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Advanced Organic Chemistry: Mass spectrum of 2-chloropropane

The mass spectrum of 2-chloropropane

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US K12 grade 11 grade 12 organic chemistry courses involving molecular spectroscopy analysing mass spectra of 2-chloropropane

C3H7Cl CH3CHClCH3 mass spectrum of 2-chloropropane fragmentation pattern of m/z m/e ions for analysis and identification of  isopropyl chloride image diagram doc brown's advanced organic chemistry revision notes 

2-chloropropane  C3H7Cl  (c) doc b  (c) doc b  (c) doc b

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.

m/z value of [fragment]+ 65 63 44 43 42 41
[molecular fragment]+ [CH3CH37Cl]+ [CH3CH35Cl]+ [C3H8]+ [C3H7]+ [C3H6]+ [C3H5]+
m/z value of [fragment]+ 40 39 29 28 27 26
[molecular fragment]+ [C3H4]+ [C3H3]+ [C2H5]+ [C2H4]+ [C2H3]+ [C2H2]+

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.

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