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Advanced Organic Chemistry: Mass spectrum of 1-bromo-2-methylpropane

Interpreting the mass spectrum of 1-bromo-2-methylpropane

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-bromo-2-methylpropane (isobutyl bromide)

See also comparing infrared, mass, 1H NMR & 13C NMR spectra of 4 halogenoalkane isomers of C4H9Br

mass spectrum of 1-bromo-2-methylpropane C4H9Br (CH3)2CHCH2Br fragmentation pattern of m/z m/e ions for analysis and identification of 1-bromo-2-methylpropane image diagram doc brown's advanced organic chemistry revision notes 

1-bromo-2-methylpropane (isobutyl bromide), C4H9Br, (CH3)2CHCH2Br

Interpreting the fragmentation pattern of the mass spectrum of 1-bromo-2-methylpropane

[M]+ is the parent molecular ion peak (M) with an m/z of 136 or 138 corresponding to [C4H9Br]+, the original 1-bromo-2-methylpropane molecule minus an electron, [(CH3)2CHCH2Br]+.

There are two possibilities because bromine consists of two isotopes, 79Br and 81Br in the ratio ~1 : 1.

Therefore the molecular ion can be [(CH3)2CHCH279Br]+  or  [(CH3)2CHCH281Br]+, which should, and does, show up as a double peak of ~equal heights (~equal abundance).

These are referred to as the M and M+2 peaks respectively, emphasising the two mass unit difference due to the bromine isotopes in the two molecular ions of 1-bromo-2-methylpropane.

The two bromine isotopes also account for the tiny 'twin peaks' of m/z ions 93 & 95 and 121 & 132.

Bromine consists of two isotopes, 79Br and 81Br in roughly equal proportions, therefore any molecular ion or fragment containing a bromine atom will show up as a double peak of similar height (abundance) two mass units apart e.g. m/z ions 93 and 95, and 121 and 123, plus small molecular ion peaks of m/z values 136 and 138 (but still, all pairs of ~equal height!) in the mass spectrum of 1-bromo-2-methylpropane.

The small M+1 peak at m/z 137, corresponds to an ionised 1-bromo-2-methylpropane molecule with one 13C atom in it i.e. an ionised 1-bromo-2-methylpropane molecule of formula [13C12C4H979Br]+

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.

1-bromo-2-methylpropane has 4 carbon atoms, so on average, ~1 in 25 molecules will contain a 13C atom.

The most abundant ion of the molecule under mass spectrometry investigation (1-bromo-2-methylpropane) 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 (including M and M+2 ions) in the fragmentation pattern of 1-bromo-2-methylpropane.

Unless otherwise indicated, assume the carbon atoms in 1-bromo-2-methylpropane are the 12C isotope.

Some of the possible positive ions, [molecular fragment]+, formed in the mass spectrometry of 1-bromo-2-methylpropane.

The parent molecular ion M of 1-bromo-2-methylpropane m/z 136: [(CH3)2CHCH2Br]+

m/z value of [fragment]+ 138 136 123 121 109 107
[molecular fragment]+ [C4H981Br]+ [C4H979Br]+ [C3H681Br]+ [C3H679Br]+ [C2H481Br]+ [C2H479Br]+
m/z value of [fragment]+ trace of 95 trace of 93 58, with 13C atom 57, all 12C atoms 56 55
[molecular fragment]+ [CH281Br]+ [CH279Br]+ [C4H9]+ [C4H9]+ [C4H8]+ [C4H7]+
m/z value of [fragment]+ 43 42 41 40 39 29 28 27 26 15
[molecular fragment]+ [C3H7]+ [C3H6]+ [C3H5]+ [C3H2]+ [C3H3]+ [C2H4]+ [C2H4]+ [C2H3]+ [C2H2]+ [CH3]+

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 1-bromo-2-methylpropane

Atomic masses: H = 1;  C = 12;  Br = 79 or 81 (1:1 isotope abundance ratio); I = 127

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

Possible equations to explain some of the most abundant ion peaks of 1-bromo-2-methylpropane (tabulated above)

Formation of m/z 121 and 123 ions:

[(CH3)2CHCH2Br]+  ===>  [C3H6Br]+  +  CH3

C-C bond scission in the parent molecular ions, mass change 136/138 - 15 = 121/123..

