Links associated with 2-iodopropane

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 33 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.

The base peak ion for the mass spectrum of 2-iodopropane is the m/z 43 ion [C₃H₇]⁺

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of 2-iodopropane.

Note: You don't get the 'twin peak' complications with organo-iodine compounds that you do with organo-chlorine and organo-bromine compounds due to them having two stable isotopes (³⁵Cl, ³⁷Cl, ⁷⁹Br and ⁸¹Br).

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 [C₃H₇I]⁺ or  [CH₃CHICH₃]⁺

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

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

Suggested 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 (M-42 ion peak)

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 (M-43 ion peak), 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 (M-127 ion peak), 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 protons to give m/z ions from 42 down to 39.

The m/z 44 ion [¹³C¹²C₂H₅]⁺ will be formed in the same way, as 1 in 25 carbon based fragments will contain a carbon-13 isotope. It is unlikely to be [C₃H₈]⁺ because the original alkyl group of the parent molecule is only C₃H₇.

Note that an accurate mass spectrometer can sort them out, it can measure relative fragment ion masses to four decimal places e.g. using very accurate relative isotopic masses,

¹H = 1.0078  ¹²C = 12.0000  ¹³C = 13.0034, you can then calculate (predict) that the accurate relative ion masses are:

[C₃H₈]⁺ = 44.0624  and [¹³C¹²C₂H₇]⁺ = 44.058, difference of 0.0044 in ion relative mass, no problem!

Formation of m/z 42 ion:

A few possibilities, and probably others too e.g.

[C₃H₇]⁺  ===>  [C₃H₆]⁺  +  H

Proton loss from the m/z 43 ion.

It could also be a carbon-13 isotope ion [¹³C¹²C₂H₅]⁺

Note that an accurate mass spectrometer can sort them out, it can measure relative fragment ion masses to four decimal places e.g. using very accurate relative isotopic masses,

¹H = 1.0078  ¹²C = 12.0000  ¹³C = 13.0034, you can then calculate (predict) that the accurate relative ion masses are:

[C₃H₆]⁺  = 42.0468  and  [¹³C¹²C₂H₅]⁺ =  42.0424, difference of 0.0044 in ion relative mass, no problem!

Formation of m/z 41 ion:

A few possibilities, and probably others too e.g.

[C₃H₇]⁺  ===>  [C₃H₅]⁺  +  H₂

[C₃H₆]⁺  ===>  [C₃H₅]⁺  +  H

Proton loss from m/z 42 or 43 ions?

Formation of m/z 39 ion:

A few possibilities, and probably others too e.g.

[C₃H₅]⁺  ===>  [C₃H₂]⁺  +  H₂

Formation of m/z 27 ions:

A few possibilities, and probably others too e.g.

[C₃H₇]⁺  ===>  [C₂H₅]⁺  +  CH₂

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 spectrometry index

ALL SPECTROSCOPY INDEXES

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