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Introductory note on the mass spectrum of 1,1,1-trichloroethane

Students and teachers please note my explanation of the mass spectrum of 1,1,1-trichloroethane is designed for advanced, but pre-university, chemistry courses.

If M represents the 1,1,1-trichloroethane 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 1,1,1-trichloroethane and only the formation of singly charged positive are considered for the mass spectrum of 1,1,1-trichloroethane.

I've included a stick diagram and table of m/z ions for the mass spectrum of 1,1,1-trichloroethane 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 1,1,1-trichloroethane.

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 1,1,1-trichloroethane. 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 1,1,1-trichloroethane, but the mass spectrometer software does!

1,1,1-trichloroethane  Cl₃CCH₃  or  CH₃CCl₃

Interpreting the fragmentation pattern of the mass spectrum of 1,1,1-trichloroethane

There are no [M]⁺ the molecular ion peak (M) with an m/z of 132, 134, 136 or 138 (too unstable) corresponding to [C₂H₃Cl₃]⁺, the original 1,1,1-trichloroethane molecule minus an electron, depending on the chlorine isotopic composition of the 1,1,1-trichloromethane molecule.

I presume these ions are too unstable, but, see discussion on possible molecular ion peaks on the mass spectrum page for 1,1,2-trichloroethane, where, unlike here, you clearly observe three of the four possible molecular ion peaks for a C₂H₃Cl₃ molecular formula molecule of different isotopic combinations.

The most abundant ion of the molecule under mass spectrometry investigation (1,1,1-trichloroethane) 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 1,1,2-trichloroethane is m/z 97 [C₂H₃³⁵Cl₂]⁺

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of 1,1,1-trichloroethane.

Unless otherwise indicated, assume the carbon atoms in 1,1,1-trichloroethane are the ¹²C isotope, but the chlorine atoms maybe ³⁵Cl or ³⁷Cl and this makes the mass spectrum even more complex to interpret.

Some of the possible positive ions, [molecular fragment]⁺, formed in the mass spectrometry of 1,1,1-trichloroethane.

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 1,1,1-trichloroethane

This is a very complex pattern because of three chlorine atoms and their two isotopes, so I've picked out a few 'obvious' patterns for the mass spectrum of 1,1,1-trichloroethane!

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;  Cl = 35 or 37 (3:1)

Bond enthalpies = kJ/mol: C-C = 348;  C-H = 412;  C-Cl = 338

Possible suggested equations to explain the most abundant ion peaks of 1,1,1-trichloroethane (tabulated above)

Formation of m/z 117, 119 and 121 ions (see m/z ion table above):

[Cl₃CCH₃]⁺  ===>  [CCl₃]⁺  +  CH₃

C-C bond scission with loss of a methyl radical from the parent molecular ion.

Formation of m/z 97, 99 and 101 ions (see m/z ion table above):

[Cl₃CCH₃]⁺  ===>  [C₂H₃Cl₂]⁺  +  Cl

C-Cl bond scission (weakest bond), loss of chlorine radical from the parent molecular ion.

The m/z 97 ion is the base peak ion, the most abundant and 'stable' ion fragment.

Alternatively, but much less probable, the Cl is ionised to give the m/z 35 and 37 ions.

[Cl₃CCH₃]⁺  ===>  [Cl]⁺  +  C₂H₃Cl₂

You can see roughly a 3:1 ratio for the m/z 35 and 37 ions.

Formation of m/z 62 and 64 ions (see m/z ion table above):

[C₂H₂Cl₂]⁺  ===>  [C₂HCl]⁺  +  HCl

C-Cl bond scission from the m/z 97 and 99 ions, loss of chlorine radical.

Alternatively, but very much less probable, the HCl is ionised to give the m/z 36 and 38 ions.

Formation of m/z 61 and 63 ions (see m/z ion table above):

[C₂H₃Cl₂]⁺  ===>  [C₂H₂Cl]⁺  +  HCl

Elimination of hydrogen chloride from the m/z 97, 99 101 ions.

You can see roughly a 3:1 ratio for the m/z 61 and 63 ions.

Alternatively, but very much less probable, the HCl is ionised to give the m/z 36 and 38 ions.

You can see roughly a 3:1 ratio for the m/z 36 and 38 ions.

Formation of m/z 60 and 62 ions (see m/z ion table above):

[?]⁺  ===>  [C₂HCl]⁺  +  ?

You don't see a 3:1 ratio for the m/z 60 and 62 ions?

Formation of m/z 35 to 38 ions (see m/z ion table above):

Whenever a chlorine atom (Cl) or hydrogen chloride molecule (HCl) are generated in the fragmentation process of 1,1,1-trichloroethane, there is a low, but real, probability of them becoming ionised.

Formation of m/z 25 to 27 ions (see m/z ion table above):

Some of the fragments must lose all their chlorine atoms, but retain the three hydrogen atoms to give [C₂H₃]⁺, which can then lose protons to give [C₂H₂]⁺ and [C₂H₂]⁺..

I can't figure out what the m/z 28 ion is, can't be [C₂H₄]+, since there are only three hydrogen atoms in the original parent molecule!

The infrared spectrum of 1,1,1-trichloroethane

The infrared spectrum of 1,1,2-trichloroethane

The mass spectrum of 1,1,1-trichloroethane

The mass spectrum of 1,1,2-trichloroethane

The H-1 NMR spectrum of 1,1,1-trichloroethane

The H-1 NMR spectrum of 1,1,2-trichloroethane

The C-13 NMR spectrum of 1,1,1-trichloroethane

The C-13 NMR spectrum of 1,1,2-trichloroethane

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