[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 spectroscopy analysis of butanone [updated RE-EDIT]

 Mass spectrometry - spectra index

Introductory note on the mass spectrum of butanone

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

If M represents the butanone 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 butanone and only the formation of singly charged positive are considered for the mass spectrum of butanone.

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

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 butanone, but the mass spectrometer software does!

Butanone C₄H₈O,  , , , ketone

The molecular structure and naming of aldehydes and ketones

Interpreting the fragmentation pattern of the mass spectrum of butanone

[M]⁺ is the molecular ion peak (M) with an m/z of 72 corresponding to [C₄H₈O]⁺, the original butanone molecule minus an electron, [CH₃COCH₂CH₃]⁺

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 73, corresponds to an ionised butanone molecule with one ¹³C atom in it i.e. an ionised butanone molecule of formula ¹³C¹²C₃H₈O

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.

Butanone has 4 carbon atoms, so on average, ~1 in 25 molecules will contain a ¹³C atom.

The most abundant (most intense, tallest) ion peak of the molecule under mass spectrometry investigation (butanone) 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 butanal is m/z 43 ion [CH₃CO]⁺

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

The parent molecular ion is the m/z 72 ion [CH₃CH₂COCH₃]⁺.

Explaining the principal fragments of the mass spectrum of butanone - suggested equations

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); O = 16; 

Bond enthalpies kJ/mol: C-C = 348;  C-H = 412;  C-O = 360; C=O = 743

Several of the most abundant ion peaks arise from scission of C-C bonds in the original molecular ion of butanone.

Possible equations to explain some of the most abundant ion peaks of butanal

Formation of m/z 57 ion:

[CH₃COCH₂CH₃]⁺  ===>  [CH₃CH₂CO]⁺  +  CH₃

C-C bond scission, mass change 72 - 15 = 57 (M-15 ion peak)

The C-C bond is the weakest in the molecule.

Formation of m/z 43 ion:

[CH₃COCH₂CH₃]⁺  ===>  [CH₃CO]⁺  +  CH₂CH₃

C-C bond scission, mass change 72 - 29 = 43 (M-29 ion peak)

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

The m/z 43 ion can lose a proton to give the m/z 42 ion  [C₂H₂O]⁺ ?

The m/z ion 44 is probably formed in the same way, but containing a ¹³C atom rather than the ion [C₃H₇]⁺.

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 43: [C₂H₃O]⁺ or [CH₃C=O]⁺ = 43.0183  and [C₃H₇]⁺ = 43.0546, a difference of 0.0363 in relative ion mass.

Database of accurate m/z values and possible identity of ions formed in mass spectrometry

Formation of m/z 29 ion: 

[CH₃COCH₂CH₃]⁺  ===>  [CH₂CH₃]⁺  +  CH₃CO

C-C bond scission, mass change 72 - 43 = 29 (M-43 ion peak)

This m/z 29 ion can lose H or H₂ to give m/z ions 28, 27 and 26

Formation of m/z 15 ion:

[CH₃COCH₂CH₃]⁺  ===>  [CH₃]⁺  +  CH₃CH₂CO

C-C bond scission, mass change 72 - 57 = 15 (M-57 ion peak)

Isomers of molecular formula C₃H₆O (M_r = 58)

The infrared spectrum of butanone

The H-1 NMR spectrum of butanal (butyraldehyde)

The C-13 NMR spectrum of butanal (butyraldehyde)

The chemistry of ALDEHYDES and KETONES revision notes INDEX

Mass spectroscopy index

ALL SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

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Index of my advanced (pre-college/university) organic chemistry revision notes

 Index of all my spectroscopy pages

 Index of all my isomerism pages

 The chemistry of alkanes and the petrochemical industry

 The chemistry of alkenes

 The chemistry of haloalkanes

 The chemistry of alcohols

 The chemistry of aldehydes and ketones

 The chemistry of carboxylic acids and derivatives

 The chemistry of organo-nitrogen compounds

 The chemistry of aromatic compounds