mass spectrum of 2-iodobutane C4H9I CH3CHICH2CH3 fragmentation pattern of m/z m/e ions for analysis and identification of sec-butyl iodide image diagram doc brown's advanced organic chemistry revision notes

Advanced Organic Chemistry: Mass spectrum of 2-iodobutane CH₃CHICH₂CH₃

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Interpreting and explaining the mass spectrum of 2-iodobutane

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

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Introductory note on the mass spectrum of 2-iodobutane

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

If M represents the 2-iodobutane 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-iodobutane.

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

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-iodobutane. 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-iodobutane, but the mass spectrometer software does!

mass spectrum of 2-iodobutane C4H9I CH3CHICH2CH3 fragmentation pattern of m/z m/e ions for analysis and identification of sec-butyl iodide image diagram doc brown's advanced organic chemistry revision notes

2-iodobutane (sec-butyl iodide), C₄H₉I, CH₃-CHI-CH₂-CH₃

The molecular structure and naming of haloalkanes

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

[M]⁺ is the parent molecular ion peak (M) with an m/z of 184 corresponding to [C₄H₉I]⁺, the original 2-iodobutane molecule minus an electron, [CH₃CHICH₂CH₃]⁺

Iodine consists of 100% of the ¹²⁷I isotope, so there are no double peak complexities with 2-iodobutane that you get with the mass spectra of organic chlorine and bromine compounds, where you get M+2 peaks due to two isotopes of the halogen and other double peaks two m/z units apart.

See Mass spectroscopy index

The small M+1 peak at m/z 185, corresponds to an ionised 2-iodobutane molecule with one ¹³C atom in it i.e. an ionised 2-iodobutane molecule of formula [¹³C¹²C₃H₉I]⁺

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.

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

You may find two peaks one unit apart with a height ratio of 25:1 e.g. m/z ions 57/58 and 184/185.

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

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

Note: You don't get the 'twin peak' complications with organo-iodine compounds that you do with organo-chlorine and organo-bromine compounds due to them having two stable isotopes (³⁵Cl, ³⁷Cl, ⁷⁹Br and ⁸¹Br).

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

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

The parent molecular ion of 2-iodobutane m/z 184: [C₄H₉I]⁺ or [CH₃CHICH₂CH₃]⁺

Data table of some of the ions formed in the fragmentation pattern of the mass spectrum of 2-iodobutane

m/z value of [fragment]⁺	185	184	155	128	127	58
[molecular fragment]⁺	[¹³C¹²C₃H₉I]⁺	[C₄H₉I]⁺	[C₂H₄I]⁺	[HI]⁺	[I]⁺	[¹³C¹²C₃H₉]⁺

m/z value of [fragment]⁺	57	55	41	39	29	28	27
[molecular fragment]⁺	[C₄H₉]⁺	[C₄H₇]⁺	[C₃H₅]⁺	[C₃H₃]⁺	[C₂H₅]⁺	[C₂H₄]⁺	[C₂H₃]⁺

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

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); I = 127

Bond enthalpies = kJ/mol: C-C = 348; C-H = 412; C-I = 238

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

Formation of m/z 155 ion:

[CH₃CHICH₂CH₃]⁺ ===> [C₂H₄I]⁺ + C₂H₅

C-C bond scission in the parent molecular ion,

mass change 184 - 29 = 155 (M-29 ion)

Very low probability of formation due to large C-C bond enthalpy compared to the smaller C-I bond enthalpy.

Formation of m/z 127 and 128 ions:

[CH₃CHICH₂CH₃]⁺ ===> [I]⁺ + C₄H₉

C-I bond scission, iodine atom freed and ionised,

mass change 184 - 57 = 127 (M-57 ion peak)

this is the weakest bond in the molecule, but the alkyl fragment is much likely to retain the positive charge (see m/z 57 ion below).

The m/z of 127 is indicative of an iodine compound.

[CH₃CHICH₂CH₃]⁺ ===> [HI]⁺ + C₄H₈

Elimination of a hydrogen iodide molecule,

mass change 184 - 56 = 128 (M-56 ion peak)

Both reactions have a low probability judging from the small abundances - small peaks.

