[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 & AP honors chemistry courses: Molecular spectroscopy of bromoethane [spectra updated Mar 19th 2026 *]

 email doc brown   Re-edit mass spectrum of CH₃CH₂Br

Introductory note on the mass spectrum of bromoethane

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

If M represents the bromoethane 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 bromoethane and only the formation of singly charged positive are considered for the mass spectrum of bromoethane.

I've included a stick diagram and table of m/z ions for the mass spectrum of bromoethane 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 bromoethane.

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 bromoethane, but the mass spectrometer software does!

Bromoethane           

Interpreting the fragmentation pattern of the mass spectrum of bromoethane

[M]⁺ is the molecular ion peak with an m/z of 108 (M peak) and 110 (M+2 peak) corresponding to the ions  [C₂H₅⁷⁹Br]⁺ and [C₂H₅⁸¹Br]⁺ the original bromoethane molecule minus an electron, [CH₃CH₂Br]⁺.

There are two molecular ion peaks because bromine as two isotopes, 50.5% ⁷⁹Br and 50.5% ⁸¹Br.

One peak is slightly shorter for the molecular ion with the slightly less abundant heavier bromine isotope.

Their average relative mass is ~80, so the relative molecular mass for bromoethane is ~109.

However, this means any fragment carrying a bromine atom should show up as twin peaks, two mass units apart and approximately of equal height (intensities), though intensity of ions with the heavier bromine isotope are slightly smaller..

The small M+1 and M+2 peaks at m/z 109 and 111, corresponds to an ionised bromoethane molecule with one ¹³C atom in it

i.e. an ionised bromoethane molecule of formula [¹³C¹²CH₅⁷⁹Br]⁺ (M ion peak) and  [¹³C¹²CH₅⁸¹Br]⁺ (M+2 ion peak), because the latter has the slightly less abundant heavier bromine isotope.

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 these ¹³C peaks.

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

They seem to show up clearly because the parent molecular ions (m/z 108 and 110) are relatively stable, their intensities are second only to the base ion peak of m/z 29.

The most abundant ion of the molecule under mass spectrometry investigation (bromoethane) is usually given an arbitrary value of 100, called the base ion peak, and all other abundances ('intensities') are measured against it.

The base ion peak in the mass spectrum of bromoethane is the m/z 29 ion [C₂H₅]⁺

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of bromoethane.

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

The small peaks at m/z 80 and 82 are due to the ionised HBr molecules formed by elimination from the parent molecular ion of bromoethane i.e. [H⁷⁹Br]⁺  and [H⁸¹Br]

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

PLEASE NOTE I have found it difficult to find 'authentic' equations to explain mass spectra fragmentation patterns and it is complex chemistry! I've identified the formulae of the ionised fragments on the mass spectrum diagram, but the equations are from the internet or my conjecture as to how the ions might be formed - please take care in using the information, especially for assignments at university or pre-university level.

Atomic masses: C = 12 (~1% 13);  H = 1; Br = 79 and 81

Bond enthalpies kJ/mol: C-C = 348;  C-H = 412;  C-Br = 276;

Possible equations to explain the most abundant ion peaks from the fragmentation of ionised bromoethane molecules.

Formation of m/z 93 and 95 ions:

[CH₃CH₂Br]⁺  ===>  [CH₂Br]⁺  +  CH₃

C-C bond scission with mass loss of methyl group,

mass changes 108 - 15 = 93, 110 - 15 = 95

(bromine can be ⁷⁹Br or ⁸¹Br, see data table of ionised fragments)

Where R is alkyl, the double RBr peaks of roughly 1 : 1 abundance ratio are characteristic of organo-bromine compounds (one m/z ion peak is slightly shorter than the other, technically 50.7 : 49.3).

Formation of m/z 79 to 81 ions:

[CH₃CH₂Br]⁺  ===>  [⁷⁹Br]⁺  or  [⁸¹Br]  +  CH₂CH₃

C-Br bond scission with mass loss of ethyl group,

mass changes 108 - 29 = 79, 110 - 29 = 81

m/z 80 and 82 ions could be formed by elimination of HBr, leaving ethene

[CH₃CH₂Br]⁺  ===>  [H⁷⁹Br]⁺  or  [H⁸¹Br]  +  CH₂=CH₂

Alternatively the ethene fragment could be ionised to give the m/z 28 ion [C₂H₄]⁺

[CH₃CH₂Br]⁺  ===>  [CH₂=CH₂]⁺  +  HBr

(see also below for m/z 29 ion - H atom)

Formation of m/z 29 ion:

[CH₃CH₂Br]⁺  ===>  [CH₃CH₂]⁺  +  Br

C-Br bond scission and the ethyl fragment carries the positive charge.

This is breaking the weakest bond in the bromoethane molecule.

The m/z 29 ion is the base peak ion, the most abundant and 'stable' ion fragment.

The ethyl cation loses successive protons to generate m/z ions of masses 28 => 27 => 26 => 25 (see data table of fragment ions).

The infrared spectrum of bromoethane

The H-1 NMR spectrum of bromoethane

The C-13 NMR spectrum of bromoethane

The chemistry of HALOGENOALKANES (haloalkanes) revision notes INDEX

 Mass spectrometry - introduction and mass spectra index

ALL SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

Use My Google search site box

Email doc b: chem55555@hotmail.com