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Advanced Organic Chemistry: Mass spectrum of Pentane

Mass spectrum of Pentane

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

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

image diagram mass spectrum of pentane fragmentation pattern of ions for analysis and identification of pentane doc brown's advanced organic chemistry revision notes 

Pentane  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 mass spectrum of pentane

[M]+ is the molecular ion peak (M), with an m/z of 72 corresponding to [C5H12]+, the original molecule minus an electron, [CH3CH2CH2CH2CH3]+.

The small M+1 peak at m/z 73, corresponds to an ionised pentane molecule with one 13C atom in it i.e. an ionised 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.

Pentane 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 pentane.

m/z value of [fragment]+ 57 43 42 41 39 29 27
molecular fragment CH3CH2CH2CH2 CH3CH2CH2 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 pentane

Possible equations to explain the most abundant ion peaks in the mass spectrum of pentane

C-C bond scission to split the linear carbon chain of the molecular ion of pentane

Formation of m/z ion 57

[CH3CH2CH2CH2CH3]+  ===>  [CH3CH2CH2CH2]+  +  CH3

Mass change 72 - 15 = 57.

Formation of m/z ion 43

[CH3CH2CH2CH2CH3]+  ===>  [CH3CH2CH2]+  +  CH3CH2

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 ion 29

[CH3CH2CH2CH2CH3]+  ===>  [CH3CH2]+  +  CH3CH2CH2

Mass change 72 - 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.

Key words & phrases: how to interpret and explain the mass spectrum of pentane,  image and diagram of the mass spectrum of pentane, details of the mass spectroscopy of pentane,  low and high resolution mass spectrum of pentane, prominent m/z peaks in the mass spectrum of pentane, comparative mass spectra of pentane, the molecular ion peak in the mass spectrum of pentane, analysing and understanding the fragmentation pattern of the mass spectrum of pentane, characteristic pattern of peaks in the mass spectrum of pentane, relative abundance of mass ion peaks in the mass spectrum of pentane, revising the mass spectrum of pentane, revision of mass spectroscopy of pentane, most abundant ions in the mass spectrum of pentane, how to construct the mass spectrum diagram for abundance of fragmentation ions in the mass spectrum of pentane

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


Associated links

The chemistry of ALKANES revision notes INDEX

The infrared spectrum of pentane

The H-1 NMR spectrum of pentane

The C-13 NMR spectrum of Pentane

Mass spectroscopy index

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