[Author ©  Dr WP Brown PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses & US K12 grade 11, grade 12 and AP honors chemistry courses: Molecular spectroscopy - analysing the mass spectrum of 2-ethylpentane [updated Nov 4th 2025]

3-ethylpentane CH(CH₂CH₃)₃ 

Interpreting the fragmentation pattern of the mass spectrum of 3-ethylpentane

[M]⁺ is the molecular ion peak (M) with an m/z of 100 corresponding to [C₇H₁₆]⁺, the original 3-ethylpentane molecule minus an electron, [(CH₃CH₂)₃CH]⁺.

The very tiny M+1 peak at m/z 101, corresponds to an ionised 3-ethylpentane molecule with one ¹³C atom in it i.e. an ionised 3-ethylpentane molecule of formula [¹³C¹²C₆H₁₆]⁺

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.

3-ethylpentane has 7 carbon atoms, so on average, ~1 in 14 molecules will contain a ¹³C atom.

This sort of argument also applies to fragment ions from the parent molecular ion of 3-ethylpentane - though the ratio will be greater:

e.g. m/z 44 ion could be [¹³C¹²C₂H₇]⁺or m/z 72 ion [¹³C¹²C₄H₁₁]⁺

These ions might be more likely than those containing only ¹²C isotope atoms

i.e. [C₃H₈]⁺or [C₅H₁₂]⁺

Either way, for identification purposes, all these peaks add uniqueness to the fragmentation pattern of the mass spectrum of 3-ethylpentane

The most abundant ion of the molecule under mass spectrometry investigation (3-ethylpentane) 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 3-ethylpentane is m/z ion 43 [C₃H₇]⁺

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of 3-ethylpentane.

Unless otherwise indicated, assume the carbon atoms in heptane are the ¹²C isotope.

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

The parent molecular ion of 3-ethylpentane m/z 100: [C₇H₁₆]⁺

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of 3-ethylpentane.

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

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.

Examples of equations to explain some of the most abundant ion peaks in the mass spectrum of heptane

Atomic masses: H = 1;  C = 12 (~1% 13)

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

Formation of m/z 71 and 72 ions:

[(CH₃CH₂)₃CH]⁺  ===>  [C₅H₁₁]⁺  +  CH₂CH₃

Here an ethyl group is broken off by C-C bond scission in the parent molecular ion,

mass change = 100 - 29 = 71 (M-29 ion peak)

Most fragmentation in alkanes arises from C-C bond scission (C-C bond weaker than C-H).

Note in the equations below that both fragments are capable of being ionised, but only one at a time.

Many other fragments are formed by hydrogen atom/molecule loss so you get sequences like 71 => 70, 57 => 56 => 55, 43 => 42 => 41 => 40 => 39 and 29 => 28 => 27 etc.

Ethene is often eliminated to give a smaller fragment e.g. the m/z ion 71 gives the m/z ion 43 ion.

[C₅H₁₁]⁺  ===>  [C₃H₇]⁺  +  C₂H₄

Mass change = 71 - 28 = 43.

Note that the m/z 72 ion can be formed in the same way as the m/z 71 ion, but containing a ¹³C carbon isotope atom i.e. [¹³C¹²C₄H₁₁]⁺ rather than [C₅H₁₂]⁺

Note that an accurate mass spectrometer can sort out 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, from which you can calculate (predict) that the accurate relative ion masses are:

For m/z 72: [C₅H₁₂]⁺ = 72.0936  and  [¹³C¹²C₄H₁₁]⁺ = 72.0892, a difference of 0.0044 in relative ion mass.

Formation of m/z 70 ion:

[C₅H₁₁]⁺  ===>  [C₅H₁₀]⁺  +  H

Hydrogen atom loss from m/z 71 ion too or hydrogen molecule loss from the parent molecular ion?

Formation of m/z 57 ion: (see also above from m/z 85 ion)

[(CH₃CH₂)₃CH]⁺  ===>  [C₄H₉]⁺  +  C₃H₇

C-C bond scission of the parent molecular ion of 3-ethylpentane.

Loss of a C₃H₇ group, mass change = 100 - 43 = 57 (M-43 ion peak)

Formation of m/z 43 ion:

[(CH₃CH₂)₃CH]⁺  ===>  [C₃H₇]⁺  +  C₄H₉

C-C bond scission of the parent molecular ion of 3-ethylpentane.