Low probability as the C-Br is a much weaker bond, hence more likely to break.

Note the twin ~1:1 peaks due to the two bromine isotopes.

Formation of m/z 93 and 95 ions:

[(CH3)2CHCH2Br]+  ===>  [CH2Br]+  +  C3H7

C-C bond scission in the parent molecular ion, mass change 136/138 - 43 = 93/95.

Note the twin ~1:1 peaks due to the two bromine isotopes.

The C-Br bond is the weakest bond in the molecule, hence the most likely bond scission is C-Br with 1-bromo-2-methylpropane (see below).

Formation of m/z 57 ion:

[(CH3)2CHCH2Br]+  ===>  [C4H9]+  +  Br

This alternative ionisation to above is much more likely, C-Br bond scission in the parent molecular ion, mass change 136/138 - 79/81 = 57.

The m/z 57 ion is the base peak ion, the most abundant and 'stable' ion fragment.

The m/z 58 ion is likely to be [13C12C3H9]+  i.e. as above but with a 13C atom in the hydrocarbon fragment.

The m/z 57 ion can lose a hydrogen atom/molecule to give m/z ions 56 and 55.

There is a very low probability that the bromine atom can also be ionised to give m/z 79 and 81 ions - you can just about make out the tiny twin peaks.

Formation of m/z 56 ion:

[(CH3)2CHCH2Br]+  ===>  [C4H8]+  +  HBr

Elimination of hydrogen bromide from the parent molecular ion can also give the m/z 56 ion.

Mass change 136/138 - 80/82 = 56.

There is a very low probability that the hydrogen bromide molecule can also be ionised to give m/z 80 and 82 ions - you can just about make out the tiny twin peaks.

Formation of m/z 41 and 39 ions:

Possible reactions include:

m/z 41: [C4H8]+  ===>  [C3H5]+  +  CH3

m/z 39: [C3H5]+  ===>  [C3H3]+  +  H2

Formation of m/z 29, 28 and 27 ions:

Possible reactions include:

[(CH3)2CHCH2Br]+  ===>  [C2H5]+  +  CH2CH2Br

From bond scission in the parent molecular ion.

m/z 27: [C4H8]+  ===>  [C2H3]+  +  C2H5

m/z 28: [C4H8]+  ===>  [C2H4]+  +  C2H4

m/z 29: [C4H8]+  ===>  [C2H5]+  +  C2H3

Comparing the infrared, mass, 1H NMR and 13C NMR spectra of the 4 halogenoalkane isomers of C4H9Br

NOTE: The images are linked to their original detailed spectral analysis pages AND can be doubled in size with touch screens to increase the definition to the original 1-bromobutane, 2-bromobutane, 1-bromo-2-methylpropane and 2-bromo-2-methylpropane image sizes.  These four molecules are structural isomers of molecular formula C4H9Br and exemplify the infrared, mass, 1H NMR and 13C NMR spectra of lower aliphatic halogenoalkanes (haloalkanes, alkyl halides, bromoalkanes, alkyl bromides).

INFRARED SPECTRA (above): Apart from the significant differences in the fingerprint region at wavenumbers 1500 to 400 cm-1, there are no other great striking differences, but each could be identified from its infrared spectrum.

MASS SPECTRA (above): All four give the parent molecular ions of m/z 136 and 138, but it is only a relatively tiny peak for 2-bromobutane and 2-bromo-2-methylpropane. All four give the base ion peak of m/z 57. All four give prominent peaks for m/z ions 27, 29, 39 and 41 and all give a tiny peak from an ionised iodine atom at m/z 127. They look quite similar to me and lack a clear fingerprint fragmentation pattern. There are small differences in the relative abundances (peak heights) for pairs of ions involving 79Br/81Br isotopes e.g. m/z 93/95, 107/109 and 121/123. 1-bromo-2-methylpropane is the only one of the four to have a prominent peak for the m/z 43 ion.

