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Interpreting the fragmentation pattern of the mass spectrum of 1-chloropropane

[M]⁺ are the molecular ion peaks (M) with a m/z values of 78 and 80 corresponding to [C₃H₇Cl]⁺, the original 1-chloropropane molecule minus an electron, [CH₃CH₂CH₂³⁵Cl]⁺ and  [CH₃CH₂CH₂³⁷Cl]⁺.

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 1-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 1-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.

1-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 (1-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 1-chloropropane.

Unless otherwise indicated, assume the carbon atoms in 1-chloropropane are the ¹²C isotope.

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

Formation of m/z 63 and 65 ions:

[CH₃CH₂CH₂Cl]⁺  ===>  [CH₂CH₂³⁵Cl]⁺  or  [CH₂CH₂³⁷Cl]⁺  +  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 49 and 51 ions:

[CH₃CH₂CH₂Cl]⁺  ===>  [CH₂³⁵Cl]⁺ or  [CH₂³⁷Cl]⁺   +  CH₃CH₂

C-C bond fission where the end ethyl 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 43 ion:

[CH₃CH₂CH₂Cl]⁺  ===>  [CH₃CH₂CH₂]⁺  +  Cl

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

Formation of m/z 42 ion:

[CH₃CH₂CH₂Cl]⁺  ===>  [C₃H₆]⁺  +  HCl

Elimination of hydrogen chloride to give a positive ion of propene.

The m/z 42 ion is the base peak ion, the most abundant and 'stable' ion fragment.

The m/z 42 and 43 ions can lose protons to give m/z ions of 42 ==> 39

Formation of m/z 41 ion:

[?]⁺  ===>  [C₃H₅]⁺  +  ?

Formation of m/z 39 ion:

[?]⁺  ===>  [C₃H₃]⁺  +  ?

Formation of m/z 35 to 38 ions: (not shown in table)

Ionisation of chlorine after C-Cl bond scission gives [³⁵Cl]⁺ or [³⁷Cl]⁺

Elimination of hydrogen chloride gives [H³⁵Cl]⁺ or [H³⁷Cl]⁺

Low probability of formation, and of little importance, the alkyl fragments tend to carry the positive charge - the more electronegative chlorine tends to retain its electrons.

Formation of m/z 29 ion:

[CH₃CH₂CH₂Cl]⁺  ===>  [CH₃CH₂]⁺  +  CH₂Cl

C-C bond scission

Formation of m/z 28 ion:

[CH₂CH₂Cl]⁺  ===>  [C₂H₄]⁺  +  Cl

C-Cl bond scission

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

ALL SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

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