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Interpreting the mass
spectrum of 2-methylbut-2-ene (2-methyl-2-butene)
[Author
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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-methylbut-2-ene
[spectra
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analysis infrared spectrum of
(CH3)2C=CHCH3
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Mass spectrometry
- introduction and spectra index
Introductory note on the mass spectrum of 2-methylbut-2-ene
Students and teachers please note
my explanation of the mass spectrum of 2-methylbut-2-ene is designed for
advanced, but pre-university, chemistry courses.
If M represents the
2-methylbut-2-ene 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-methylbut-2-ene and only the formation of singly charged
positive are considered for the mass spectrum of
2-methylbut-2-ene.
I've included a stick diagram and table of m/z ions for the mass spectrum of
2-methylbut-2-ene
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-methylbut-2-ene.
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!
2-methylbut-2-ene C5H10
(CH3)2C=CHCH3
 ,
The molecular structure and naming of alkenes
Interpreting the fragmentation pattern of the mass spectrum of
2-methylbut-2-ene (2-methyl-2-butene)
[M]+ is the molecular ion peak (M) with an m/z of
70 corresponding to [C5H10]+, the original 2-methylbut-2-ene
(2-methyl-2-butene) molecule minus an electron,
[(CH3)2C=CHCH3]+
The small M+1 peak at m/z 71, corresponds to an ionised
2-methylbut-2-ene (2-methyl-2-butene)
molecule with one 13C atom in it
i.e. an ionised
2-methylbut-2-ene (2-methyl-2-butene) molecule of
formula
[13C12C4H10]+
Identifying the species giving the most prominent peaks
(apart from M) in the fragmentation pattern of 2-methylbut-2-ene
(2-methyl-2-butene).
The most abundant ion of the molecule under mass
spectrometry investigation (2-methylbut-2-ene) 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-methylbut-2-ene is the m/z 55 ion
[C4H7]+
The parent molecular ion is the m/z of 70 ion
[C5H10]+
or
[(CH3)2C=CHCH3]+
|
m/z value of
[fragment]+ |
69 |
56 |
55 |
53 |
43 |
42 |
41 |
39 |
29 |
27 |
|
[molecular fragment]+ |
[C5H9]+ |
[C4H8]+ |
[C4H7]+ |
[C4H5]+ |
[C3H7]+ |
[C3H6]+ |
[C3H5]+ |
[C3H3]+ |
[C2H5]+ |
[C2H3]+ |
Analysing and explaining the principal ions in the
fragmentation pattern of the mass spectrum of 2-methylbut-2-ene
(2-methyl-2-butene)
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.
Suggested equations to explain
some of the most abundant ion peaks of
2-methylbut-2-ene (2-methyl-2-butene)
Formation of m/z 69 ion
[(CH3)2C=CHCH3]+ ===> [C5H9]+
+ H
Loss of a proton from various sites on the molecule
mass change 70 - 1 =
69 (M-1 ion)
Formation of m/z 55
and 56 ions
[(CH3)2C=CHCH3]+ ===> [C4H7]+
+ CH3
C-C bond scission to
lose a methyl group from the molecular ion.
Mass change 70 - 15 =
55 (M-15 ion)
The m/z 55 ion is the base peak ion, the most
abundant and 'stable' ion fragment and formed by the loss of a
methyl group from the parent molecular ion.
The m/z 55 ion can
lose hydrogen atoms to give the m/z 53 ion.
The m/z 56 ion could
be
[C4H8]+
OR formed as
but containing a 13C isotope i.e.
[13C12C3H7]+
An accurate mass
spectrometer sorts this out, measuring relative fragment ion
masses to four decimal places e.g. using v ery 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 56:
[13C12C3H7]+
= 56.0580,
[C4H8]+
= 56.0624,
a difference of 0.0044 in relative ion mass.
Formation of m/z
42 ion
[(CH3)2C=CHCH3]+ ===> [C3H6]+
+ C2H4
mass change 70 - 28 = 42 (M-28 ion)
Formed by C-C bond scission.
proton migration, and loss of non-ionised
ethene molecule.
The m/z 42 and 43 ions can lose hydrogen atoms to
give the m/z 41 and 39 ions.
Formation of m/z
29 and 27 ions
[C2H5]+
and [C2H5]+ will
be formed by the fragmentation and/or loss of hydrogen atoms for
larger ions e.g.
[C3-4H5-10]+
Summary of key points for the mass spectrum of 2-methylbut-2-ene plus
extra exam revision comments and practice questions
A structured breakdown of the mass spectrum of 2-methylbut-2-ene
(C5H10), tailored for advanced A-level
chemistry revision.
This includes key peaks, fragmentation origins, common
misconceptions, and exam strategies across major exam boards.
Key
Concepts in Mass Spectrometry of 2-Methylbut-2-ene
- Molecular ion (M⁺): Formed by electron impact
ionization, giving the full molecular mass.
