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

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

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Interpreting explaining the mass spectrum of 1-bromo-2-methylpropane (CH₃)₂CHCH₂Br

[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: Molecular spectroscopy analysis of 1-bromo-2-methylpropane (mass spectra) [spectra page updated Mar 30th 2026 *]

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Introductory note on the mass spectrum of 1-bromo-2-methylpropane

Students and teachers please note my explanation of the mass spectrum of 1-bromo-2-methylpropane is designed for advanced, but pre-university, chemistry courses.

If M represents the 1-bromo-2-methylpropane molecule, the initial ionisation to give the molecular ion is:

M_(g) + high KE e^- ==> [M•]⁺_(g) + 2e^- and 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 are formed in the fragmentation pattern for 1-bromo-2-methylpropane.

I've included stick diagram and table of m/z ions for the mass spectrum of 1-bromo-2-methylpropane and conducting the mass spectrum analysis under standard conditions, a database can be built up based complex fingerprint patterns, often involving relative intensities of many fragment ions, that can be used to identify compounds including 1-bromo-2-methylpropane.

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 and compared the accurate ion masses if appropriate for 1-bromo-2-methylpropane. 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-bromo-2-methylpropane, but the mass spectrometer software does!

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

The molecular structure and naming of haloalkanes

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 has two isotopes, ⁷⁹Br and ⁸¹Br in the ratio ~1 : 1.

Therefore the molecular ion can be [(CH₃)₂CHCH₂⁷⁹Br]⁺ or [(CH₃)₂CHCH₂⁸¹Br]⁺, which should, and do, 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 (details in the analysis below).

% isotopic composition of isotopes in the naturally occurring element:

⁷⁹Br = 78.9183 (50.7%) and ⁸¹Br = 80.9163 (49.3%)

The peak for molecular/fragment ions with the heavier, slightly less abundant ⁸¹Br isotope, are slight shorter (slightly smaller intensity).

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.

The base ion peak for the mass spectrum of 1-bromo-2-methylpropane is m/z ion 57 [C₄H₉]⁺

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)₂CHCH₂⁷⁹Br]⁺

but the M+2 m/z 138 molecular ion [(CH₃)₂CHCH₂⁸¹Br]⁺ will also fragment in the same way.

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 (~1% 13); Br = 79 or 81 (~1:1 isotope abundance ratio)

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 (M-15 'twin' ion peaks)

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.

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 93 and 95 ions:

[(CH₃)₂CHCH₂Br]⁺ ===> [CH₂Br]⁺ + C₃H₇

C-C bond scission in the parent molecular ion,

mass change 136/138 - 43 = 93/95 (M-43 'twin' ion peaks)

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).

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

Again, the double RBr peaks of roughly 1 : 1 ratio are characteristic of organo-bromine compounds (one m/z ion peak is slightly shorter than the other, technically 50.6 : 49.4).

Formation of m/z ions of mass 79 to 82

There are a set of tiny peaks associated with the bromine atoms of the parent molecule, and note there are only two stable isotopes of bromine ⁷⁹Br and ⁸¹Br (50% abundance for each, giving a ratio of 1:1).

[R-Br]⁺ ===> [R] + ⁷⁹Br⁺ or ⁸¹Br⁺

These two ions of mass 79 and 81 (ratio 1:1), derived from the ionised molecule/fragment are very tiny, but characteristic peaks of the two isotopes of bromine, observed in the mass spectrum of any organo-bromine compound like 1-bromo-2-methylpropane.

(Note that R could represent the rest of the original 1-bromo-2-methylpropane molecule or any smaller fragment retaining a bromine atom).

There are two other tiny peaks, but a bit larger than those for the ionised bromine atoms.

They have m/z values of 80 and 82, that is 1 more mass unit than the ionised bromine atoms.

These two peaks can arise from the formation of hydrogen bromide (HBr) by an elimination reaction.

[(CH₃)₂CHCH₂Br]⁺ ===> [H⁷⁹Br] or [H⁸¹Br] + (CH₃)₂CH==CH₂

Again, note there are two peaks expected in the expected 1:1 ratio from the mass spectrum of 1-bromo-2-methylpropane.

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

You can also get m/z ions 26 to 28 from proton loss from the m/z 29 ion.

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 Explanatory diagram of the mass spectrum of the 1-bromo-2-methylpropane molecule in terms of its molecular structure. Listing data of the prominent main peaks in the mass spectrum of 1-bromo-2-methylpropane. How to explain the mass spectrum of 1-bromo-2-methylpropane. The m/z value of the molecular ion peak in the mass spectrum of 1-bromo-2-methylpropane. Identifying 1-bromo-2-methylpropane from its mass spectrum pattern. The m/z m/e peak analysis of the mass spectrum of the 1-bromo-2-methylpropane molecule. The uses of the mass spectrum of the 1-bromo-2-methylpropane molecule. The distinctive features of the mass spectrum of the 1-bromo-2-methylpropane molecule explained. explaining the fragmentation pattern of the mass spectrum of 1-bromo-2-methylpropane equations showing the formation of the ionised fragments in the mass spectrum of 1-bromo-2-methylpropane what does the mass spectrum tell you about the structure and properties of the 1-bromo-2-methylpropane molecule?(CH3)2CHCH2Br How do you interpret the mass spectrum of 1-bromo-2-methylpropane How to interpret the mass spectrum of 1-bromo-2-methylpropane Explanatory diagram of the mass spectrum of the 1-bromo-2-methylpropane molecule in terms of its molecular structure. Table listing data of the m/z ion prominent main peaks in the mass spectrum of 1-bromo-2-methylpropane. How to explain the mass spectrum of 1-bromo-2-methylpropane. The m/z value of the molecular ion peak in the mass spectrum of 1-bromo-2-methylpropane. Identifying 1-bromo-2-methylpropane from its mass spectrum pattern. The m/z m/e peak analysis interpretation diagram of the mass spectrum of the 1-bromo-2-methylpropane molecule. The uses of the mass spectrum of the 1-bromo-2-methylpropane molecule. The distinctive features of the mass spectrum of the 1-bromo-2-methylpropane molecule explained. explaining the fragmentation pattern of the mass spectrum of 1-bromo-2-methylpropane equations showing the formation of the ionised fragments in the mass spectrum of 1-bromo-2-methylpropane what does the mass spectrum tell you about the structure and properties of the 1-bromo-2-methylpropane molecule? Data table of ionised fragments in the mass spectrum of 1-bromo-2-methylpropane and equations for their formation in the fragmentation of the ionised 1-bromo-2-methylpropane molecule.

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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)

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