mass spectrum of 2-methylbutane fragmentation pattern of m/z m/e ions for analysis and identification of isopentane image diagram doc brown's advanced organic chemistry revision notes

Advanced Organic Chemistry: Mass spectrum of 2-methylbutane (CH₃)₂CHCH₂CH₃

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Interpreting the mass spectrum of 2-methylbutane

[Author © Dr Phil 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 spectrometry - analysing the mass spectrum of 2-methylbutane [spectra page updated Mar 13th 2026 *]

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Mass spectrometry - introduction and spectra index

Introductory note on the mass spectrum of 2-methylbutane

Students and teachers please note my explanation of the mass spectrum of 2-methylbutane is designed for advanced, but pre-university, chemistry courses.

If M represents the 2-methylbutane 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 2-methylbutane and only the formation of singly charged positive are considered for the mass spectrum of 2-methylbutane.

I've included a stick diagram and table of m/z ions for the mass spectrum of 2-methylbutane 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 2-methylbutane.

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, but the mass spectrometer software does!

mass spectrum of 2-methylbutane fragmentation pattern of m/z m/e ions for analysis and identification of 2-methylbutane image diagram doc brown's advanced organic chemistry revision notes

2-methylbutane C₅H₁₂ , alkanes structure and naming (c) doc b ,

For more see The molecular structure and naming of alkanes

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

[M]⁺ is the molecular ion peak (M) with an m/z of 72 corresponding to [C₅H₁₂]⁺, the original 2-methylbutane molecule minus an electron, [(CH₃)₂CHCH₂CH₃]⁺

The very tiny M+1 peak at m/z 73, corresponds to an ionised 2-methylbutane molecule with one ¹³C atom in it i.e. an ionised 2-methylbutane 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.

2-methylbutane has 5 carbon atoms, so on average, ~1 in 20 molecules will contain a ¹³C atom.

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

The most abundant ion of the molecule under mass spectrometry investigation (2-methylbuane) 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 peak ion for the mass spectrum of 2-methylbutane is the m/z 43 ion [C₃H₇]⁺

The parent molecular ion is m/z of 72 corresponding to [C₅H₁₂]⁺ or [(CH₃)₂CHCH₂CH₃]⁺

m/z value of [fragment]⁺	71 [C₅H₁₁]⁺ or	58, with ¹³C atom	57	56	55	44
[molecular fragment]⁺	[(CH₃)₂CCH₂CH₃]⁺	[C₄H₉]⁺	[C₄H₉]⁺	[C₄H₈]⁺	[C₄H₇]⁺	[C₃H₈]⁺

m/z value of [fragment]⁺	43 [C₃H₇]⁺	42	41	39	29	27
[molecular fragment]⁺	[(CH₃)₂CH]⁺	[C₃H₆]⁺	[C₃H₅]⁺	[C₃H₃]⁺	[CH₃CH₂]⁺	[C₂H₃]⁺

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

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

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

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.

Possible equations to explain some of the most abundant ion peaks in the mass spectrum of 2-methylbutane

Formation of m/z 71 ion:

[(CH₃)₂CHCH₂CH₃]⁺ ===> [C₅H₁₁]⁺ + H

C-H bond scission and proton loss from the parent molecular ion,

mass change 72 - 1 = 71 (M-1 ion peak)

Formation of m/z 57 ion:

[(CH₃)₂CHCH₂CH₃]⁺ ===> [(CH₃)₂CHCH₂CH₃]⁺ + CH₃

C-C bond scission in the parent molecular ion,

mass change 72 - 15 = 57 (M-15 ion peak) [C₄H₉]⁺

The m/z 58 ion could be [C₄H₁₀]⁺ or more likely the [¹³C¹²C₃H₉]⁺ ion formed in the same way as the m/z 57 ion, but containing a ¹³C carbon-13 isotope atom..

An accurate mass spectrometer sorts this out, measuring relative fragment ion masses to four decimal places e.g. using very 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:

[(CH₃)₂CHCH₂CH₃]⁺ ===> [C₄H₈]⁺ + CH₄

C-C bond scission in the parent molecular ion and proton rearrangement ???,

mass change 72 - 14 = 43 (M-29 ion peak)

maybe hydrogen loss from the m/z 57 ion ??

