[Author © Dr Phil Brown PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses & US K12 grade 11, grade 12 and AP honors chemistry courses: Molecular spectroscopy - analysing the mass spectra of propan-2-amine (isopropylamine) [spectra page updated April 4th 2026 *]

Students and teachers please note my explanation of the mass spectrum of propan-2-amine is designed for advanced, but pre-university, chemistry courses.

If M represents the propan-2-amine molecule, the initial ionisation to give the molecular ion is:

M_(g) + high KE e^-  ==> [M•]⁺_(g) + 2e^- and fragmentation equations assume [M]⁺ is the start of the processes and all species are in a gaseous state.

I've not usually shown an unpaired electron on e.g. an ion or a non-ionised alkyl radical R e.g.

[M•]⁺ ==> [X]⁺  +  R•, but you should be aware this is a more accurate depiction of some processes.

I've used simplified equations to show how some of the ions that might be formed in the fragmentation pattern for the mass spectrum of propan-2-amine and only the formation of singly charged positive are considered for the mass spectrum of propan-2-amine.

I've included a stick diagram and table of m/z ions for the mass spectrum of propan-2-amine and doing the mass spectrum analysis under standard conditions, databases can be compiled based on complex fingerprint patterns, often involving the relative intensities of many fragment ions, and used to identify compounds including propan-2-amine.

In selected cases, where two different fragment ions have the same integer m/z value, I've pointed out that modern mass spectrometers can measure relative ion mass to four decimal places. So, using accurate isotopic masses, I've calculated and compared the accurate ion masses if appropriate for this primary amine. BUT strictly speaking, 0.0005 should be deducted for singly charged ions to account for the loss of the electron in their formation. I have NOT done this for this primary amine, but the mass spectrometer software does!

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.

The base ion peak for the mass spectrum of propan-2-amine is the m/z 44 ion [C₂H₆N]⁺

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).

The parent molecular ion of propan-2-amine is m/z 59: [C₃H₉N]⁺  or  [(CH₃)₂CHNH₂]⁺

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

PLEASE NOTE I have found it difficult to find 'authentic' equations to explain mass spectra fragmentation patterns and it is complex chemistry! I've identified the formulae of the ionised fragments on the mass spectrum diagram, but the equations are from the internet or my conjecture as to how the ions might be formed - please take care in using the information, especially for assignments at university or pre-university level.

Atomic masses: H = 1;  C = 12 (~1% 13);  N = 14

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

I'm not sure which fragment species predominates for some pairs of ions with the same integer m/z value, which might be formed by other fragmentation reactions, BUT an accurate mass spectrometer can sort them out, and can measure relative fragment ion masses to four decimal places.

Using accurate relative isotopic masses you can calculate and predict an accurate relative ion mass:

¹H = 1.0078  ¹²C = 12.0000  ¹³C = 13.0034  ¹⁴N = 14.0031 

Possible equations to explain the most abundant ion peaks of propan-2-amine (2-aminopropane) (tabulated above)

Formation of m/z 59 ion:

[(CH₃)₂CHNH₂]⁺  ===>  [C₃H₈N]⁺  +  H

C-H or N-H bond scission, loss of hydrogen atom from parent molecular ion,

mass change 59 - 1 = 58 (M-1 ion peak)

Formation of m/z 44 ion:

[(CH₃)₂CHNH₂]⁺  ===>  [C₂H₆N]⁺  +  CH₃

C-C bond scission, loss of methyl group from parent molecular ion,

mass change 59 - 15 = 44 (M-15 ion peak)

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.

The m/z 45 ion [¹³C¹²CH₆N]⁺ could formed in the same process, but containing a ¹³C isotope ion.

Formation of m/z 43 ion:

[?]⁺  ===>  [C₂H₅N]⁺  +  ?

or more likely

[(CH₃)₂CHNH₂]⁺  ===>  [C₃H₇]⁺  +  NH₂

C-N bond scission, loss of amine group from parent molecular ion,

mass change 59 - 16 = 43 (M-16 ion peak)

Using accurate relative isotopic masses you can calculate and predict an accurate relative ion mass for the two possibilities:

For m/z 43: [C₃H₇]⁺ = 43.0546  and  [C₂H₅N]⁺ = 43.0421, a relative mass difference of 0.0125

Theoretically, both these m/z 43 ions can lose hydrogen atoms to give m/z ions down to m/z 39 (see data table of possibilities)

Formation of m/z 42 ion:

[?]⁺  ===>  [C₂H₄N]⁺  +  ?

or

[?]⁺  ===>  [C₃H₆]⁺  +  ?

Theoretically, both these m/z 42 ions can lose hydrogen atoms to give m/z ions down to m/z 39 (see data table of possibilities)

Using accurate relative isotopic masses you can calculate and predict an accurate relative ion mass for the two possibilities:

For m/z 42: [C₃H₆]⁺  = 42.0468  and  [C₂H₄N]⁺ = 42.0343, a difference of 0.0125 in relative ion mass/

Formation of m/z 41 ion:

[?]⁺  ===>  [C₂H₃N]⁺  +  ?

or

[?]⁺  ===>  [C₃H₅]⁺  +  ?

Theoretically, both these m/z 41 ions can lose hydrogen atoms to give m/z ions down to m/z 39 (see data table of possibilities)

Using accurate relative isotopic masses you can calculate and predict an accurate relative ion mass for the two possibilities:

For m/z 41: [C₃H₅]⁺ = 41.0390  and [C₂H₃N]⁺ = 41.0265, a difference of 0.0125 in relative ion mass.

Formation of m/z 40 ion:

[?]⁺  ===>  [C₂H₂N]⁺  +  ?

or

[?]⁺  ===>  [C₃H₄]⁺  +  ?

The m/z 40 ion [C₃H₄]⁺ can lose a hydrogen atom to give the m/z 39 ion [C₃H₃]⁺

Using accurate relative isotopic masses you can calculate and predict an accurate relative ion mass for the two possibilities:

For m/z 40: [C₃H₄]⁺ = 40.0312 and. [C₂H₂N]⁺ = 40.0187, mass difference 0.0125

Formation of m/z 28 ion:

[?]⁺  ===>  [C₂H₄]⁺  +  ?

or

[?]⁺  ===>  [CH₂N]⁺  +  ?

Using accurate relative isotopic masses you can calculate and predict an accurate relative ion mass for the two possibilities:

For m/z 28: [C₂H₄]⁺ = 28.0312  and  [CH₂N]⁺ = 28.0187, mass difference of 0.0125

Formation of m/z 27 ion:

[?]⁺  ===>  [C₂H₃]⁺  +  ?

or

[?]⁺  ===>  [CHN]⁺  +  ?

Using accurate relative isotopic masses you can calculate and predict an accurate relative ion mass for the two possibilities:

For m/z 27: [C₂H₃]⁺ = 27.0234, [CHN]⁺ = 27.0109, a difference of 0.0125 in relative ion mass.

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 (M-44 ion peak)

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

The infrared spectra of propan-2-amine (2-aminopropane, 2-propylamine, isopropylamine)

The H-1 NMR spectra of propan-2-amine (2-aminopropane, 2-propylamine, isopropylamine)

The C-13 NMR spectra of propan-2-amine (2-aminopropane, 2-propylamine, isopropylamine)

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