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2,2,3-trimethylbutane  C₇H₁₆   

Interpreting the fragmentation pattern of the mass spectrum of 2,2,3-trimethylbutane

[M]⁺ is the molecular ion peak (M) with an m/z of 100 corresponding to [C₇H₁₆]⁺, the original 2,2,3-trimethylbutane molecule minus an electron, [(CH₃)₃CCH(CH₃)₂]⁺.

The molecular ion peak of 2,2,3-trimethylbutane is very small, suggesting it is a very unstable ion.

You might detect an M+1 peak at m/z 101, corresponds to an ionised 2,2,3-trimethylbutane molecule with one ¹³C atom in it i.e. an ionised 2,2,3-trimethylbutane 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.

2,2,3-trimethylbutane 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 2,2,3-trimethylbutane - though the ratio will be greater:

e.g. m/z 44 ion could be [¹³C¹²C₂H₇]⁺, m/z 58 ion [¹³C¹²C₃H₉]⁺, and m/z 86 ion [¹³C¹²C₅H₁₃]⁺

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

i.e. [C₃H₈]⁺, [C₄H₁₀]⁺ and [C₆H₁₄]⁺

Either way, for identification purposes, all these peaks add uniqueness to the fragmentation pattern of the mass spectrum of 2,2,3-trimethylbutane.

The most abundant ion of the molecule under mass spectrometry investigation (2,2,3-trimethylbutane) 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 2,2,3-trimethylbutane is the m/z ion 57 [C₄H₉]⁺

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of 2,2,3-trimethylbutane.

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 2,2,3-trimethylbutane.

The parent molecular ion of 2,3-dimethylpentane m/z 100: [C₇H₁₆]⁺

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of 2,2,3-trimethylbutane.

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 2,2,3-trimethylbutane

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 possible equations to explain some of the most abundant ion peaks in the mass spectrum of 2,2,3-trimethylbuttane

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

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

Formation of m/z 85 ion:

[(CH₃)₃CCH(CH₃)₂]⁺  ===>  [C₆H₁₃]⁺  +  CH₃

C-C bond scission in the parent molecular ion of 2,2,3-trimethylbutane to release a methyl group,

mass change = 100 - 15 = 85 (M-15 ion peak)

The m/z 86 ion is probably formed in the same way but containing a ¹³C atom rather than the alternative ion [C₆H₁₄]⁺

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:

m/z 86: [C₆H₁₄]⁺ = 86.1092  and  [¹³C¹²C₅H₁₃]⁺ = 86.1048, a relative ion mass difference of 0.0044

Formation of m/z 69 ion:

[?]⁺  ===>  [C₅H₉]⁺  +  ?

Formation of m/z 57 ion:

[(CH₃)₃CCH(CH₃)₂]⁺  ===>  [(CH₃)₃C]⁺  +  C₃H₇

C-C bond scission in the parent molecular ion,

mass change 100 - 43 = 57 (M-43 ion peak)

The m/z 57 ion is the base peak ion, the most abundant and 'stable' ion fragment.

The m/z 57 ion can also be formed from the elimination of ethene from the m/z 85 ion.

[(CH₃)₃C]⁺  ===>  [C₄H₉]⁺  +  C₂H₄

Mass change = 85 - 28 = 57

Many other fragments are formed by hydrogen atom/molecule loss, so you get m/z ion sequences like 71 ==>70, 57 ==> 56 ==> 55, 43 ==> 39 and 29 ==> 27 etc. (see examples below).

The m/z 58 ion is probably formed in the same way but containing a ¹³C atom rather than the alternative ion [C₄H₁₀]⁺

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

Formation of m/z 56 ion:

[C₆H₁₃]⁺  ===>  [C₄H₈]⁺  +  C₂H₅

Perhaps C-C bond scission and proton rearrangement?

85 - 29 = 56, or the other fragment could be ionised to give the m/z 29 ion.

[C₆H₁₃]⁺  ===>  [C₂H₅]⁺  +  C₄H₈

Formation of m/z 43 ion:

[(CH₃)₃CCH(CH₃)₂]⁺  ===>  [CH(CH₃)₂]⁺  +  (CH₃)₃C

C-C bond scission in the parent molecular ion.

Ion mass change: 100 - 57 = 43 (M-57 ion)

Note this is the same C-C bond fission as the m/z 57 ion formation, but the other fragment is ionised.

The m/z 44 ion is probably formed in the same way but containing a ¹³C atom rather than the alternative ion [C₃H₈]⁺

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.

The m/z 43 ion can lose hydrogen atoms to give the m/z 42, 41 and then 39 ions (see ion data table).

Formation of m/z 41 ion:

[?]⁺  ===>  [C₃H₅]⁺  +  ?

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Key Peaks in the Mass Spectrum of 2,2,3-trimethylbutane

m/z	Fragment	Origin	Notes
100	Molecular ion (M⁺)	C7H16⁺• — intact molecule	Often weak or absent in alkanes
85	[M – CH3]⁺	Loss of methyl radical (•CH3)	Common in branched alkanes
57	isobutyl cation (C4H9⁺)	Further fragmentation of C5H11⁺	Common in branched alkanes, base peak ion
43	propyl cation (C3H7⁺)	Smaller alkyl fragment	Often seen in alkanes
29	ethyl cation (C2H5⁺)	Small fragment from multiple routes	Low intensity

Base peak = m/z 57 → most stable tertiary carbocation (C₄H₉⁺)

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Common Misconceptions

Misconception	Clarification
"The molecular ion is always the base peak"	Not true—alkanes often have weak or absent M⁺ due to fragmentation
"Mass spec shows all atoms"	Only positive ions are detected—not radicals or neutral fragments
"All fragments are random"	Fragmentation favours stable carbocations (e.g. tertiary > secondary > primary)
"Mass spec gives structural formula"	It gives molecular weight and fragmentation pattern, not full structure alone

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Exam Revision Tips

• Know the M⁺ peak: For C₇H₁₆, M⁺ = 100. If absent, look for logical fragments.
• Know common alkyl fragment ions: m/z 15 (CH₃⁺), 29 (C₂H₅⁺), 43 (C₃H₇⁺), 57 (C₄H₉⁺),  are frequently tested, but not always diagnostic.
• Look for base peak: Often corresponds to the most stable carbocation.
• Use fragmentation logic: Predict which bonds break to form stable ions.
• Combine with IR/NMR: Mass spec gives molecular mass and fragments; use other spectra for full structure.
• Practice with isomers: Isomers have same M⁺ but different fragmentation patterns.

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Practice Question

Q: The mass spectrum of a hydrocarbon shows a weak molecular ion peak at m/z = 100 and a base peak at m/z = 71. Other peaks appear at m/z = 85, 57, and 43.
Which of the following is the most likely identity of the compound?

1. Heptane
2. 2,2,3-Trimethylbutane
3. Cycloheptane
4. 3-Methylhexane

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Model Answer

Correct answer: B. 2,2,3-Trimethylbutane

Justification:

• Molecular ion at 100 → matches C₇H₁₆
• Base peak at 71 → indicates formation of a tertiary carbocation, consistent with 2,2,3-trimethylbutane
• Other peaks (85, 57, 43) → match expected alkyl fragments from branched structure
• Heptane and 3-methylhexane would show different fragmentation patterns (less stable carbocations)
• Cycloheptane would show a strong M⁺ and different fragmentation due to ring strain

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 2,2,3-trimethylbutane

The H-1 NMR spectrum of 2,2,3-trimethylbutane

The C-13 NMR spectrum of 2,2,3-trimethylbutane

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