[C₄H₉]⁺ ===> [C₄H₈]⁺ + H

OR even more likely from the m/z 71 ion

[C₅H₁₁]⁺ ===> [C₄H₈]⁺ + CH₃

C-C bond fission and loss of methyl group,

mass change 71 - 15 = 56

Formation of m/z 43 ion:

[(CH₃)₂CHCH₂CH₃]⁺ ===> [C₃H₇]⁺ + CH₂CH₃

C-C bond scission in the parent molecular ion, mass change 72 - 29 = 43 (M-29 ion)

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 atom H or H₂ molecule to give the m/z 42, 41 and 39 ions.

The m/z 44 ion is probably formed in the same way, but containing a ¹³C carbon isotope atom i.e. [¹³C¹²C₂H₇]⁺ and not [C₃H₈]⁺

An accurate mass spectrometer sorts this out, measuring relative fragment ion masses to four decimal places e.g. using very accurate relative isotopic masses,

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

For m/z 44: [C₃H₈]⁺ = 44.0624 and [¹³C¹²C₂H₇]⁺ = 44.058, a difference of 0.0044 in relative ion mass.

Formation of m/z 29 ion:

[(CH₃)₂CHCH₂CH₃]⁺ ===> [CH₂CH₃]⁺ + C₃H₇

C-C bond scission from the parent molecular ion, mass change 72 - 43 = 29 (M-43 ion)

The m/z 29 ion can lose hydrogen atoms to give the m/z 27 ion [C₂H₃]⁺

Key Features of the Mass Spectrum of 2-Methylbutane and practice questions

2-methylbutane (C₅H₁₂) is a branched alkane. Its mass spectrum shows:

A molecular ion peak (M⁺) at m/z = 72, often weak due to fragmentation.
Several prominent fragment ions from cleavage of C–C bonds and loss of alkyl groups.
Base peak (most intense) typically at m/z = 43, from a stable carbocation.

Prominent m/z Ions and their origins

m/z	Ion Formula	Fragment Origin	Notes
72	C₅H₁₂⁺	Molecular ion (M⁺)	Often weak due to instability
57	C₄H₉⁺	Loss of CH₃ (methyl group)	Common in branched alkanes
43	C₃H₇⁺	Loss of C₂H₅ (ethyl group) or rearrangement	Base peak — very stable ion
41	C₃H₅⁺	Further fragmentation of C₃H₇⁺	Less intense, m/z ions 39 and 42 quite prominent
29	C₂H₅⁺	Ethyl cation from deeper fragmentation	Often present
15	CH₃⁺	Methyl cation	Small peak

Sources: NIST Mass Spectrometry Database, AQA/Edexcel/OCR specimen papers

Common Misconceptions in Mass Spectrometry

Confusing base peak with molecular ion: The base peak is the tallest, not necessarily the molecular ion.
Assuming M⁺ is always strong: In alkanes, M⁺ is often weak or absent due to fragmentation.
Ignoring rearrangements: Branched alkanes like 2-methylbutane can undergo rearrangement before fragmentation.
Thinking all fragments are neutral: Only positive ions are detected — neutral fragments are invisible.

Exam Revision Tips for Mass Spectra

What to Focus On

Identify M⁺ peak: Look for the peak at the molecular mass (C₅H₁₂ = 72).
Spot the base peak: Usually m/z = 43 for 2-methylbutane.
Understand fragmentation logic: Break C–C bonds to form stable carbocations.
Use peak differences: Subtract m/z values to deduce lost groups (e.g., 72 → 57 = loss of CH₃).

Memory Aids

“Base peak = most stable ion” — not the biggest fragment.
“M⁺ is the full molecule” — helps anchor your analysis.
“Alkanes fragment easily” — expect weak M⁺ and strong alkyl ions.

