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Advanced Organic Chemistry: Mass spectrum of 2-bromopropane

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The mass spectrum of 2-bromopropane

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 2-bromopropane

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Mass spectroscopy - spectra index

C3H7Br CH3CHBrCH3 mass spectrum of 2-bromopropane fragmentation pattern of m/z m/e ions for analysis and identification of isopropyl bromide image diagram doc brown's advanced organic chemistry revision notes 

2-bromopropane, C3H7Br, CH3CHBrCH3(c) doc b , (c) doc b

Interpreting the fragmentation pattern of the mass spectrum of 2-bromopropane

[M]+ is the molecular ion peaks (M) with an m/z of 122 and 124 corresponding to [C3H7Br]+, the original 2-bromopropane molecule minus an electron, [CH3CHBrCH3]+.

There are two 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 2-bromopropane is ~123.

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).

The very tiny M+1 peaks at m/z 123 and 124, corresponds to an ionised 2-bromopropane molecule with one 13C atom in it i.e. an ionised 2-bromopropane molecule of formula [13C12C2H7Br]+

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 M+1 peaks.

2-bromopropane has 3 carbon atoms, so on average, ~1 in 33 molecules will contain a 13C atom.

The most abundant ion of the molecule under mass spectrometry investigation (2-bromopropane) is usually given an arbitrary 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 2-bromopropane.

Unless otherwise indicated, assume the carbon atoms in 2-bromopropane are the 12C isotope.

m/z value of [fragment]+ 124 122 109 107 82 81 80 79
[molecular fragment]+ [C3H781Br]+ [C3H779Br]+ [C2H481Br]+ [C2H479Br]+ [H81Br]+ [81Br]+ [H79Br]+ [79Br]+
m/z value of [fragment]+ 44 43 42 41 39 27 15
[molecular fragment]+ [13CC2H7]+ [C3H7]+ [C3H6]+ [C3H5]+ [C3H3]+ [C2H3]+ [CH3]+

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 2-bromopropane

Atomic masses: C = 12;  H = 1; O = 16;  Cl = 35/37;  Br = 79 or 81

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

Equations to explain the most abundant ion peaks of 2-bromopropane

Formation of m/z 107 and 109 ions:

[CH3CHBrCH3]+  ===>  [C2H4Br]+  +  CH3

C-C bond scission to lose an end methyl group.

Mass loss = 124 - 15 = 109  and  122 - 15 = 107

Very small peaks since C-C bond bond scission is much less likely than C-Br scission - the latter having a significantly lower bond enthalpy.

Formation of m/z 80 and 82 ions:

[CH3CHBrCH3]+  ===>  [H79Br]+  or  [H81Br]+  +  C3H6

Elimination of hydrogen bromide from the parent molecular ion.

Formation of m/z 79 and 81 ions:

[CH3CHBrCH3]+  ===>  [79Br]+  or  [81Br]+  +  C3H7

C-Br bond scission, but the alkyl fragment is more likely to carry the positive charge (see below).

Formation of m/z 43 ion:

[CH3CHBrCH3]+  ===>  [C3H7]+  +  Br

C-Br bond fission to give the ionised alkyl ion (bond weaker than C-C).

Mass loss 124 - 81 = 43  and  122 - 79 = 43

The m/z 43 ion is the base peak ion, the most abundant and 'stable' ion fragment.

It undergoes success proton loss e.g. 43 => 42 => 41 => 40 => 39

Formation of m/z 27 ion:

[C2H4Br]+  ===>  [C2H3]+  +  HBr

Elimination of hydrogen bromide.

Mass loss 109 - 82 = 27  and  107 - 80 = 27

Formation of m/z 15 ion:

[CH3CHBrCH3]+  ===>  [CH3]+  +  C2H4Br

C-C bond scission.

Mass loss 124 - 109 = 15  and  122 - 107 = 15


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