Interpreting the fragmentation pattern of the mass spectrum of chloroethane

[M]⁺ is the molecular ion peak (M) with an m/z of 64 or 66 and corresponding to [C₂H₅Cl]⁺, the original chloroethane molecule minus an electron, [CH₃CH₂³⁵Cl]⁺ and [CH₃CH₂³⁷Cl]⁺.

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 64 and 66.

The small M+1 peak at m/z 65 (and minute at m/z 67), corresponds to an ionised chloroethane molecule with one ¹³C atom in it i.e. an ionised chloroethane molecule of formula [¹³C¹²C₂H₅Cl]⁺

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.

chloroethane has 2 carbon atoms, so on average, ~1 in 50 molecules will contain a ¹³C atom.

The most abundant ion of the molecule under mass spectrometry investigation (chloroethane) 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 chloroethane.

Unless otherwise indicated, assume the carbon atoms in chloroethane are the ¹²C isotope.

Interpreting the mass spectrum of 1-chlorobutane

Atomic masses: H = 1; C = 12; N = 14; O = 16;  Cl = 35 or 37 (3:1);  Br = 79 or 81 (1:1);

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

Equations to explain the most abundant ion peaks of chloroethane

Base ion peak m/z 64

The m/z 64 ion is the base peak ion, the most abundant and 'stable' ion fragment.

[CH₃CH₂³⁵Cl]⁺ is also the most abundant of the two molecular ion peaks, carrying the more abundant ³⁵Cl isotope.

Formation of m/z 51 and 49 ions:

[CH₃CH₂Cl]⁺  ===>  [CH₂³⁵Cl]⁺  or   [CH₂³⁷Cl]⁺  +   CH₃CH₂

C-C bond scission, strong bond and much less likely to happen than fission of the weaker C-Cl bond (see below)

Formation of m/z 35 and 37 ions:

[CH₃CH₂Cl]⁺  ===>  [³⁵Cl]⁺  or   [³⁷Cl]⁺  +   CH₃CH₂

C-Cl bond scission, more likely to happen, the weakest bond in the molecule.

Formation of m/z 36 and 38 ion:

[CH₃CH₂Cl]⁺  ===>  [H³⁵Cl]⁺  or  [H³⁷Cl]⁺  +  C₂H₄

A small probability of elimination of hydrogen chloride from the parent molecular ion, or the elimination of an ethene molecule, either can be ionised (see m/z 28)

Formation of m/z 29 ion:

[CH₃CH₂Cl]⁺  ===>  [CH₃CH₂]⁺  +  Cl

C-Cl bond scission, this time the alkyl group carries the positive charge.

Formation of m/z 28 ion:

[CH₃CH₂Cl]⁺  ===>  [C₂H₄]⁺  + HCl

Again, elimination of hydrogen chloride, but the ethene fragment carries the positive charge.

The chemistry of HALOGENOALKANES (haloalkanes) revision notes INDEX

Mass spectroscopy index

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