- Fragmentation: Occurs via cleavage of bonds, often
forming carbocations.
- Base peak: The most intense peak, often from the most
stable fragment.
- Isotopic peaks: Minor peaks due to natural abundance of
isotopes (e.g. ¹³C).
Prominent
m/z Peaks and Fragment Origins in
the mass spectrum of 2-methylbut-2-ene
|
m/z |
Ion Fragment |
Origin of
Fragmentation |
Notes |
| 70 |
C5H10₀⁺ (M⁺) |
Molecular ion |
Confirms molecular mass of 70 |
| 55 |
C4H7⁺ |
Loss of CH3• (methyl
radical) |
Allylic carbocation, relatively
stable, base ion peak |
| 41 |
C3H5⁺ |
Further cleavage of alkyl chain |
Common fragment in alkenes, m/z ions
39 and 42 quite prominent |
| 29 |
C2H5⁺ |
Ethyl fragment |
Often seen in hydrocarbon mass spectra |
| 15 |
CH3⁺ |
Methyl cation |
Small, stable fragment |
The base peak is typically m/z = 55,
due to the stability of the allylic carbocation formed after methyl loss.
Common
Misconceptions about
the mass spectrum of 2-methylbut-2-ene
(see also below)
| Misconception |
Clarification |
| The molecular ion is always the base
peak |
Not true — base peak reflects most
stable fragment, not necessarily M⁺ |
| All fragments are radicals |
Only ions are detected; neutral
radicals are not seen in the spectrum |
| Fragmentation is random |
It follows predictable patterns based
on bond strength and carbocation stability |
| Isotopic peaks are impurities |
They reflect natural isotope abundance
(e.g. ¹³C at m/z = 71) |
Exam
Revision Tips for questions involving
the mass spectrum of 2-methylbut-2-ene
(AQA, Edexcel, OCR, WJEC, CCEA, CIE, IB)
(see also above)
What to Focus On:
- Identify M⁺ peak: Confirms molecular mass (m/z = 70 for
C5H10).
- Recognise base peak: Often the most stable carbocation
(m/z = 55 in this case).
- Use fragmentation logic: Predict fragments based on
bond cleavage and stability.
- Isotope peaks: Small peaks at M+1 due to ¹³C — useful
for estimating carbon count.
Exam-Style Strategy:
- Step 1: Locate M⁺ peak and base peak.
- Step 2: Deduce possible fragments and their origin.
- Step 3: Use fragmentation patterns to confirm
structure.
- Step 4: Avoid overinterpreting minor peaks unless asked
about isotopes.
Practice questions
based on the mass spectrum of 2-methylbut-2-ene
Question 1 — Base
peak origin and stabilisation
based on the EI mass spectrum of 2-methyl-2-butene
Which explanation best accounts for the base peak at m/z 55 in the EI
mass spectrum of 2‑methylbut‑2‑ene (C5H10)?
- Formation of an acylium ion after oxygen insertion leading to m/z 55
- Direct formation of a saturated tert‑butyl cation C4H9+
(m/z 57) by simple homolytic cleavage
- Loss of a methyl radical from M•+ producing a resonance‑stabilised C4H7+
(m/z 55)
- Loss of ethene (C2H4, 28) from M•+ giving C3H6+
(m/z 42) which rearranges to m/z 55
Correct answer: C
Model answer
- EI produces the molecular radical cation M•+ (m/z 70). A favourable
fragmentation is loss of •CH3 (15 u) from positions adjacent to
the internal double bond, yielding C4H7+
(m/z 55). That C4 fragment is stabilised by alkenyl/allylic delocalisation
and by branching at C‑2, so this fragment is particularly abundant and
appears as the base peak.
Distractor analysis
- A is incorrect because acylium ions require oxygen/carbonyl chemistry,
not present here.
- B is incorrect because
C4H9+
(m/z 57) is a saturated carbocation and is not the dominant EI fragment for
this alkene.
- D is incorrect because loss of C2H4 yields m/z 42;
rearrangement to 55 is not the principal EI route.
Question 2 — Isomer
discrimination by relative fragment intensities
based on the EI mass spectrum of 2-methyl-2-butene
Which spectral feature most reliably distinguishes 2‑methylbut‑2‑ene from
its positional isomer 2‑methylbut‑1‑ene under EI conditions?
- Different molecular ion masses because branching changes M+
- A dominant base peak at m/z 55 for 2‑methylbut‑2‑ene due to formation of
stabilised C4H7+
- Presence of a chlorine isotope pattern in 2‑methylbut‑2‑ene but not in
2‑methylbut‑1‑ene
- A dominant peak at m/z 41 for 2‑methylbut‑2‑ene from allylic cleavage
Correct answer: B
Model answer
- Both isomers have M+ = 70. The diagnostic difference is fragmentation:
the internal, branched alkene (2‑methylbut‑2‑ene) favours loss of •CH3
to give a stabilised C4H7+ (m/z 55). The
terminal isomer (2‑methylbut‑1‑ene) more commonly produces allylic C3
fragments (C3H5+, m/z 41). Thus a
relatively intense m/z 55 indicates the 2‑ene.
