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

[M]⁺ is the molecular ion peak (M) with an m/z of 86 corresponding to [C₆H₁₄]⁺, the original 2-methylpentane molecule minus an electron, [(CH₃)₂CHCH₂CH₂CH₃]⁺.

The very tiny M+1 peak at m/z 87, corresponds to an ionised 2-methylpentane molecule with one ¹³C atom in it i.e. an ionised 2-methylpentane molecule of formula ¹³C¹²C₅H₁₄

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-methylpentane has 6 carbon atoms, so on average, ~1 in 17 molecules of will contain a ¹³C atom.

A similar argument applies to fragment ions from the breakdown of the parent molecular ion of 2-methylpentane - though the ratio will be greater.

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

The most abundant ion of the molecule under mass spectrometry investigation is usually given an arbitrary abundance value of 100, called the base ion peak, and all other abundances ('intensities') are measured against it.

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

Atomic masses: H = 1;  C = 12 (13 for ~1 in 100)

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

Examples of possible equations to explain some of the most abundant ion peaks in the mass spectrum of 2-methylpentane

Formation of m/z 71 ion:

[(CH₃)₂CHCH₂CH₂CH₃]⁺  ===>  [C₅H₁₁]⁺  +  CH₃

C-C bond scission in the parent molecular ion, leading to loss of a methyl group from the parent molecular ion (mass change = 86 - 15 = 71)

There are several possible structures of the [C₅H₁₁]⁺ ion.

Formation of m/z 57 ion:

[(CH₃)₂CHCH₂CH₂CH₃]⁺  ===>  [(CH₃)₂CHCH₂]⁺  +  CH₂CH₃

C-C bond scission in the parent molecular ion, leading to loss of a ethyl group from the parent molecular ion (mass change = 86 - 29 = 57)

Formation of m/z 43 ion:

[(CH₃)₂CHCH₂CH₂CH₃]⁺  ===>  [(CH₃)₂CH]⁺  +  CH₂CH₂CH₃

C-C bond scission in the parent molecular ion, mass change = 86 - 43 = 43, this secondary carbocation is more stable than [ CH₂CH₂CH₃]⁺, but this ionisation, although less likely, also forms the less stable propyl carbocation.

[(CH₃)₂CHCH₂CH₂CH₃]⁺  ===>  [CH₂CH₂CH₃]⁺  +  (CH₃)₂CH

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

Formation of m/z 29 ion:

[(CH₃)₂CHCH₂CH₂CH₃]⁺  ===>  [CH₂CH₃]⁺  +  (CH₃)₂CHCH₂

Like the equations above, its another example of chain scission of the parent molecular ion.

The smaller fragments like the ethyl ion, can also be formed by chain scission of bigger fragments, but still smaller than the parent molecular ion of 2-methypentane.

Sequences including m/z values of 42, 41, 40, 39 or 28, 28, 27, 26, indicate successive hydrogen atom/molecule loss from the m/z 43 or 29 ions.

Comparing the infrared, mass, 1H NMR and 13C NMR spectra of the five structural alkane isomers of C6H14 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 hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane and 2,3-dimethylbutane image sizes.  These five molecules are structural isomers of saturated alkanes of molecular formula C6H14 and exemplify the infrared, mass, 1H NMR and 13C NMR spectra of lower aliphatic alkanes (non-cyclic alkanes).
Infrared spectra below.
	INFRARED SPECTRA: 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. All the absorption bands are typical of molecules containing saturated alkyl structure and there are no characteristic infrared absorptions due to a specific functional group.
Infrared spectra above, mass spectra below.
	MASS SPECTRA: Base ion peaks plus m/z comments. Hexane: m/z 57, 42 and 56 prominent 2-methylpentane: m/z 43, 42 and 71 prominent 3-methylpentane: m/z 57, 41 and 56 prominent 2,2-dimethylbutane: m/z 43, 41, 57 and 71 prominent 2,3-dimethylbutane: m/z 43, 41, 42 and 71 prominent
Mass spectra above, 1H NMR spectra below.
	1H NMR SPECTRA: They can all be distinguished by their different integrated proton ratios - need very high resolution. Hexane: 3 1H δ shifts, H ratio 3:2:2 (6:4:4 in formula) 2-methylpentane: 5 1H δ shifts, H ratio 6:3:2:2:1 3-methylpentane: 4 1H δ shifts, H ratio 6:4:3:1 2,2-dimethylbutane: 3 1H δ shifts, H ratio 9:3:2 2,3-dimethylbutane: 2 1H δ shifts, H ratio 6:1 (12:2 in formula)
1H NMR spectra above, 13C NMR spectra below.
	13C NMR SPECTRA: From the number of shifts, you can't distinguish (iii) and (iv) but you can distinguish them from (i), (ii) and (v). (i) Hexane: 3 13C δ shifts (ii) 2-methylpentane: 5 13C δ shifts (iii) 3-methylpentane: 4 13C δ shifts (iv) 2,2-dimethylbutane: 4 13C δ shifts (v) 2,3-dimethylbutane: 2 13C δ shifts
13C NMR spectra above.

Key words & phrases:: isohexane image diagram on how to interpret and explain the mass spectrum of 2-methylpentane m/z m/e base peaks, image and diagram of the mass spectrum of 2-methylpentane, details of the mass spectroscopy of 2-methylpentane,  low and high resolution mass spectrum of 2-methylpentane, prominent m/z peaks in the mass spectrum of 2-methylpentane, comparative mass spectra of 2-methylpentane, the molecular ion peak in the mass spectrum of 2-methylpentane, analysing and understanding the fragmentation pattern of the mass spectrum of 2-methylpentane, characteristic pattern of peaks in the mass spectrum of 2-methylpentane, relative abundance of mass ion peaks in the mass spectrum of 2-methylpentane, revising the mass spectrum of 2-methylpentane, revision of mass spectroscopy of 2-methylpentane, most abundant ions in the mass spectrum of 2-methylpentane, how to construct the mass spectrum diagram for abundance of fragmentation ions in the mass spectrum of 2-methylpentane, how to analyse the mass spectrum of 2-methylpentane, how to describe explain the formation of fragmented ions in the mass spectra of 2-methylpentane equations for explaining the formation of the positive ions in the fragmentation of the ionised molecule of 2-methylpentane recognising the base ion peak of 2-methylpentane interpreting interpretation the mass spectrum of 2-methylpentane: isohexane Stick diagram of the relative abundance of ionised fragments in the fingerprint pattern of the mass spectrum of 2-methylpentane. Table of the m/e m/z values and formula of the ionised fragments in the mass spectrum of 2-methylpentane. The m/e m/z value of the molecular ion peak in the mass spectrum of 2-methylpentane.  The m/e m/z value of the base ion peak in the mass spectrum of 2-methylpentane. Possible examples of equations showing the formation of the ionised fragments in 2-methylpentane. Revision notes on the mass spectrum of 2-methylpentane. Matching and deducing the structure of the 2-methylpentane molecule from its mass spectrum. Mass spectroscopy of  aliphatic alkanes, mass spectra of 2-methylpentane, an isomer of molecular formula C6H14

---

The chemistry of ALKANES revision notes INDEX

The infrared spectrum of 2-methylpentane

The H-1 NMR spectrum of 2-methylpentane

The C-13 NMR spectrum of 2-methylpentane

Mass spectroscopy index

ALL SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

Use My Google search site box

Email doc b: chem55555@hotmail.com