[Author © Dr Phil Brown PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses & US K12 grade 11, grade 12 and AP honors chemistry courses: Molecular spectroscopy analysis of chloroethane [spectra page updated Mar 22nd 2026 *]

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Introductory note on the mass spectrum of chloroethane

Students and teachers please note my explanation of the mass spectrum of chloroethane is designed for advanced, but pre-university, chemistry courses.

If M represents the chloroethane molecule, the initial ionisation to give the molecular ion is:

M_(g) + high KE e^-  ==> [M•]⁺_(g) + 2e^- and for fragmentation equations assume [M]⁺ is the start of the processes and all species are in a gaseous state.

I've not usually shown an unpaired electron on e.g. an ion or a non-ionised alkyl radical R e.g.

[M•]⁺ ==> [X]⁺  +  R•, but you should be aware this is a more accurate depiction of some processes.

I've used simplified equations to show how some of the ions that might be formed in the fragmentation pattern for the mass spectrum of chloroethane and only the formation of singly charged positive are considered for the mass spectrum of chloroethane.

I've included a stick diagram and table of m/z ions for the mass spectrum of chloroethane and doing the mass spectrum analysis under standard conditions, databases can be compiled based on complex fingerprint patterns, often involving the relative intensities of many fragment ions, and used to identify compounds including chloroethane.

In selected cases, where two different fragment ions have the same integer m/z value, I've pointed out that modern mass spectrometers can measure relative ion mass to four decimal places. So, using accurate isotopic masses, I've calculated the accurate ion masses, BUT strictly speaking, 0.0005 should be deducted for singly charged ions to account for the loss of the electron in their formation. I have NOT done this for chloroethane, but the mass spectrometer software does!

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

the M ion  [C₂H₅Cl]⁺, the original chloroethane molecule minus an electron,

the M ion is m/z 64 [CH₃CH₂³⁵Cl]⁺   and  the M+2 ion m/z 66 [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 at M+3), corresponds to an ionised chloroethane molecule with one ¹³C atom in it i.e. an ionised chloroethane molecule of formula [¹³C¹²CH₅Cl]⁺

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.

The base ion peak and molecular ion peak are the same m/z 64 ion [CH₃CH₂³⁵Cl]⁺

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 chloroethane

Atomic masses: H = 1; C = 12 (~1% 13);  Cl = 35 or 37 (~ratio 3:1)

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

Suggested 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. The smaller peak at m/z 66 is due to [CH₃CH₂³⁷Cl]⁺

Where R is alkyl, the appearance of double RCl m/z ion peaks of roughly 3 : 1 abundance ratio are characteristic of organo-chlorine compounds i.e. caused by the 3 : 1 isotope ratio of ³⁵Cl : ³⁷Cl.

Formation of m/z 51 and 49 ions:

[CH₃CH₂Cl]⁺  ===>  [CH₂³⁵Cl]⁺  or   [CH₂³⁷Cl]⁺  +   CH₃

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

Mass changes 64 - 15  = 49  and  66 - 15 = 51 for the M-15 ions

Again note where R is alkyl, the double RCl m/z ion peaks of roughly 3 : 1 abundance ratio are characteristic of organo-chlorine compounds i.e. caused by the 3 : 1 isotope ratio of ³⁵Cl : ³⁷Cl.

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.

BUT the alkyl group is more likely to be ionised (see m/z 29 below).

Mass changes 64 - 29  = 35  and  66 - 29 = 37 for the M-29 and M+2-29 ions

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.

Mass changes 64 - 35  = 29  and  66 - 37 = 29 for the M-35 and M+2-37 ions

The m/z 29 ion can lose hydrogen atoms to give the ions from 28 down to 25 (see data table of ions).

Formation of m/z 28 ion:

[CH₃CH₂Cl]⁺  ===>  [C₂H₄]⁺  + HCl

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

Mass changes 64 - 36  = 28  and  66 - 38 = 28 for the M-36 and M+2-38 ions.

The m/z 28 ion can lose hydrogen atoms to give the ions from 27 down to 25 (see data table of ions).

The infrared spectrum of chloroethane

The H-1 NMR spectrum of chloroethane

The C-13 NMR spectrum of chloroethane

The chemistry of HALOGENOALKANES (haloalkanes) revision notes INDEX

Mass spectroscopy index

ALL SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

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