mass spectrum of 2-bromobutane C4H9Br CH3CHBrCH2CH3 fragmentation pattern of m/z m/e ions for analysis and identification of sec-butyl bromide image diagram doc brown's advanced organic chemistry revision notes

Advanced Organic Chemistry: Mass spectrum of 2-bromobutane CH₃CH₂CHBrCH₃

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Interpreting the mass spectrum of 2-bromobutane (sec-butyl bromide)

[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 2-bromobutane (mass spectra) [spectra page updated April 1st 2026 *]

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Introductory note on the mass spectrum of 2-bromobutane

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

If M represents the 2-bromobutane 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 that might be formed in the fragmentation pattern for the mass spectrum of 2-bromobutane.

I've included a stick diagram and table of m/z ions for the mass spectrum of 2-bromobutane 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 2-bromobutane.

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 2-bromobutane. 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 2-bromobutane, but the mass spectrometer software does!

mass spectrum of 2-bromobutane C4H9Br CH3CHBrCH2CH3 fragmentation pattern of m/z m/e ions for analysis and identification of sec-butyl bromide image diagram doc brown's advanced organic chemistry revision notes

2-bromobutane, C₄H₉Br, CH₃CHBrCH₂CH₃, CH₃-CH(Br)-CH₂-CH₃

The molecular structure and naming of haloalkanes

Secondary haloalkane/halogenoalkane and old names: sec-butyl bromide, sec-butyl bromide (secondary alkyl halide)

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

[M]⁺ is the parent molecular ion peaks (M and M+2) have an m/z of 136 and 138 corresponding to [C₄H₉Br]⁺, the original 2-bromobutane molecule minus an electron, [CH₃CHBrCH₂CH₃]⁺

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

Therefore the molecular ion can be [CH₃CH⁷⁹BrCH₂CH₃]⁺ or [CH₃CH⁸¹BrCH₂CH₃]⁺, which should, and does, show up as a double tiny peak of ~equal heights (~equal abundance, intensity).

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 2-bromobutane.

The two bromine isotopes also account for the tiny 'twin peaks' of m/z ions 107 and 109 (details discussed later).

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 107 and 109 and the tiny molecular ion peaks of m/z values 136 and 138 (but still of similar height!) in the mass spectrum of 2-bromobutane.

The actual isotopic composition for naturally occurring bromine is ⁷⁹Br (50.7%) and ⁸¹Br (49.3%), which is why the molecular/fragment ions with the heavier bromine-81 isotope have slightly less intensity peaks - see the small peaks for m/z ions 107 and 109.

Theoretically there are very tiny M+1 and M+3 peaks at m/z 137 and 139, corresponds to an ionised 2-bromobutane molecule with one ¹³C atom in it i.e. an ionised 2-bromobutane molecule of formula [¹³C¹²C₃H₉Br]⁺

However, they are barely observed because the parent molecular ions are highly unstable with very low abundances i.e. very small peaks at m/z values 136 and 138.

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

However, fragments containing a ¹³C atom are observed e.g. the m/z 58 ion [¹³C¹²C₃H₉]⁺

The most abundant ion of the molecule under mass spectrometry investigation (2-bromobutane) 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 2-bromobutane is m/z 57 [C₄H₉]⁺

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of 2-bromobutane.

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

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

The parent molecular ions of 2-bromobutane m/z 136: [C₄H₉⁷⁹Br]⁺ & m/z 138 [C₄H₉⁸¹Br]⁺

m/z value	138	136	109	107	58, with ¹³C atom	57, all ¹²C atoms	56	55
[frag.]⁺	[C₄H₉⁸¹Br]⁺	[C₄H₉⁷⁹Br]⁺	[C₂H₄⁸¹Br]⁺	[C₂H₄⁷⁹Br]⁺	[¹³C¹²C₃H₉]⁺	[C₄H₉]⁺	[C₄H₈]⁺	[C₄H₇]⁺

m/z value of [fragment]⁺	79	80	81	82	m/z ions 79 to 82 have a very low abundance, just tiny peaks in the mass spectrum of 2-bromobutane.
[molecular fragment]⁺	[⁷⁹Br]⁺	[H⁷⁹Br]⁺	[⁸¹Br]⁺	[H⁸¹Br]⁺

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 2-bromobutane

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 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 2-bromobutane (tabulated above)

Formation of m/z 107 and 109 ion:

[CH₃CHBrCH₂CH₃]⁺ ===> [C₂H₄Br]⁺ + C₂H₅

C-C bond scission in the parent molecular ion and proton rearrangement, mass change 136/138 - 29 = 107/109.

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).

The C-Br bond is the weakest bond in the molecule, hence the most likely bond scission, but the alternative ionisation (below) is more likely with 2-bromobutane, since only tiny traces of these ions are observed.

Formation of m/z 79 to 82 ions:

These are mentioned in conjunction with the formation of the m/z 57 and 56 ions (see table of ions above and fragmentation equations below for m/z 56 and 57 ions).

There are tiny peaks for [⁷⁹Br]⁺, [H⁷⁹Br]⁺, [⁸¹Br]+ and [H⁸¹Br]⁺

The positive charge is much more likely to be carried by alkyl fragment ions.

Formation of m/z 56 and 57 ions:

[CH₃CHBrCH₂CH₃]⁺ ===> [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 protons to give m/z ions 56 and 55.

There is a 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.

There is also a real, but low, possibility of elimination of hydrogen bromide from the parent molecular ions to also give the m/z 56 ion.

[C₄H₉Br]⁺ ===> [C₄H₈]⁺ + HBr

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

Therefore, as above for bromine atoms, there is a low probability that the hydrogen bromide molecules can also be ionised to give m/z 80 and 82 ions - you can just about make out the tiny twin peaks.

Note the m/z 58 ion is NOT C₄H₁₀

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:

[CH₃CHBrCH₂CH₃]⁺ ===> [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.

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Links associated with 2-bromobutane

The chemistry of HALOGENOALKANES (haloalkanes) revision notes INDEX

The infrared spectrum of 2-bromobutane (sec-butyl bromide)

The H-1 NMR spectrum of 2-bromobutane (sec-butyl bromide)

The C-13 NMR spectrum of 2-bromobutane (sec-butyl bromide)

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