[Author © Dr Phil Brown GRIC, PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses, IB chemistry & US K12 grade 11, grade 12 and AP honors chemistry courses: Molecular spectrometry analysis of propanone [spectra updated RE-EDIT]

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 Mass spectrometry - spectra index

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

If M represents the propanone molecule, the initial ionisation to give the molecular ion is:

M_(g) + high KE e^-  ==> [M•]⁺_(g) + 2e^- and for 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 propanone and only the formation of singly charged positive are considered for the mass spectrum of propanone.

I've included a stick diagram and table of m/z ions for the mass spectrum of propanone 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 propanone.

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 the accurate ion masses, 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 propanone, but the mass spectrometer software does!

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

A ketone The molecular structure and naming of aldehydes and ketones

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]⁺.

Unless otherwise stated, C means a ¹²C atom, if not, the isotopic carbon atom ¹³C will be indicated.

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 may also account for the m/z 44 ion which could be the  ¹³C¹²CH₃O  ion (see base peak ion below).

The most abundant ion of the molecule under mass spectrometry investigation (propanone) 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 propanal is the m/z 43 ion [CH₃CO]⁺

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.

The parent molecular ion for propanone is the m/z 58 ion  [CH₃COCH₃]⁺  or   [C₃H₆O]⁺

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

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 (13 for ~1 in 100);  O = 16

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

Examples of suggested possible equations to explain some of the most abundant ion peaks of the mass spectrum of propanone

Formation of m/z 58 ion peak

This is the complete molecule ionised by the loss of one electron.

[CH₃COCH₃]⁺  or  [C₃H₆O]⁺  because you can't always assume the molecular ion retains the original molecular structure.

Formation of m/z 43 and 42 ions:

[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.

The m/z 44 ion is likely to be formed in the same way as the m/z 43 ion i.e. this ion's formula could be [¹³C¹²H₃CO]⁺ rather than [C₂H₄O]⁺.

Note that an accurate mass spectrometer can sort out ions with the same integer m/z value because they can measure relative fragment ion masses to four decimal places.

e.g. using accurate relative isotopic masses:

¹H = 1.0078  ¹²C = 12.0000  ¹³C = 13.0034   ¹⁶O = 15.9949, from which you can calculate (predict) that the accurate relative ion masses are:

For m/z 44:[C₂H₄O]⁺ = 44.0261, [¹³C¹²CH₃O]⁺ = 44.0217,a difference of 0.0044 in relative ion mass.

Formation of m/z 39 ion:

A common ion in many organic molecule mass spectra, but origin here?

Its formation is equivalent to the change

[CH₃COCH₃]⁺  ===>  [C₃H₃]⁺  +  H₃O

but ???

Formation of m/z 27 and 29 ion:

(i) Two possibilities for the m/z 29 ion, not sure which predominates.

[CH₃COCH₃]⁺  ===>  [C₂H₅]⁺  +  CHO

or

[CH₃COCH₃]⁺  ===>  [CHO]⁺  +  C₂H₅

In both cases C-C bond fission of parent molecular ion and proton rearrangement and mass change = 58 - 29 = 29. (see note (iii) below)

(ii) The m/z 29 ion [C₂H₅]⁺ can lose protons to give the m/z 27 ion [C₂H₃]⁺

(iii) I'm not sure which fragment species predominates for these pairs, which I'm sure can be formed by other fragmentation reactions, BUT an accurate mass spectrometer can sort them out - can measure relative fragment ion masses to four decimal places.

e.g. using accurate relative isotopic masses:

¹H = 1.0078  ¹²C = 12.0000 ¹⁶O = 15.9949

Accurate relative ion masses: [CHO]⁺ = 29.0027  and [C₂H₅]⁺ = 29.0390

A difference of 0.0363 in relative ion mass - no problem!

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