Loss of a C₄H₉ group, mass change = 100 - 57 = 43 (M-57 ion peak)

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

Will also be formed by: [C₅H₁₁]⁺  ===>  [C₃H₇]⁺  +  C₂H₄

Formation of m/z 29 ion:

[(CH₃CH₂)₃CH]⁺  ===>  [CH₂CH₃]⁺  +  C₅H₁₁

Loss of a C₄H₉ group by C-C bond scission of the parent molecular ion,

mass change = 100 - 71 = 29 (M-71 ion peak)

Again, an ethyl group is broken off, but this has become ionised.

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Key Mass Spectral Features of 2-Ethylpentane

2-Ethylpentane (C₇H₁₆) is a branched alkane.

Its fragmentation pattern reflects typical alkyl chain cleavage and rearrangement:

m/z	Ion Formula	Fragment Origin	Notes
100	C7H16⁺	Molecular ion (M⁺)	Often weak in alkanes due to easy fragmentation
85	C6H13⁺	Loss of CH3 (–15)	Common alkyl cleavage
71	C5H11⁺	Loss of C2H5 (–29)	Stable pentyl cation
70	C5H10⁺	Loss of H from 71?	As prominent as m/z 71
57	C4H9⁺	Loss of C3H7 (–43)	Tends to be intense due to stable butyl cation
43	C3H7⁺	Propyl fragment	Very common in alkane spectra, base peak ion
29	C2H5⁺	Ethyl fragment	Minor but diagnostic
15	CH3⁺	Methyl cation	Often present in alkyl compounds

Sources: LibreTexts Chemistry, OpenStax Organic Chemistry

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Common Misconceptions in Exams

• Expecting a strong molecular ion peak: Alkanes often fragment easily, so M⁺ may be weak or absent.
• Confusing fragment ions with molecular ions: Students may misidentify intense peaks like m/z 57 or 43 as M⁺.
• Assuming all fragments are radicals: Only positively charged ions are detected; radicals are neutral and invisible.
• Ignoring rearrangement possibilities: Branched alkanes can undergo complex fragmentation not seen in straight chains.

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Exam Revision Tips

• Use m/z values to deduce neutral losses: Subtract fragment m/z from M⁺ to identify lost groups.
• Learn common alkyl fragment ions: m/z 57 (butyl), 43 (propyl), and 29 (ethyl) appear frequently in hydrocarbon spectra.
• Compare with straight-chain alkanes: Branching affects fragmentation pathways—use this to distinguish isomers.
• Practice with real spectra: Annotate peaks with fragment structures and neutral losses to reinforce understanding.
• Combine with IR or NMR data: Mass spectra alone may not confirm structure—use complementary techniques in exams.

Comparing the 1H NMR and 13C NMR spectra of the nine alkane structural isomers of C7H16 You can distinguish all 9 isomers from a data combination of their number of 1H NMR chemical shifts, and their resulting integrated 1H proton ratios, plus, their number of 13C chemical shifts.
Name of the alkane structural isomer of molecular formula C7H16	Abbreviated structural formulae of the nine isomers of molecular formula C7H16 (interpretation complications with 3-methylhexane and 2,3-dimethylpentane because they exhibit R/S isomerism due to a chiral carbon)	Skeletal formula of the nine alkane isomers of  molecular formula C7H16	Number of 1H NMR chemical shifts (δ) and proton ratio (links to spectrum)	Number of 13C chemical shifts (δ) (links to spectrum)
heptane			4 δ: proton ratio: 3:2:2:1 (6:4:4:2 in the molecule)	4 δ shifts
2-methylhexane			6 δ: proton ratio : 6:3:2:2:2:1	6 δ shifts
3-methylhexane			7 δ: proton ratio: 3:3:3:2:2:2:1 (simplification) !	7 δ shifts
3-ethylpentane			3 δ: proton ratio: 9:6:1	3 δ shifts
2,2-dimethylpentane			4 δ: proton ratio: 9:3:2:2	5 δ shifts
2,3-dimethylpentane			6 δ: proton ratio: 6:3:3:2:1:1 (simplification) !	6 δ shifts (simplification) !!!
2,4-dimethylpentane			3 δ: proton ratio: 12:2:2	3 δ shifts
3,3-dimethylpentane			3 δ: proton ratio: 3:3:2 (6:4:4 in the molecule)	4 δ shifts
2,2,3-trimethylbutane			3 δ: proton ratio: 9:6:1	4 δ shifts

The infrared spectrum of 3-ethylpentane

The H-1 NMR spectrum of 3-ethylpentane

The C-13 NMR spectrum of 3-ethylpentane

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