[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 analysis of ethanal [spectra page updated Mar 23rd 2026 *]

 email doc brown   Re-edit mass spectrum of CH₃CHO

 This is a BIG website, PLEASE take time to explore it

 Mass spectrometry - spectra index

Introductory note on the mass spectrum of ethanal

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

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

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

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 ethanal, but the mass spectrometer software does!

Ethanal (acetaldehyde), C₂H₄O, , , , an aliphatic aldehyde

The molecular structure and naming of aldehydes and ketones

Interpreting the fragmentation pattern of the mass spectrum of ethanal

[M]⁺ is the molecular ion peak (M) with an m/z of 44 corresponding to [C₂H₄O]⁺, the original ethanal molecule minus an electron, [CH₃CHO]⁺

The small M+1 peak at m/z 45, corresponds to an ionised ethanal molecule with one ¹³C atom in it i.e. an ionised ethanal molecule of formula ¹³C¹²CH₄O

BUT, the m/z peak of 45 seems to big to be just due to a 13C molecule, only ~1% of carbon atoms are the ¹³C isotope, so is something else formed?

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.

Ethanal 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 (ethanal) 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 peak ion for ethanal is the m/z 44 ion [CH₃CHO]⁺, the same as molecular ion.

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

Suggested equations explaining the principal fragments of the mass spectrum of ethanoic acid

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.

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

Atomic masses: H = 1;  C = 12 (1 in ~100 is 13);  O = 16

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

Equations to explain the most abundant ion peaks of ethanal

Formation of m/z 43 ion

[CH₃CHO]⁺  ===>  [CH₃C=O]⁺  +  H

Proton loss from the parent molecular ion

mass change 44 - 1 = 43 (M-1 ion peak)

Formation of m/z 42 ion

[CH₃CHO]⁺  ===>  [CH₂C=O]⁺  +  H₂

Elimination of hydrogen from the parent molecular ion

mass change 44 - 2 = 42 (M-2 ion peak)

Formation of m/z 29 ion

[CH₃CHO]⁺  ===>  [CHO]⁺  +  CH₃

The m/z 29 ion is the base peak, the most abundant 'stable' ion fragment formed by C-C bond fission.

mass change 44 - 15 = 29 (M-15 ion peak)

Formation of m/z 15 ion

[CH₃CHO]⁺  ===>  [CH₃]⁺  +  CHO

Less likely alternative ion from C-C bond fission.

mass change 44 - 29 = 15 (M-29 ion peak)

The infrared spectrum of ethanal (acetaldehyde)

The H-1 NMR spectrum of ethanal (acetaldehyde)

The C-13 NMR spectrum of ethanal (acetaldehyde)

The chemistry of ALDEHYDES and KETONES revision notes INDEX

Mass spectrometry index

ALL SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

Use My Google search site box

Email doc b: chem55555@hotmail.com