Exam questions strategy

Annotate spectra with m/z values and fragment origins.
Use molecular formula to calculate expected M⁺.
Practice with isomers: Compare spectra of pentane versus 2-methylbutane.
Link to structure: Use skeletal formula to predict fragmentation routes.

alkanes structure and naming (c) doc b Practice questions based on the mass spectrum of 2-methylbutane

Three varied, technically accurate multiple-choice questions on the mass spectrum of 2-methylbutane (C₅H₁₂), suitable for AQA, Edexcel, OCR, WJEC, CCEA, CIE, IB, and US AP/Honors chemistry exams.

These questions explore fragmentation logic, ion stability, and isomer comparison, with model answers and distractor analysis.

Question 1: Base Peak Identification and Fragment Stability based on the mass spectrum of 2-methylbutane

In the EI mass spectrum of 2-methylbutane, the base peak appears at m/z 57.

What is the most likely identity and origin of this fragment?

C₄H₉⁺ formed by loss of a methyl radical from the molecular ion
C₃H₃⁺ formed by loss of an ethyl radical
C₅H₁₂⁺ molecular ion with no fragmentation
C₂H₅⁺ formed by cleavage of the central carbon

Correct Answer: A

Model Answer:
The molecular ion of 2-methylbutane (M⁺ = 72) fragments by losing a methyl radical (•CH₃), forming a C₄H₉⁺ ion (m/z 57). This ion is a secondary carbocation, which is relatively stable and thus appears as the base peak.

Distractor Analysis:

Option	Why It’s Incorrect
B	C₃H₃⁺ (m/z 43) is a possible fragment but less stable and less intense than C₄H₃⁺.
C	The molecular ion (m/z 72) is often weak or absent in alkanes due to rapid fragmentation.
D	C₂H₅⁺ (m/z 29) is too small and not the dominant fragment in this molecule.

Question 2: Isomer Comparison by Fragmentation Pattern based on the mass spectrum of 2-methylbutane

Which of the following best explains why the mass spectrum of 2-methylbutane differs from that of pentane, even though both have the same molecular formula (C₅H₁₂)?

2-methylbutane produces a more stable secondary carbocation at m/z 57, while pentane favours m/z 43
Pentane contains a double bond, which alters its fragmentation pattern
2-methylbutane has a higher molecular ion peak due to branching
Pentane contains chlorine, which introduces isotope peaks

Correct Answer: A

Model Answer:
Both molecules have M⁺ = 72, but 2-methylbutane fragments to form a stable secondary carbocation (C₄H₉⁺, m/z 57), while pentane more commonly fragments to form C₃H₇⁺ (m/z 43). The difference in fragment stability and branching leads to distinct base peaks.

Distractor Analysis:

Option	Why It’s Incorrect
B	Neither molecule contains a double bond; both are saturated alkanes.
C	Branching affects fragmentation, not the molecular ion mass.
D	No halogens are present in either molecule.

Question 3: Fragmentation Mechanism and Ion Formation based on the mass spectrum of 2-methylbutane

Which of the following best describes the fragmentation mechanism that leads to the m/z 57 peak in the mass spectrum of 2-methylbutane?

Homolytic cleavage of a C–C bond adjacent to the branched carbon, forming a secondary carbocation
Loss of a hydrogen radical from the central carbon, forming a tertiary carbocation
Rearrangement to form a cyclic ion with m/z 57
Loss of a chlorine atom from the molecular ion

Correct Answer: A

Model Answer:
The m/z 57 fragment arises from homolytic cleavage of a C–C bond, typically adjacent to the branched carbon. This produces a secondary carbocation (C₄H₉⁺), which is stabilized by hyperconjugation and appears as the base peak.

Distractor Analysis:

Option	Why It’s Incorrect
B	Loss of hydrogen would give m/z 71, but this tertiary carbocation is not favoured in this case.
C	Cyclic rearrangements are rare in simple alkanes under EI conditions.
D	No chlorine is present in 2-methylbutane.

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 C_nH_2n+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 C_nH_2n+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 [C₂H_x]⁺ (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 C_nH_2n+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 C_nH_2n+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.

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