Advanced Organic Chemistry: Mass spectrum of 1,1-dibromoethane

SITEMAP * HOME PAGE * SEARCH * GCSE Level Chemistry age ~14-16 * Advanced Level Chemistry age ~16-19

The mass spectrum of 1,1-dibromoethane

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 1,1-dibromoethane

email doc brown

Use your mobile phone or ipad etc. in 'landscape' mode

This is a BIG website, you need to take time to explore it

Mass spectroscopy - spectra index

C2H4Br2 CH3CHBr2 mass spectrum of 1,1-dibromoethane fragmentation pattern of m/z m/e ions for analysis and identification of 1,1-dibromoethane image diagram doc brown's advanced organic chemistry revision notes 

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 three possibilities (and in a 1:2:1 ratio, see below) are:

m/z 186 [CH3CH79Br79Br]+, m/z 188 [CH3CH79Br81Br]+, and m/z 190 [CH3CH81Br81Br]+

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

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

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

79Br79Br, 79Br81Br,  81Br79Br and 81Br81Br (think of a 2 x 2 random probability square).

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

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.

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 atoms in 1,1-dibromoethane are the 12C isotope.

3 m/z values of [M]+ 190 188 186
[molecule M]+ [CH3CH79Br79Br]+ [CH3CH79Br81Br]+ [CH3CH81Br81Br]+
m/z value of [fragment]+ 175 173 171 109 107
[molecular fragment]+ [CH79Br79Br]+ [CH79Br81Br]+ [CH81Br81Br]+ [C2H481Br]+ [C2H479Br]+
m/z value of [fragment]+ 82 81 80 79 28 27 26
[molecular fragment]+ [H81Br]+ [81Br]+ [H79Br]+ [79Br]+ [C2H4]+ [C2H3]+ [C2H2]+

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

Atomic masses: H = 1; C = 12; Br = 79 or 81 (1:1)

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

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.

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

[CH3CHBr2]+  ===>  [CHBr2]+  +  CH3

C-C bond scission to release a methyl group.

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.

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.

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).

Formation of m/z 80 and 82 ions:

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

Elimination of hydrogen bromide from parent molecular ion.

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 m/z 107 and 109 ions.

Key words & phrases: C2H4Br2 CH3CHBr2 image diagram on how to interpret and explain the mass spectrum of 1,1-dibromoethane m/z m/e base peaks, image and diagram of the mass spectrum of 1,1-dibromoethane, details of the mass spectroscopy of 1,1-dibromoethane,  low and high resolution mass spectrum of 1,1-dibromoethane, prominent m/z peaks in the mass spectrum of 1,1-dibromoethane, comparative mass spectra of 1,1-dibromoethane, the molecular ion peak in the mass spectrum of 1,1-dibromoethane, analysing and understanding the fragmentation pattern of the mass spectrum of 1,1-dibromoethane, characteristic pattern of peaks in the mass spectrum of 1,1-dibromoethane, relative abundance of mass ion peaks in the mass spectrum of 1,1-dibromoethane, revising the mass spectrum of 1,1-dibromoethane, revision of mass spectroscopy of 1,1-dibromoethane, most abundant ions in the mass spectrum of 1,1-dibromoethane, how to construct the mass spectrum diagram for abundance of fragmentation ions in the mass spectrum of 1,1-dibromoethane, how to analyse the mass spectrum of 1,1-dibromoethane, how to describe explain the formation of fragmented ions in the mass spectra of 1,1-dibromoethane equations for explaining the formation of the positive ions in the fragmentation of the ionised molecule of 1,1-dibromoethane recognising the base ion peak of 1,1-dibromoethane interpreting interpretation the mass spectrum of 1,1-dibromoethane

Links associated with 1,1-dibromoethane

The chemistry of HALOGENOALKANES (haloalkanes) revision notes INDEX

Mass spectroscopy index


All Advanced Organic Chemistry Notes

Use My Google search site box

Email doc b: