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Advanced Organic Chemistry: Mass spectrum of chloroethane

The mass spectrum of chloroethane

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 chloroethane

C2H5Cl CH3CH2Cl mass spectrum of chloroethane fragmentation pattern of m/z m/e ions for analysis and identification of ethyl chloride image diagram doc brown's advanced organic chemistry revision notes 

chloroethane   CH3CH2Cl   CH3-CH2-Cl

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

m/z value of [fragment]+ 51 49 38 37 36 35 29 28 27 26
[molecular fragment]+ CH237Cl CH235Cl [H37Cl]+ [37Cl]+ [H35Cl]+ [35Cl]+ [C2H5]+ [C2H4]+ [C2H3]+ [C2H2]+

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of chloroethane

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.


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Links associated with chloroethane

The chemistry of HALOGENOALKANES (haloalkanes) revision notes INDEX

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