1H NMR SPECTRA (above): The 1H NMR spectra of all four molecules give different integrated proton ratios i.e.1-bromobutane four peaks of ratio 3:2:2:2; 2-bromobutane four peaks of ratio 3:3:2:1, 1-bromo-2-methylpropane three peaks of ratio 6:2:1 and 2-bromo-2-methylpropane gives just one peak '1' (effectively no ratio involved), so all four molecular structures can be distinguished from each other by their 1H NMR spectra proton ratios, numbers of peaks and (n+1) rule splitting patterns.

13C NMR SPECTRA (above): The 13C NMR spectra of the four molecules show various numbers of carbon-13 chemical environments i.e 1-bromobutane and 2-bromobutane show four 13C NMR resonances, 1-bromo-2-methylpropane three 13C NMR resonances and 2-bromo-2-methylpropane only two 13C resonances. Therefore 1-bromo-2-methylpropane and 2-bromo-2-methylpropane can be distinguished from the other three by their number of resonances in their 13C NMR spectra, but 1-bromobutane and 2-bromobutane cannot be distinguished from each other from their number of 13C NMR resonance lines - other data would be required.

Key words & phrases: C4H9Br (CH3)2CHCH2Br image diagram on how to interpret and explain the mass spectrum of 1-bromo-2-methylpropane m/z m/e base peaks, image and diagram of the mass spectrum of 1-bromo-2-methylpropane, details of the mass spectroscopy of 1-bromo-2-methylpropane,  low and high resolution mass spectrum of 1-bromo-2-methylpropane, prominent m/z peaks in the mass spectrum of 1-bromo-2-methylpropane, comparative mass spectra of 1-bromo-2-methylpropane, the molecular ion peak in the mass spectrum of 1-bromo-2-methylpropane, analysing and understanding the fragmentation pattern of the mass spectrum of 1-bromo-2-methylpropane, characteristic pattern of peaks in the mass spectrum of 1-bromo-2-methylpropane, relative abundance of mass ion peaks in the mass spectrum of 1-bromo-2-methylpropane, revising the mass spectrum of 1-bromo-2-methylpropane, revision of mass spectroscopy of 1-bromo-2-methylpropane, most abundant ions in the mass spectrum of 1-bromo-2-methylpropane, how to construct the mass spectrum diagram for abundance of fragmentation ions in the mass spectrum of 1-bromo-2-methylpropane, how to analyse the mass spectrum of 1-bromo-2-methylpropane, how to describe explain the formation of fragmented ions in the mass spectra of 1-bromo-2-methylpropane equations for explaining the formation of the positive ions in the fragmentation of the ionised molecule of 1-bromo-2-methylpropane recognising the base ion peak of 1-bromo-2-methylpropane interpreting interpretation the mass spectrum of 1-bromo-2-methylpropane isobutyl bromide alkyl halide alkyl bromide functional group haloalkane halogenoalkane bromoalkane

Stick diagram of the relative abundance of ionised fragments in the fingerprint pattern of the mass spectrum of 1-bromo-2-methylpropane. Table of the m/e m/z values and formula of the ionised fragments in the mass spectrum of 1-bromo-2-methylpropane. The m/e m/z value of the molecular ion peak in the mass spectrum of 1-bromo-2-methylpropane.  The m/e m/z value of the base ion peak in the mass spectrum of 1-bromo-2-methylpropane. Possible examples of equations showing the formation of the ionised fragments in 1-bromo-2-methylpropane. Revision notes on the mass spectrum of 1-bromo-2-methylpropane. Matching and deducing the structure of the 1-bromo-2-methylpropane molecule from its mass spectrum. Mass spectroscopy of aliphatic halogenoalkanes bromoalkanes alkyl bromides, mass spectra of 1-bromo-2-methylpropane, an isomer of molecular formula C4H9Br


Links associated with 1-bromo-2-methylpropane

The chemistry of HALOGENOALKANES (haloalkanes) revision notes INDEX

The infrared spectrum of 1-bromo-2-methylpropane (isobutyl bromide)

The H-1 NMR spectrum of 1-bromo-2-methylpropane (isobutyl bromide)

The C-13 NMR spectrum of 1-bromo-2-methylpropane (isobutyl bromide)

Mass spectroscopy index

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