[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: Mass spectrometry - analysing the mass spectrum of 1,1-dibromoethane [spectra page updated Mar 16th 2026 *]

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Introductory note on the mass spectrum of 1,1-dibromoethane

Students and teachers please note my explanation of the mass spectrum of 1,1-dibromoethane is designed for advanced, but pre-university, chemistry courses.

If M represents the 1,1-dibromoethane 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 1,1-dibromoethane and only the formation of singly charged positive are considered for the mass spectrum of 1,1-dibromoethane.

I've included a stick diagram and table of m/z ions for the mass spectrum of 1,1-dibromoethane 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 1,1-dibromoethane.

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 1,1-dibromoethane, but the mass spectrometer software does!

1,1-dibromoethane, C₂H₄Br₂, CH₃CHBr₂

Interpreting the fragmentation pattern of the mass spectrum of 1,1-dibromoethane

[M]⁺ are the molecular ion peaks (M) with m/z values of 186, 188 and 190 corresponding to [C₂H₄Br₂]⁺, the original 1,1-dibromoethane molecule minus an electron.

The four possibilities (and in a 1:2:1 ratio, shown below) are:

M-2 ion: m/z 186 [CH₃CH⁷⁹Br⁷⁹Br]⁺

M ion: m/z 188 [CH₃CH⁷⁹Br⁸¹Br]⁺  or  [CH₃CH⁸¹Br⁷⁹Br]⁺ 

M+2 ion: m/z 190 [CH₃CH⁸¹Br⁸¹Br]⁺

There are three molecular ion peaks because bromine as two isotopes, 50.7% ⁷⁹Br and 49.3% ⁸¹Br.

Their average relative isotopic mass is ~80, so the relative molecular mass for 1,1-dibromoethane is ~188.

There are four possible isotopic bromine permutations in the 1,1-dibromoethane molecule:

⁷⁹Br⁷⁹BrCHCH₃, ⁷⁹Br⁸¹BrCHCH₃,  ⁸¹Br⁷⁹BrCHCH₃  and  ⁸¹Br⁸¹BrCHCH₃

(think of a 2 x 2 random probability square).

However, this means any fragment from 1,1-dibromoethane carrying two bromine atoms should show up as three peaks, two mass units apart and approximately in a ratio of  ~ 1:2:1 in heights (intensities).

Fragments from 1,1-dibromoethane with one bromine atom, will give twin peaks in a ~1 : 1 ratio because the isotopic ratio ⁷⁹Br : ⁸¹Br is a ~ 1 : 1 ratio (slightly shorter for the less abundant heavier ⁸¹Br isotope).

The peaks for any ionised ⁷⁹Br fragments will be slightly higher than for any ionised ⁸¹Br ionised fragment peaks in the mass spectrum of 1,1-dibromoethane because there is a slightly higher % of the bromine-79 isotope compared to the bromine-81 - this is quite clear on the mass spectrum.

The very tiny M+1 peaks at m/z 187 and 189, corresponds to an ionised 1,1-dibromoethane molecule with one ¹³C atom in it i.e. an ionised 1,1-dibromoethane molecule of formula [¹³C¹²CH₄Br₂]⁺

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 peaks.

1,1-dibromoethane 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 (1,1-dibromoethane) 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 for the mass spectrum of 1,1-dibromoethane is m/z 107 ion [C₂H₄⁷⁹Br]⁺

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of 1,1-dibromoethane.

Unless otherwise indicated, assume the carbon atom containing ionised fragments from the mass spectrum of 1,1-dibromoethane are the ¹²C isotope.

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 1,1-dibromoethane

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: H = 1; C = 12 ~1% 13); Br = 79 or 81 (~ 1:1 isotope ratio)

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

Suggested equations to explain the most abundant ion peaks of 1,1-dibromoethane

The three molecular ions exist due to the permutations of the two bromine isotopes.

Their occurrence and ~ 1:2:1 ratio have already been described above.

 

Note that any fragment containing a single bromine atom, will show up as twin peaks because of the two bromine isotopes (and one peak is slightly shorter than the other).

 

Formation of m/z 171, 173 and 175 ions:

[CH₃CHBr₂]⁺  ===>  [CHBr₂]⁺  +  CH₃

C-C bond scission to release a methyl group.

Tiny peaks, low probability due to the strong C-C bond, but note the approximate 1:2:1 ratio which fits in with ratio expected due to the two bromine isotopes.

[CH⁷⁹Br⁷⁹Br]⁺, [CH⁷⁹Br⁸¹Br]⁺, [CH⁸¹Br⁷⁹Br]⁺, and [CH⁸¹Br⁸¹Br]⁺

Formation of m/z 107 and 109 ions:

[CH₃CHBr₂]⁺  ===>  [C₂H₄⁷⁹Br]⁺  or  [C₂H₄⁸¹Br]⁺  +  Br

Scission of the C-Br bond, the weakest bond in the molecule.

The weaker C-Br bond is more likely to be broken than the stronger C-C or C-H bonds, hence the peaks are much greater in intensity.

Note the approximate 1:1 ratio, but the 109 peak is slightly smaller.

In fact, and as a consequence, apart from the m/z 27 ion peak, all other ion peaks, formation described below, are of a much lower intensity, but I offer some suggestions of how they may be formed in the fragmentation of 1,1-dibromoethane.

The m/z 107 ion is the base peak ion, the most abundant and 'stable' ion fragment.

Formation of m/z 79 and 81 ions:

[CH₃CHBr₂]⁺  ===>  [⁷⁹Br]⁺  or  [⁸¹Br]⁺  +  BrCH₂CH₂

C-Br bond scission of parent molecular ion (or other bromine containing fragment).

Small peaks, low probability of ionising the freed Br atom, but still observe ~1:1 ratio of intensities (peak height).

Formation of m/z 80 and 82 ions:

[CH₃CHBr₂]⁺  ===>  [H⁷⁹Br]⁺  or   [H⁸¹Br]⁺  +  C₂H₃Br

Elimination of hydrogen bromide from parent molecular ion.

Small peaks, low probability of ionising the eliminated HBr molecule, but still observe ~1:1 ratio of intensities (peak height).

Formation of m/z 28 ion:

[C₂H₄Br]⁺  ===>  [C₂H₄]⁺  +  Br

C-Br bond scission of the m/z 107 or 109 ions to give an ionised ethene molecule.

Formation of m/z 27 ion:

[C₂H₄Br]⁺  ===>  [C₂H₃]⁺  +  HBr

Elimination of hydrogen bromide from the most abundant m/z 107 and 109 ions.

The m/z 28 ion may also lose a H to give the m/z 27 ion.

The m/z 27 and 28 ions may also lose hydrogen to give the m/z 26 ion.

Note the, not unexpected, absence of an m/z 29 ion [C₂H₅]⁺

The infrared spectrum of 1,1-dibromoethane

The infrared spectrum of 1,2-dibromoethane

The mass spectrum of 1,2-dibromoethane

The H-1 NMR spectrum of 1,1-dibromoethane

The H-1 NMR spectrum of 1,2-dibromoethane

The C-13 NMR spectrum of 1,1-dibromoethane

The C-13 NMR spectrum of 1,2-dibromoethane

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