Interpreting & explaining the mass spectrum of Pentane

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

Introductory note on the mass spectrum of pentane

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

If M represents the pentane molecule, the initial ionisation to give the molecular ion is:

M_(g) + high KE e^-  ==> [M•]⁺_(g) + 2e^- and for 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 pentane and only the formation of singly charged positive are considered for the mass spectrum of pentane.

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

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 the accurate ion masses, 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 pentane, but the mass spectrometer software does!

Pentane  C₅H₁₂,  , ,  an alkane

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.

The most abundant ion of the molecule under mass spectrometry investigation (pentane) 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 pentane is the m/z 43 ion [C₃H₇]⁺

Some of the possible positive ions, [molecular fragment]⁺, formed in the mass spectrometry of pentane based on the parent molecular ion, m/z 74  [C₅H₁₂]⁺ or [CH₃CH₂CH₂CH₂CH₃]⁺

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

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of pentane

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)

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

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

C-C bond scission and loss of end methyl group.

Mass change 72 - 15 = 57 (M-15 ion peak)

Formation of m/z ion 55

[C₄H₉]⁺  ===>  [C₄H₇]⁺  +  CH₃

Elimination of hydrogen from the m/z 57 ion

Mass change 57 - 2 = 55

Formation of m/z ion 43

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

Mass change 72 - 29 = 43 (M-29 ion peak)

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 to give the m/z 42, 41 and 39 ions.

Note that an accurate mass spectrometer can sort out (resolve) pairs of ions with the same integer m/z value because they can measure relative fragment ion masses to four decimal places,

e.g. using accurate relative isotopic masses:

¹H = 1.0078  ¹²C = 12.0000   ¹³C = 13.0034: you can then calculate (predict) that the accurate relative ion masses are:

For m/z 44: [C₃H₈]⁺ = 44.0624  and [¹³C¹²C₂H₇]⁺ = 44.0580, a difference of 0.0044 in relative ion mass.

Formation of m/z ion 29

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

C-C bond scission of the parent molecular ion.

Mass change 72 - 43 = 29 (M-43 ion peak)

The m/z 29 ion can lose hydrogen atoms to give the m/z 27 and 28 ions.

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.