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Advanced Organic Chemistry: Mass spectrum of iodoethane

Interpreting the mass spectrum of iodoethane (ethyl iodide)

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US K12 grade 11 grade 12 organic chemistry courses involving molecular spectroscopy analysing mass spectra of iodoethane

mass spectrum of iodoethane C2H5I CH3CH2I fragmentation pattern of m/z m/e ions for analysis and identification of ethyl iodide image diagram doc brown's advanced organic chemistry revision notes 

iodoethane, C2H5I, CH3CH2I, CH3-CH2-I

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.

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.

m/z value of [fragment]+ 156 141 128 127 30 29 28 27 26 15
[molecular fragment]+ [C2H5I]+ [CH2I]+ [HI]+ [I]+ [13C12CH5]+ [C2H5]+ [C2H4]+ [C2H3]+ [C2H2]+ [CH3]+

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

Atomic masses: H = 1;  C = 12; 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.

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).

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.

Formation of m/z 128 ion:

[CH3CH2I]+  ===>  [HI]+  +  C2H4

Elimination of hydrogen iodide from the parent molecular ion, mass change 156 - 28 = 128.

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, but containing a 13C 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.


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Links associated with iodoethane

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)

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