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

[M]⁺ are the molecular ion peaks (M) with a m/z values of 78 and 80 corresponding to [C₃H₇Cl]⁺, the original 2-chloropropane molecule minus an electron, [CH₃CH³⁵ClCH₃]⁺ and  [CH₃CH³⁷ClCH₃]⁺.

Since chlorine has two common isotopes of ³⁵Cl and ³⁷Cl in the ratio 3 : 1, you should observe double peaks in the intensity ratio 3 : 1, two mass units apart for molecular fragments containing a chlorine atom from the fragmentation pattern of 2-chlorobutane.

This also applies to the molecular ion, so two molecular ions are observed at m/z 78 and 80.

The tiny M+1 peak at m/z 79, corresponds to an ionised 2-chloropropane molecule with one ¹³C atom in it i.e. an ionised 1-chloropropane molecule of formula [¹³C¹²C₂H₇³⁵Cl]⁺

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-chloropropane 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-chloropropane) 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-chloropropane.

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

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

Atomic masses: H = 1; C = 12; Cl = 35 or 37 (3:1)

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

Equations to explain the most abundant ion peaks of 2-chloropropane

Formation of m/z 63 and 65 ions:

[CH₃CHClCH₃]⁺  ===>  [CH₃CH³⁵Cl]⁺  or  [CH₃CH³⁷Cl]⁺  +  CH₃

C-C bond fission where the end methyl group has broken off.

Low probability because of the strong C-C bond enthalpy.

Note the expected 3:1 ratio of chlorine containing fragments.

Formation of m/z 44 ion:

[CH₃CHClCH₃]⁺  ===>  [C₃H₆]⁺  +  HCl

Elimination of hydrogen chloride from the parent molecular ion.

Formation of m/z 43 ion:

[CH₃CHClCH₃]⁺  ===>  [CH₃CHCH₃]⁺  +  Cl

Bond scission of the C-Cl bond, the weakest bond in the molecule.

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

Formation of m/z 42 ion:

[CH₃CHClCH₃]⁺  ===>  [CH₃CHCH₃]⁺  +  HCl

Elimination of hydrogen chloride.

Formation of m/z 41 ion:

[?]⁺  ===>  [C₃H₅]⁺  +  ?

Formation of m/z 39 ion:

[?]⁺  ===>  [C₃H₃]⁺  +  ?

Formation of m/z 27 ion:

[CH₃CHCl]⁺  ===>  [C₂H₃]⁺  +  HCl

Elimination of hydrogen chloride from the m/z 63 and 65 ions.

The chemistry of HALOGENOALKANES (haloalkanes) revision notes INDEX

Mass spectroscopy index

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