and all other abundances ('intensities') are measured against it.

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

Unless otherwise indicated, assume the carbon atoms in ethylamine are the 12C isotope.

Some of the possible positive ions, [molecular fragment]+, formed in the mass spectrometry of ethylamine.

m/z value of [fragment]+ 44 43 42 41 40
[molecular fragment]+ [C2H6N]+ [C2H5N]+ [C2H4N]+ [C2H3N]+ [C2H2N]+
m/z value of [fragment]+ 30 29 29 28 28 27 27 26 26
[molecular fragment]+ [CH4N]+ [C2H5]+ [CH3N]+ [C2H4]+ [CH2N]+ [C2H3]+ [CHN]+ [C2H2]+ [CN]+

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

Atomic masses: H = 1;  C = 12;  N = 14

Bond enthalpies kJ/mol: C-C = 348;  C-H = 412;  C-N = 305;  N-H = 391

Possible equations to explain the most abundant ion peaks of ethylamine (tabulated above)

Formation of m/z 44 ion:

[CH3CH2NH2]+  ===>  [C2H6N]+  +  H

N-H or C-H bond scission to lose a hydrogen atom, mass change 45 - 1 = 44.

The m/z 44 ion can lose hydrogen atoms to give m/z ions of 43, 42, 41 and 40.

Formation of m/z 30 ion:

[CH3CH2NH2]+  ===>  [CH4N]+  +  CH3

C-C bond scission, loss of methyl group, mass change 45 - 15 = 30.

The m/z 30 ion is the base peak ion, the most abundant and 'stable' ion fragment.

This is typical for the base peak of alkyl amines.

Formation of m/z 29 ion:

[CH3CH2NH2]+  ===>  [CH3CH2]+  +  NH2

C-N bond scission and ionisation of the ethyl group, mass change 45 - 30 = 15.

The m/z 29 ion can lose hydrogen atoms to give m/z ions of 28, 27 and 26.

I don't know if the ions of similar m/z vales of 29 to 26, [CH3N]+  to  [CN]+, are formed?


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