mass spectrum of propan-2-amine (2-aminopropane) C3H9N (CH3)2CHNH2 fragmentation pattern of m/z m/e ions for analysis and identification of isopropylamine 2-propylamine 2-propanamine image diagram doc brown's advanced organic chemistry revision notes

Advanced Organic Chemistry: Mass spectrum of propan-2-amine

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Interpreting the mass spectrum of propan-2-amine (2-aminopropane, 2-propanamine, 2-propylamine, isopropylamine)

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Mass spectroscopy - spectra index

mass spectrum of propan-2-amine (2-aminopropane) C3H9N (CH3)2CHNH2 fragmentation pattern of m/z m/e ions for analysis and identification of isopropylamine 2-propylamine 2-propanamine image diagram doc brown's advanced organic chemistry revision notes

propan-2-amine, 2-aminopropane, 2-propylamine, 2-propanamine, isopropylamine, ,

Interpreting the fragmentation pattern of the mass spectrum of propan-2-amine (2-aminopropane)

[M]⁺ is the molecular ion peak with an m/z of 59 corresponding to [C₃H₉N]⁺, the original propan-2-amine (2-aminopropane) molecule minus an electron, [(CH₃)₂CHNH₂]⁺.

The tiny M+1 peak at m/z 60, corresponds to an ionised propan-2-amine (2-aminopropane) molecule with one ¹³C atom in it i.e. an ionised propan-2-amine (2-aminopropane) molecule of formula [¹³C¹²C₂H₉N]⁺

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.

propan-2-amine (2-aminopropane) has 3 carbon atoms, so on average, ~1 in 33 molecules will contain a ¹³C atom.

The most abundant ion of the molecule under mass spectrometry investigation (propan-2-amine (2-aminopropane)) 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 propan-2-amine (2-aminopropane).

Unless otherwise indicated, assume the carbon atoms in propan-2-amine (2-aminopropane) are the ¹²C isotope.

Some of the possible positive ions, [molecular fragment]⁺, formed in the mass spectrometry of propan-2-amine (2-aminopropane).

m/z value of [fragment]⁺	58	45	44	43	42	41	40
[molecular fragment]⁺	[C₃H₈N]⁺	[¹³C¹²CH₆N]⁺	[C₂H₆N]⁺	[C₂H₅N]⁺	[C₂H₄N]⁺	[C₂H₃N]⁺	[C₂H₂N]⁺

m/z value of [fragment]⁺	39	30	28	28 ?	27	27	18 ?	15
[molecular fragment]⁺	[?]⁺	[CH₄N]⁺	[CH₂N]⁺	[C₂H₄]⁺	[CHN]⁺	[C₂H₃]⁺	[?]⁺	[CH₃]⁺

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of propan-2-amine (2-aminopropane)

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 propan-2-amine (2-aminopropane) (tabulated above)

Formation of m/z 44 ion:

[(CH₃)₂CHNH₂]⁺ ===> [C₂H₆N]⁺ + CH₃

C-C bond scission, loss of methyl group from parent molecular ion, 59 - 15 = 44.

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

This is a characteristic ion formed in the mass spectrum of aliphatic amines.

Further hydrogen atom losses will give the 43, 42, 41, 40 and 39 ions.

Formation of m/z 15 ion:

[(CH₃)₂CHNH₂]⁺ ===> [CH₃]⁺ + C₂H₆N

Loss of methyl group from parent molecular ion, but carries the positive charge, mass change 59 - 44 = 15.

The positive methyl ion could be formed from other fragments too.

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