Distractor analysis
- A is false because branching does not change nominal molecular mass;
both are C5H10 → m/z 70.
- C is irrelevant; no halogen present.
- D is reversed: m/z 41 (allylic fragment) is more characteristic and
stronger for terminal 1‑enes, not for the branched internal 2‑ene.
Question 3 —
Competing fragmentation channels and mechanistic detail based on the EI mass
spectrum of 2-methyl-2-butene
The EI spectrum shows a strong peak at m/z 55 and a moderate peak at m/z
41.
Which mechanistic statement best explains their relative intensities?
- m/z 41 is the molecular ion and therefore should be the base peak; m/z
55 is an oxidation artefact
- m/z 55 arises from halogen loss; m/z 41 from halogenated side products
- m/z 55 (C4H7+) is favoured because loss
of •CH3 adjacent to the internal double bond yields a delocalised
alkenyl/allylic cation; m/z 41 (C3H5+)
arises from alternative allylic cleavage and is less stabilised here
- m/z 55 and m/z 41 are produced by identical symmetric cleavages and
therefore must be equal in intensity
Correct answer: C
Model answer
- Two principal EI fragmentation channels compete: (1) loss of •CH3
→ C4H7+ (m/z 55), stabilised by delocalisation across
the alkene and branching, producing the base peak; (2) C–C cleavage to give
C3H5+ (m/z 41) (allylic fragment) which is possible
but yields a less stabilised fragment here, so appears with lower intensity.
Distractor analysis
- A is incorrect: the molecular ion is m/z 70, not 41; EI fragments are
not “oxidation artifacts.”
- B is chemically irrelevant because no halogens are present.
- D is incorrect because different bond cleavages give fragments of
different stabilities and therefore different abundances.
Key words & phrases: C5H10
image diagram on how to interpret and explain the mass spectrum of
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2-methylbut-2-ene (2-methyl-2-butene), details of the mass spectroscopy of
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2-methylbut-2-ene (2-methyl-2-butene), comparative
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2-methylbut-2-ene (2-methyl-2-butene),
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mass spectra of 2-methylbut-2-ene (2-methyl-2-butene) equations for explaining the formation of the positive ions
in the fragmentation of the ionised molecule of 2-methylbut-2-ene
(2-methyl-2-butene) recognising the base ion peak of 2-methylbut-2-ene
(2-methyl-2-butene) interpreting interpretation the
mass spectrum of 2-methylbut-2-ene (2-methyl-2-butene) C5H10
How do you interpret the mass spectrum of
2-methylbut-2-ene (2-methyl-2-butene) How to interpret
the mass spectrum of 2-methylbut-2-ene (2-methyl-2-butene) Explanatory diagram of the mass spectrum of the
2-methylbut-2-ene (2-methyl-2-butene) molecule in
terms of its molecular structure.
Listing data of the prominent main peaks in the mass spectrum of
2-methylbut-2-ene (2-methyl-2-butene). How to explain the mass spectrum of
2-methylbut-2-ene (2-methyl-2-butene). The m/z value of the
molecular ion peak in the mass spectrum of 2-methylbut-2-ene
(2-methyl-2-butene). Identifying 2-methylbut-2-ene (2-methyl-2-butene) from
its mass spectrum pattern. The m/z m/e peak analysis of the mass
spectrum of the 2-methylbut-2-ene (2-methyl-2-butene) molecule. The uses of the mass spectrum of the
2-methylbut-2-ene (2-methyl-2-butene) molecule. The distinctive features of the mass spectrum of
the 2-methylbut-2-ene (2-methyl-2-butene) molecule explained. explaining the fragmentation pattern of the mass spectrum of
2-methylbut-2-ene (2-methyl-2-butene) equations showing the
formation of the ionised fragments in the mass spectrum of
2-methylbut-2-ene (2-methyl-2-butene)
what does the mass spectrum tell you about the structure and
properties of the 2-methylbut-2-ene (2-methyl-2-butene) molecule?
Data table of ionised fragments in the mass spectrum of 2-methylbut-2-ene
(2-methyl-2-butene) and equations for their formation in the fragmentation of
2-methylbut-2-ene (2-methyl-2-butene) molecules
Links associated
with
2-methylbut-2-ene (2-methyl-2-butene)
The infrared spectrum of
2-methylbut-2-ene
The H-1 NMR spectrum of
2-methybut-2-ene
The C-13 NMR spectrum of
2-methylbut-2-ene
The chemistry of ALKENES
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- introduction and spectra index
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