[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: Mass spectrometry - analysing the mass spectrum of 2,2-dimethylbutane [updated Nov 4th 2025]

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 2,2-dimethylbutane C₆H₁₄,  , ,

For more see The molecular structure, classification and naming of alkanes

Interpreting the fragmentation pattern of the mass spectrum of 2,2-dimethylbutane

[M]⁺ is the molecular ion peak (M) with an m/z of 86 corresponding to [C₆H₁₄]⁺, the original 2,2-dimethylbutane molecule minus an electron, [(CH₃)₃CCH₂CH₃]⁺, BUT it doesn't seem to show up!

The molecular ion of 2,2-dimethylbutane must be very unstable.

An M+1 peak at m/z 87, could correspond to an ionised 2,3-dimethylbutane molecule with one ¹³C atom in it i.e. an ionised 2,2-dimethylbutane molecule of formula ¹³C¹²C₅H₁₄, BUT this is even less likely to show up!

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.

2,2-dimethylbutane 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 2,2-dimethylbutane - though the ratio will be greater e.g. the m/z 57 and 58 ions.

e.g. the m/z 58 ion can be [¹³C¹²C₃H₉]⁺ rather than  [C₄H₁₀]⁺

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

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.

The base ion peak for 2,2-dimethylbutane is m/z ion 43 [C₃H₇]⁺

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

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 (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 2,2-dimethybutane

Formation of m/z 71 ion:

[(CH₃)₃CCH₂CH₃]⁺  ===>  [C₅H₁₁]⁺  +  CH₃

 C-C bond chain scission, methyl fragment lost from parent molecular ion,

mass change = 86 - 15 = 71 (M-15 ion)

Some possible structures of the [C₅H₁₁]⁺ ion.

e.g. [CH₃)₃CCH₂]⁺ or [(CH₃)₂CCH₂CH₃]⁺

Formation of m/z 57 ion:

[(CH₃)₃CCH₂CH₃]⁺  ===>  [(CH₃)₃C]⁺  +  CH₂CH₃

 C-C bond chain scission, ethyl fragment lost, mass change = 86 - 29 = 57 (M-29 ion)

The 2nd most abundant ion, a relatively stable tertiary carbocation.

The m/z 57 ion intensity is nearly as intense as the base peak ion and a tertiary carbocation, stabilised by the +I inductive effect of the three methyl groups.

The m/z 58 ion is probably formed in the same way i.e. [¹³C¹²C₃H₉]⁺ rather than the [C₄H₁₀]⁺ ion.

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 58: [¹³C¹²C₃H₉]⁺ = 58.0736, [C₄H₁₀]⁺ = 58.0780, a difference of 0.0044 in relative ion mass.

Formation of m/z 43 ion:

[(CH₃)₃CCH₂CH₃]⁺  ===>  [(CH₃)₂CH]⁺  +  C₃H₇

 C-C bond chain scission of parent molecular ion,

mass change = 86 - 43 = 43 (M-43 ion peak)

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

The m/z 44 ion is probably formed in the same way i.e. [¹³C¹²C₂H₇]⁺ rather than the [C₃H₆]⁺ ion.

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

Formation of m/z 29 ion:

[(CH₃)₃CCH₂CH₃]⁺  ===>  [CH₂CH₃]⁺  +  (CH₃)₃C

 C-C bond chain scission of parent molecular ion, here it is the ethyl fragment ionised,

mass change = 86 - 57 = 29 (M-57 ion peak)

Sequences including m/z values of 57, 56 55 or 43, 42, 41, 40, 39 or 29, 28, 28, 27, 26, indicate successive hydrogen atom loss from the m/z 57, 43 or 29 ions.

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Key points about the mass spectrum of 2,2-dimethylbutane

2,2-Dimethylbutane shows a distinctive mass spectrum dominated by fragmentation peaks rather than a strong molecular ion.

Key ions include m/z 57 (base peak), 43, and 71, reflecting its branched alkane structure.

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Key Features of the Mass Spectrum of 2,2-Dimethylbutane

• Molecular formula: C₆H₁₄

• Molecular ion (M⁺): m/z = 86 (very weak peak due to instability)

• Base peak: m/z = 57 — most stable carbocation fragment

• Fragmentation pattern: Dominated by cleavage around the central quaternary carbon

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Prominent m/z Ions and Their Origins

m/z	Fragment	Origin / Explanation
57	C4H9⁺ (tert-butyl)	Loss of ethyl group (C2H5); forms stable tertiary carbocation — base peak
43	C3H7⁺ (propyl or isopropyl)	Common alkyl fragment; formed by further cleavage of larger fragments
71	C5H11⁺	Loss of methyl group (CH3); less stable than m/z 57 but still prominent
29	C2H5⁺	Ethyl fragment; often seen in alkanes
41	C3H5⁺	Allylic-type fragment; less common in saturated alkanes
86	C6H14⁺ (M⁺)	Molecular ion; very weak due to extensive fragmentation

Common Misconceptions in Exams

• Assuming the molecular ion is always the base peak: Not true for branched alkanes like 2,2-dimethylbutane.