Formation of m/z 57 ion:

[CH₃CHICH₂CH₃]⁺ ===> [C₄H₉]⁺ + I

Scission of the weakest bond in the molecule, breakage of the C-I bond releases an iodine atom and forms the base peak ion.

The most probable bond scission (see bond enthalpies above for 2-bromobutane) and the mass change is 184 - 127 = 57.

The alkyl carbocation is much more likely to carry the positive charge than the iodine atom.

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

The m/z 58 ion is probably, [¹³C¹²C₃H₉]⁺, i.e. the base peak ion containing a ¹³C isotope.

Formation of m/z 56 ion:

[CH₃CHICH₂CH₃]⁺ ===> [C₄H₈]⁺ + HI

Elimination of hydrogen bromide molecule from the parent molecular ion, mass change 184 - 128 = 56.

Formation of m/z 55 ion:

[C₄H₉]⁺ ===> [C₄H₇]⁺ + H₂

Elimination of a hydrogen molecule from the m/z 57 ion.

Loss of a H atom from the same ion can also form the m/z 55 ion.

Low probability for these fragmentation reactions due to the high C-H bond enthalpy.

Formation of m/z 41 and 39 ions:

Possible reactions include:

m/z 41: [C₄H₈]⁺ ===> [C₃H₅]⁺ + CH₃

C-C bond scission of fragment ion.

m/z 39: [C₃H₅]⁺ ===> [C₃H₃]⁺ + H₂

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

Possible reactions include:

m/z 29: [CH₃CHICH₂CH₃I]⁺ ===> [CH₃CH₂]⁺ + CH₂CH₂I

From bond scission in the parent molecular ion (above) or C-C bond scission of the fragmentation ions.

m/z 27: [C₄H₈]⁺ ===> [C₂H₃]⁺ + C₂H₅

m/z 28: [C₄H₈]⁺ ===> [C₂H₄]⁺ + C₂H₄

m/z 28: [C₄H₉]⁺ ===> [C₂H₄]⁺ + C₂H₅

m/z 29: [C₄H₈]⁺ ===> [C₂H₅]⁺ + C₂H₃

m/z 29: [C₄H₉]⁺ ===> [C₂H₅]⁺ + C₂H₄

There are many possibilities including proton loss from the m/z 29 ion to give m/z ions 26, 27 and 28

Comparing the infrared, mass, 1H NMR and 13C NMR spectra of the 4 halogenoalkane isomers of C₄H₉I 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-iodobutane, 2-iodobutane, 1-iodo-2-methylpropane and 2-iodo-2-methylpropane image sizes. These four molecules are structural isomers of molecular formula C₄H₉I and exemplify the infrared, mass, 1H NMR and 13C NMR spectra of lower aliphatic halogenoalkanes (haloalkanes, alkyl halides, iodoalkanes, alkyl iodides).


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 ion of m/z 184, but it is only a relatively tiny peak for 2-iodo-2-methylpropane. All four give the base ion peak of m/z 57. All four give prominent peaks for m/z ions 29 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.


1H NMR SPECTRA (above): The 1H NMR spectra of all three molecules give different proton ratios i.e.1-iodobutane four peaks 3:2:2:2, 2-iodobutane four peaks 3:3:2:1, 1-iodo-2-methylpropane three peaks 6:2:1 and 2-iodo-2-methylpropane 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-iodobutane and 2-iodobutane show four 13C NMR resonances, 1-iodo-2-methylpropane three 13C NMR resonances and 2-iodo-2-methylpropane only two 13C resonances. Therefore 1-iodo-2-methylpropane and 2-iodo-2-methylpropane can be distinguished from the other three by their number of resonances in their 13C NMR spectra, but 1-iodobutane and 2-iodobutane cannot be distinguished from each other from their number of 13C NMR resonance lines - other data would be required.

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The chemistry of HALOGENOALKANES (haloalkanes) revision notes INDEX

The infrared spectrum of 2-iodobutane (sec-butyl iodide)

The H-1 NMR spectrum of 2-iodobutane (sec-butyl iodide)

The C-13 NMR spectrum of 2-iodobutane (sec-butyl iodide)

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