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Advanced Organic Chemistry: Mass spectrum of 2-methylpropane

Interpreting the mass spectrum of 2-methylpropane (isobutane)

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-methylpropane

See also comparing the IR, mass, 1H NMR and 13C NMR spectra of 2-methylpropane and butane

mass spectrum of 2-methylpropane C4H10 (CH3)3CH fragmentation pattern of m/z m/e ions for analysis and identification of isobutane image diagram doc brown's advanced organic chemistry revision notes 

2-methylpropane  C4H10  alkanes structure and naming (c) doc b   alkanes structure and naming (c) doc b   alkanes structure and naming (c) doc b 

For more see The molecular structure, classification and naming of alkanes

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

[M]+ is the molecular ion peak (M) with an m/z of 58 corresponding to [C4H10]+, the original 2-methylpropane molecule minus an electron, [(CH3)3CH]+.

You might see a tiny M+1 peak at m/z 59, corresponds to an ionised methylpropane molecule with one 13C atom in it i.e. an ionised butane molecule of formula [13C12C3H10]+

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.

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

Not clear with the molecular ion peak for 2-methylpropane, but check out the ratio of the relative abundances of the m/z 43 and 44 fragment ions (see the mass spectrum diagram above and table of fragment ions below).

The most abundant ion of the molecule under mass spectrometry investigation (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 (apart from M) in the fragmentation pattern of 2-methylpropane.

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

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

The molecular ion peak is from the m/z 58 ion: [(CH3)3CH]+.

m/z value of [fragment]+ 57 44 43 42 41 39 29 28 27 15
[molecular fragment]+ [C4H9]+ [13C12C2H7]+ [C3H7]+ [C3H6]+ [C3H5]+ [C3H3]+ [C2H5]+ [C2H4]+ [C2H3]+ [CH3]+

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

Atomic masses: H = 1; C = 12;

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

Possible equations to explain some of the most abundant ion peaks in the mass spectrum of 2-methylpropane (tabulated above)

For branched alkanes, C-C bond scission of the parent molecule tends to occur at the point of branching (automatically the case with methylpropane) and note that the C-C bond is weaker than the C-H bond.

Formation of m/z 43 ion:

[(CH3)3CH]+  ===>  [(CH3)2CH]+  +  CH3

C-C bond scission, loss of methyl group, mass change 58 - 15 = 43.

This is a secondary carbocation, stabilised by the +I effect of the methyl groups.

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

The m/z 44 ion peak for 2-methylpropane is probably [13C12C2H7]+ and should be about 1/25th of the m/z 43 ion peak (see note on isotopes in the introduction above).

Formation of m/z 29 ion:

[(CH3)3CH]+  ===>  [C2H5]+  +  C2H5

C-C bond scission in the parent molecular ion, proton rearrangement, mass change 58 - 29 = 29.

Or from C-C bond scission in the m/z 43 ion and proton rearrangement.

[(CH3)2CH]+   ===>  [ C2H5]+  +  CH2

Formation of m/z 28 ion:

[C3H7]+  ===>  [C2H4]+  +  CH3

C-C bond scission of the m/z 43 base peak ion, loss of methyl group.

Formation of m/z 27 ion:

[C2H5]+  ===>  [C2H3]+  +  H2

Elimination of a hydrogen molecule from the m/z 29 ion, mass change 29 - 2 = 27.

Formation of m/z 15 ion:

[(CH3)3CH]+  ===>  [CH3]+  +  CH3CH2CH2

C-C bond scission, mass change 58 - 43 = 15, but this time the methyl group carries the positive charge.

See also comparing the IR, mass, 1H NMR and 13C NMR spectra of 2-methylpropane and butane

Comparing the infrared, mass, 1H NMR and 13C NMR spectra of the 2 alkane isomers of C4H10

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 butane and 2-methylpropane image sizes.

The infrared spectra of butane and 2-methyl propane are quite similar, mainly due to C-H stretching and deformation vibrations, but you can see significant differences in the fingerprint region at wavenumbers 1500 to 600 cm-1.

The mass spectra of butane and 2-methyl propane are quite similar and both have a base ion peak of m/z 43 [C3H7]+, but here significant differences in the ratios of the m/z ions 27 to 29 [C2H3,4,5]+. The mass spectrum of 2-methylpropane has much lower relative abundances of the m/z 28 and 29 ions.

The 1H NMR spectra of butane and 2-methyl propane are quite similar in that both show the 8 hydrogen atoms exist in only 2 different chemical environment. However, they can be distinguished from each other by the different integrated proton ratios. Butane gives a (2) : (3) proton ratio and 2-methylbutane a (1) : (9) proton ratio.

The 13C NMR spectra of butane and 2-methyl propane are quite similar in that both show the 4 carbon atoms exist in only 2 different chemical environments.

Key words & phrases: C4H10 (CH3)3CH image diagram on how to interpret and explain the mass spectrum of 2-methylpropane m/z m/e base peaks, image and diagram of the mass spectrum of 2-methylpropane, details of the mass spectroscopy of 2-methylpropane,  low and high resolution mass spectrum of 2-methylpropane, prominent m/z peaks in the mass spectrum of 2-methylpropane, comparative mass spectra of 2-methylpropane, the molecular ion peak in the mass spectrum of 2-methylpropane, analysing and understanding the fragmentation pattern of the mass spectrum of 2-methylpropane, characteristic pattern of peaks in the mass spectrum of 2-methylpropane, relative abundance of mass ion peaks in the mass spectrum of 2-methylpropane, revising the mass spectrum of 2-methylpropane, revision of mass spectroscopy of 2-methylpropane, most abundant ions in the mass spectrum of 2-methylpropane, how to construct the mass spectrum diagram for abundance of fragmentation ions in the mass spectrum of 2-methylpropane, how to analyse the mass spectrum of 2-methylpropane, how to describe explain the formation of fragmented ions in the mass spectra of 2-methylpropane equations for explaining the formation of the positive ions in the fragmentation of the ionised molecule of 2-methylpropane recognising the base ion peak of 2-methylpropane interpreting interpretation the mass spectrum of 2-methylpropane isobutane methylpropane CH(CH3)3

Stick diagram of the relative abundance of ionised fragments in the fingerprint pattern of the mass spectrum of 2-methylpropane. Table of the m/e m/z values and formula of the ionised fragments in the mass spectrum of 2-methylpropane. The m/e m/z value of the molecular ion peak in the mass spectrum of 2-methylpropane.  The m/e m/z value of the base ion peak in the mass spectrum of 2-methylpropane. Possible examples of equations showing the formation of the ionised fragments in 2-methylpropane. Revision notes on the mass spectrum of 2-methylpropane. Matching and deducing the structure of the 2-methylpropane molecule from its mass spectrum. Mass spectroscopy of  aliphatic alkanes, mass spectra of 2-methylpropane, a structural isomer of molecular formula C4H10


Links associated with 2-methylpropane

The chemistry of ALKANES revision notes INDEX

The infrared spectrum of 2-methylpropane (methylpropane, isobutane)

The H-1 NMR spectrum of 2-methylpropane (methylpropane, isobutane)

The C-13 NMR spectrum of 2-methylpropane (methylpropane, isobutane)

Mass spectroscopy index

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