• Confusing m/z 57 with other alkyl group: It's actually a tert-butyl cation (C₄H₉⁺).

• Overlooking fragmentation stability: Tertiary carbocations are more stable and thus more abundant.

• Thinking all alkanes fragment similarly: Branching significantly alters fragmentation patterns.

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Exam Revision Tips (AQA, Edexcel, OCR, WJEC, CCEA, CIE, IB, AP)

• Know your fragmentation logic: Focus on stability — tertiary > secondary > primary.

• Practice with isomers: Compare spectra of n-hexane vs. 2,2-dimethylbutane to understand structural effects.

• Use peak tables: Learn common m/z values for alkyl fragments m/z 15 = CH₃⁺, 29 = C₂H₅⁺, 43 = C₃H₇⁺, 57 = C₄H₉⁺, but not always of diagnostic use..

• Sketch fragmentation routes: Helps visualize how ions form.

• Watch for base peak vs. molecular ion: They are often different in branched hydrocarbons.

• For IB and AP: Emphasize interpretation, not just memorization — explain why certain peaks dominate.

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: image diagram on how to interpret and explain the mass spectrum of 2,2-dimethylbutane m/z m/e base peaks, image and diagram of the mass spectrum of 2,2-dimethylbutane, details of the mass spectroscopy of 2,2-dimethylbutane,  low and high resolution mass spectrum of 2,2-dimethylbutane, prominent m/z peaks in the mass spectrum of 2,2-dimethylbutane, comparative mass spectra of 2,2-dimethylbutane, the molecular ion peak in the mass spectrum of 2,2-dimethylbutane, analysing and understanding the fragmentation pattern of the mass spectrum of 2,2-dimethylbutane, characteristic pattern of peaks in the mass spectrum of 2,2-dimethylbutane, relative abundance of mass ion peaks in the mass spectrum of 2,2-dimethylbutane, revising the mass spectrum of 2,2-dimethylbutane, revision of mass spectroscopy of 2,2-dimethylbutane, most abundant ions in the mass spectrum of 2,2-dimethylbutane, how to construct the mass spectrum diagram for abundance of fragmentation ions in the mass spectrum of 2,2-dimethylbutane, how to analyse the mass spectrum of 2,2-dimethylbutane, how to describe explain the formation of fragmented ions in the mass spectra of 2,2-dimethylbutane equations for explaining the formation of the positive ions in the fragmentation of the ionised molecule of 2,2-dimethylbutane recognising the base ion peak of 2,2-dimethylbutane interpreting interpretation the mass spectrum of 2,2-dimethylbutane Stick diagram of the relative abundance of ionised fragments in the fingerprint pattern of the mass spectrum of 2,2-dimethylbutane. Table of the m/e m/z values and formula of the ionised fragments in the mass spectrum of 2,2-dimethylbutane. The m/e m/z value of the molecular ion peak in the mass spectrum of 2,2-dimethylbutane.  The m/e m/z value of the base ion peak in the mass spectrum of 2,2-dimethylbutane. Possible examples of equations showing the formation of the ionised fragments in 2,2-dimethylbutane. Revision notes on the mass spectrum of 2,2-dimethylbutane. Matching and deducing the structure of the 2,2-dimethylbutane molecule from its mass spectrum. Mass spectroscopy of  aliphatic alkanes, mass spectra of 2,2-dimethylbutane, a structural isomer of molecular formula C6H14 How do you interpret the mass spectrum of  2,2-dimethylbutane How to interpret the mass spectrum of  2,2-dimethylbutane Explanatory diagram of the mass spectrum of the  2,2-dimethylbutane molecule in terms of its molecular structure. Listing data of the prominent main peaks in the mass spectrum of  2,2-dimethylbutane. How to explain the mass spectrum of  2,2-dimethylbutane. The m/z value of the molecular ion peak in the mass spectrum of  2,2-dimethylbutane. Identifying  2,2-dimethylbutane from its mass spectrum pattern. The m/z m/e peak analysis interpretation diagram of the mass spectrum of the  2,2-dimethylbutane molecule. The uses of the mass spectrum of the  2,2-dimethylbutane molecule.  The distinctive features of the mass spectrum of the  2,2-dimethylbutane molecule explained. explaining the fragmentation pattern of the mass spectrum of  2,2-dimethylbutane equations showing the formation of the ionised fragments in the mass spectrum of  2,2-dimethylbutane  what does the mass spectrum tell you about the structure and properties of the  2,2-dimethylbutane molecule? Data table of ionised fragments in the mass spectrum of  2,2-dimethylbutane and equations for their formation in the fragmentation of  2,2-dimethylbutane molecules

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The chemistry of ALKANES revision notes INDEX

The infrared spectrum of 2,2-dimethylbutane

The H-1 NMR spectrum of 2,2-dimethylbutane

The C-13 NMR spectrum of 2,2-dimethylbutane

Mass spectrometry index

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