[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 NMR spectra of ethylmethylamine [spectra page updated April 4th 2026 *]

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

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

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

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 iodoethane. 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 iodoethane, but the mass spectrometer software does!

Interpreting the fragmentation pattern of the mass spectrum of iodoethane

[M]⁺ is the parent molecular ion peak (M) with an m/z of 156 corresponding to [C₂H₅I]⁺, the original iodoethane molecule minus an electron, [CH₃CH₂I]⁺

The small M+1 peak at m/z 157, corresponds to an ionised iodoethane molecule with one ¹³C atom in it i.e. an ionised iodoethane molecule of formula [¹³C¹²CH₅I]⁺

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.

Iodoethane 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 (iodoethane) 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 iodoethane is the m/z 156 ion [CH₃CH₂I]⁺

Note that the m/z 29 ion [C₂H₅]⁺ is nearly as abundant.

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

Unless otherwise indicated, assume the carbon atoms in iodoethane are the ¹²C isotope.

Some of the possible positive ions, [molecular fragment]⁺, formed in the mass spectrometry of iodoethane.

The parent molecular ion of iodoethane m/z 156: [CH₃CH₂I]⁺, it is also the base peak ion too.

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

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) ; I = 127

Bond enthalpies = kJ/mol: C-C = 348;  C-H = 412;  C-I = 238

Possible equations to explain some of the most abundant ion peaks of iodoethane (tabulated above)

Formation of m/z 141 ion:

[CH₃CH₂I]⁺  ===>  [CH₂I]⁺  +  CH₃

C-C bond scission in the parent molecular ion,

mass change 156 - 15 = 141 (M-15 ion peak)

A much lower probability ionisation process than that forming the m/z ion 127 from the scission of the much weak C-I bond (see below).

All fragments containing an iodine atom give single peaks, unlike with chlorine/bromine organic compounds where you get twin peaks due to pairs of Cl/Br isotopes.

Formation of m/z 127 ion:

[CH₃CH₂I]⁺  ===>  [I]⁺  +  C₂H₅

C-I bond scission in the parent molecular ion,

mass change 156 - 29 = 127, an ionised iodine atom, M-29 ion peak.

Formation of m/z 128 ion:

[CH₃CH₂I]⁺  ===>  [HI]⁺  +  C₂H₄

Elimination of hydrogen iodide from the parent molecular ion,

mass change 156 - 28 = 128 (M-28 ion peak)

Formation of m/z 29 ion:

[CH₃CH₂I]⁺  ===>  [C₂H₅]⁺  +  C₂H₅I

C-I bond scission in the parent molecular ion, mass change 156 - 29 = 127.

The m/z 30 ion is most likely to be [C₂H₅]⁺ too, formed by the same process, but containing a ¹³C isotope atom.

Formation of m/z 28 ion:

[CH₃CH₂I]⁺  ===>  [C₂H₄]⁺  +  HI

Again, elimination of hydrogen iodide from the parent molecular ion, but this time the hydrocarbon fragment is ionised.

The m/z 28 ion can also be formed by hydrogen atom loss from the m/z 29 ion, and also hydrogen loss for either of these ions gives rise to m/z ions 27 and 26 ions.

The chemistry of HALOGENOALKANES (haloalkanes) revision notes INDEX

The infrared spectrum of iodoethane (ethyl iodide)

The H-1 NMR spectrum of iodoethane (ethyl iodide)

The C-13 NMR spectrum of iodoethane (ethyl iodide)

Mass spectroscopy index

ALL SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

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