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Advanced Organic Chemistry: Mass spectrum of 2-methylbutane

The mass spectrum of 2-methylbutane

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-methylbutane

See also comparing the infrared, mass, 1H NMR and 13C NMR spectra of the 3 alkane isomers of C5H12

mass spectrum of 2-methylbutane fragmentation pattern of m/z m/e ions for analysis and identification of 2-methylbutane image diagram doc brown's advanced organic chemistry revision notes 

2-methylbutane C5H12 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 and naming of alkanes

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

[M]+ is the molecular ion peak (M) with an m/z of 72 corresponding to [C5H12]+, the original 2-methylbutane molecule minus an electron, [(CH3)2CHCH2CH3]+.

The very tiny M+1 peak at m/z 73, corresponds to an ionised 2-methylbutane molecule with one 13C atom in it i.e. an ionised 2-methylbutane molecule of formula 13C12C4H12

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-methylbutane has 5 carbon atoms, so on average, ~1 in 20 molecules will contain a 13C atom.

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

m/z value of [fragment]+ 71 58, with 13C atom 57 56 55 44
[molecular fragment]+ [(CH3)2CCH2CH3]+ [C4H9]+ [C4H9]+ [C4H8]+ [C4H7]+ [C3H8]+
m/z value of [fragment]+ 43 42 41 39 29 27
[molecular fragment]+ [(CH3)2CH]+ [C3H6]+ [C3H5]+ [C3H3]+ [CH3CH2]+ [C2H3]+

Atomic masses: H = 1;  C = 12

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

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

Possible equations to explain the most abundant ion peaks in the mass spectrum of 2-methylbutane

Formation of m/z 71 ion:

[(CH3)2CHCH2CH3]+  ===>  [C5H11]+  +  H

C-H bond scission and proton loss from the parent molecular ion, mass change 72 - 1 = 71.

Formation of m/z 57 ion:

[(CH3)2CHCH2CH3]+  ===>  [(CH3)2CHCH2CH3]+  +  CH3

C-C bond scission in the parent molecular ion, mass change 72 - 15 = 57.

Formation of m/z 43 ion:

[(CH3)2CHCH2CH3]+  ===>  [(CH3)2CH]+  +  CH2CH3

C-C bond scission in the parent molecular ion, mass change 72 - 29 = 43.

The m/z 43 ion is the base peak ion, the most abundant and 'stable' ion fragment, formed by loss of an ethyl group from the parent molecular ion.

The m/z 43 ion can lose a hydrogen atoms/molecule to give the m/z 42, 41 and 39 ions.

Formation of m/z 29 ion:

[(CH3)2CHCH2CH3]+  ===>  [CH2CH3]+  +  C3H7

C-C bond scission from the parent molecular ion, mass change 71 - 43 = 29.

Comparing the infrared, mass, 1H NMR and 13C NMR spectra of the 3 alkane isomers of C5H12

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 pentane, 2-methylbutane and 2,2-dimethylpropane image sizes.

Comparing the infrared spectra of pentane, 2-methylbutane and 2,2-dimethylpropane

Pentane, 2-methylbutane and 2,2-dimethylpropane are structural isomers of molecular formula C5H12

Pentane, 2-methylbutane and 2,2-dimethylpropane exemplify infrared spectra of  the alkane homologous series CnH2n+2  hydrocarbon molecules, where n = 5

INFRARED SPECTRA (above): There are, as expected, differences in the fingerprint region at wavenumbers 1500 to 400 cm-1, but there is no specific infrared absorption band for a functional group. The infrared spectra of pentane and 2-methylbutane seem very similar, but that of 2,2-dimethylpropane seems much simpler.

Comparing the mass spectra of pentane, 2-methylbutane and 2,2-dimethylpropane

Pentane, 2-methylbutane and 2,2-dimethylpropane are structural isomers of molecular formula C5H12

Pentane, 2-methylbutane and 2,2-dimethylpropane exemplify the mass spectra of  the alkane series CnH2n+2  hydrocarbon molecules, where n = 5

MASS SPECTRA (above): All three hydrocarbons show some similarities in their mass spectra e.g. m/z ions 27 to 29 for [C2Hx]+ (x = 2 and 4). The molecular ion peaks will be the same for all three isomers (m/z 72), but it is very tiny for 2,2-dimethypropane. The pattern ratios for m/z 39 to 43 are similar for pentane and 2-methylbutane, but m/z 42 and 43 ions are almost absent from the 2,2-dimethylpropane spectrum. The base peak ion for pentane is m/z 43, but for 2-methylbutane and 2,2-dimethylpropane it is m/z 57.

Comparing the 1H proton NMR spectra of pentane, 2-methylbutane and 2,2-dimethylpropane

Pentane, 2-methylbutane and 2,2-dimethylpropane are structural isomers of molecular formula C5H12

Pentane, 2-methylbutane and 2,2-dimethylpropane exemplify the 1H proton NMR spectra of the alkane homologous series CnH2n+2  hydrocarbon molecules where, n = 5

1H NMR SPECTRA (above): The 1H NMR spectra of all three molecules give different proton ratios for the different 1H chemical environments i.e. pentane's proton ratio is 3:2:1 (from 6:4:2 H's in the molecule). 2-methylbutane's proton ratio is 6:1:2:3 and 2,2-dimethylpropane's doesn't have a proton ratio, all hydrogen atoms are equivalent. This means all three isomeric C5H12 hydrocarbons can be distinguished from their 1H NMR spectra.

Comparing the carbon-13 NMR spectra of pentane, 2-methylbutane and 2,2-dimethylpropane

Pentane, 2-methylbutane and 2,2-dimethylpropane are structural isomers of molecular formula C5H12

Pentane, 2-methylbutane and 2,2-dimethylpropane exemplify the carbon-13 NMR spectra of members of  the alkane homologous series CnH2n+2  hydrocarbon molecules, where n = 5

13C NMR SPECTRA (above): The 13C NMR spectra of the three molecules show different numbers of carbon-13 chemical environments i.e different numbers of 13C NMR resonance lines. So, pentane gives three 13C chemical shifts, 2-methylbutane four and 2,2-dimethylpropane two. This means all three isomeric C5H12 hydrocarbons can be distinguished from their 13C NMR spectra.

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Stick diagram of the relative abundance of ionised fragments in the fingerprint pattern of the mass spectrum of 2-methylbutane. Table of the m/e m/z values and formula of the ionised fragments in the mass spectrum of 2-methylbutane. The m/e m/z value of the molecular ion peak in the mass spectrum of 2-methylbutane.  The m/e m/z value of the base ion peak in the mass spectrum of 2-methylbutane. Possible examples of equations showing the formation of the ionised fragments in 2-methylbutane. Revision notes on the mass spectrum of 2-methylbutane. Matching and deducing the structure of the 2-methylbutane molecule from its mass spectrum. Mass spectroscopy of aliphatic alkanes, mass spectra of 2-methylbutane, an isomer of molecular formula C5H12


Links associated with 2-methylbutane

The chemistry of ALKANES revision notes INDEX

The infrared spectrum for 2-methylbutane

The H-1 NMR spectrum for 2-methylbutane

The C-13 NMR spectrum for 2-methylbutane

Mass spectroscopy index

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