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Interpreting the fragmentation pattern of the mass spectrum of 2-iodopropane

[M]⁺ is the parent molecular ion peak (M) with an m/z of 170 corresponding to [C₃H₇I]⁺, the original 2-iodopropane molecule minus an electron, [CH₃CHICH₃]⁺.

The small M+1 peak at m/z 171, corresponds to an ionised 2-iodopropane molecule with one ¹³C atom in it i.e. an ionised 2-iodopropane molecule of formula [¹³C¹²C₃H₇I]⁺

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.

2-iodopropane has 3 carbon atoms, so on average, ~1 in 3 molecules will contain a ¹³C atom.

The most abundant ion of the molecule under mass spectrometry investigation (2-iodopropane) 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 2-iodopropane.

Unless otherwise indicated, assume the carbon atoms in 2-iodopropane are the ¹²C isotope.

Some of the possible positive ions, [molecular fragment]⁺, formed in the mass spectrometry of 2-iodopropane.

The parent molecular ion of 2-iodopropane m/z 170: [CH₃CHICH₃]⁺

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 2-iodopropane

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 2-iodopropane (tabulated above)

Formation of m/z 128 ion:

[CH₃CHICH₃]⁺  ===>  [HI]⁺  +  C₃H₆

Elimination of hydrogen iodide from the parent molecular ion, mass change 170 - 42 = 128.

There is a tiny peak for the m/z 42 [C₃H₆]⁺ ion.

Formation of m/z 127 ion:

[CH₃CHICH₃]⁺  ===>  [I]⁺  +  C₃H₇

C-I bond scission in the parent molecular ion, loss of the alkyl group, mass change 170 - 43 = 127, to give a positively ionised iodine atom.

Both the m/z 127 and 128 ions have a low probability of formation, evidenced by relatively small peaks of very low abundance.

Formation of m/z 43 ion:

[CH₃CHICH₃]⁺  ===>  [C₃H₇]⁺  +  I

C-I bond scission in the parent molecular ion, loss of the iodine atom, mass change 170 - 127 = 43, to give a positively ionised iodine atom.

The C-I bond is the weakest bond in the molecule, therefore the most easily broken and not surprisingly leads to the formation of the base ion peak of highest abundance in the mass spectrum of 2-iodopropane.

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

The m/z 43 ion can lose a hydrogen atom/molecule to give m/z ions from 42 down to 39.

Formation of m/z 27 and 15 ion:

Two possibilities, and probably others too.

[C₃H₇]⁺  ===>  [C₂H₅]⁺  +  CH₂

or

[C₃H₇]⁺  ===>  [CH₃]⁺  +  C₂H₄

These could be formed by C-C bond scission of the m/z 43 ion, either fragment could be ionised, but the m/z 27 ion seems to be the more likely formed judging from their relative abundances.

A comparative footnote on the mass spectra of organic iodine compounds like 2-iodopropane

Iodine consists of 100% of the isotope ¹²⁷I, and therefore the mass spectra of organic iodine compounds does not show the complexity of the mass spectra of organo bromine or organo chlorine compounds where you are dealing with twin molecular ion/fragment peaks from two isotopes:

i.e. ³⁵Cl : ³⁷Cl (3:1) and ⁷⁹Br : ⁸¹Br (1:1).

The chemistry of HALOGENOALKANES (haloalkanes) revision notes INDEX

The infrared spectrum of 2-iodopropane (isopropyl iodide)

The H-1 NMR spectrum of 2-iodopropane (isopropyl iodide)

The C-13 NMR spectrum of 2-iodopropane (isopropyl iodide)

Mass spectroscopy index

ALL SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

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