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Interpreting the mass
spectrum of 2-methylbut-1-ene
[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-methylbut-1-ene
[spectra
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mass spectrum of
CH3CH2C(CH3)=CH2
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Mass spectrometry
- introduction and spectra index
Introductory note on the mass spectrum of 2-methylbut-1-ene
(2-methyl-1-butene)
Students and teachers please note
my explanation of the mass spectrum of 2-methylbut-1-ene is designed for
advanced, but pre-university, chemistry courses.
If M represents the
2-methylbut-1-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-1-ene and only the formation of singly charged
positive are considered for the mass spectrum of
2-methylbut-1-ene.
I've included a stick diagram and table of m/z ions for the mass spectrum of
2-methylbut-1-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-1-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-1-ene C5H10,
 ,
The molecular structure and naming of alkenes
Interpreting the fragmentation pattern of the mass spectrum of
2-methylbut-1-ene
[M]+ is the molecular ion peak (M) with an m/z of
70 corresponding to [C5H10]+, the original 2-methylbut-1-ene molecule minus an electron,
[H2C=C(CH3)CH2CH3]+
The small M+1 peak at m/z 71, corresponds to an ionised
2-methylbut-1-ene
molecule with one 13C atom in it i.e. an ionised
2-methylbut-1-ene molecule of
formula [13C12C4H10]+
Identifying the species giving the most prominent peaks
(apart from M) in the fragmentation pattern of 2-methylbut-1-ene.
The most abundant ion of the molecule under mass
spectrometry investigation (2-methylbut-1-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-1-ene is the m/z 55 ion
[C4H7]+
The parent molecular ion is m/z of
70 corresponding to [C5H10]+
or
[H2C=C(CH3)CH2CH3]+
|
m/z value of
[fragment]+ |
70 |
56 |
55
[C4H7]+ |
53 |
42 |
|
[molecular fragment]+ |
[C5H10]+ |
[C4H8]+ |
[H2C=C(CH3)CH2]+ |
[C4H5]+ |
[C3H6]+ |
|
m/z value of
[fragment]+ |
41 |
39 |
29 |
27 |
|
[molecular fragment]+ |
[C3H5]+ |
[C3H3]+ |
[CH3CH2]+ |
[C2H3]+ |
Analysing and explaining the principal ions in the
fragmentation pattern of the mass spectrum of 2-methylbut-1-ene
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-1-ene
Formation of m/z 55 ion:
[H2C=C(CH3)CH2CH3]+ ===> [H2C=C(CH3)CH2]+
+ CH3
The m/z 55 ion is the base peak ion, the most
abundant and 'stable' ion fragment.
Formed by C-C bond scission and loss of methyl group.
mass change 70 - 15 = 55
(M-15 ion).
The m/z 55 ion can
lose hydrogen atoms to give the m/z 54 or 53 ion.
Note the m/z peak of 56 could correspond with the
ion [13C12C3H7]+
rather than the ion
[C4H8]+,
and formed in the same way as the m/z 55 ion.
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,
12C
= 12.0000 13C = 13.0034, 1H = 1.0078, 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
[H2C=C(CH3)CH2CH3]+ ===> [C3H6]+
+ CH2=CH2
Formed by C-C bond scission.
proton migration, and loss of non-ionised
ethene molecule.
mass change
70 - 28 = 42 (M-28 ion)
The m/z 42 ion can
lose hydrogen atoms to give the m/z 41 or 39 ions.
Formation of m/z 41 ion
[H2C=C(CH3)CH2CH3]+ ===> [C3H5]+
+ CH2CH3
Formed by C-C bond scission and loss of non-ionised
ethyl group
mass change 70 - 29 = 41
(M-29 ion)
Formation
of m/z 39 ion
[C3H5]+ ===> [C3H3]+
+ H2
Formation of m/z 29 ion
[H2C=C(CH3)CH2CH3]+ ===> [CH2CH3]+
+ C3H5
Formed by C-C bond scission and loss of ionised
ethyl group.
mass change 70 - 41 = 29
(M-41 ion).
The m/z 29 ion can
lose hydrogen atoms to give the m/z 27 ion.
Summary of key points for the mass spectrum of 2-methylbut-1-ene plus
extra exam revision comments
A structured breakdown of the mass spectrum of
2-methylbut-1-ene, tailored for advanced A-level and IB
Chemistry exam boards. This includes key fragment ions,
misconceptions, and revision strategies.
Overview of
2-Methylbut-1-ene
- Molecular formula: C5H10
- Molecular ion (M⁺): m/z = 70
- Structure: CH2=C(CH3)CH2CH3
- Functional groups: Terminal alkene, methyl and
ethyl substituents
Prominent Fragment
Ions in
the mass spectrum of 2-methylbut-1-ene
|
m/z |
Ion Formula |
Fragment
Origin |
Notes |
|
70 |
C5H10+ |
Molecular ion (M⁺) |
May be weak or absent |
|
55 |
C4H7+ |
Loss of CH3 (–15) |
Allylic or alkyl fragment, base
peak ion |
|
41 |
C3H5+ |
Allyl cation |
Common in alkenes, m/z 39 and 42 ions
also quite prominent |
|
29 |
C2H5+ |
Ethyl fragment |
Stable alkyl ion |
|
27 |
C2H3+ |
Vinyl cation |
Indicates unsaturation |
|
15 |
CH3+ |
Methyl fragment |
Often intense |
Common
Misconceptions about
the mass spectrum of 2-methylbut-1-enersus
(see also below)
- Assuming M⁺ is always the tallest peak: The
base peak is often a stable fragment, not the molecular ion.
- Confusing alkyl fragments: C2H5⁺
(m/z 29) v C3H5⁺ (m/z 41) — both common but
arise from different cleavages.
- Overinterpreting low m/z peaks: Peaks like m/z
15 (CH3⁺) are ubiquitous and not diagnostic alone.
- Ignoring rearrangements: Allylic rearrangements
can lead to unexpected fragment ions.
Exam Revision Tips
for questions involving
the mass spectrum of 2-methylbut-1-ene
(AQA, Edexcel, OCR, WJEC,
CCEA, CIE, IB) (see also above)
Molecular Ion Identification
- Look for M⁺ peak at m/z 70 — confirms molecular
mass.
- If absent, use base peak and known fragments to
deduce structure.
Fragmentation Logic
- Practice predicting fragment ions by breaking
C–C bonds near functional groups.
- Use mass differences to deduce lost groups
(e.g. 70 → 55 = loss of CH₃).
Isomer Comparison
- Compare spectra of 2-methylbut-1-ene versus
2-methylbut-2-ene:
- Different fragmentation due to position of double bond.
- Allylic versus vinylic cleavage patterns.
Integration with IR/NMR
- Use mass spec to confirm molecular formula.
- Combine with IR for functional group and NMR
for structural detail.
Data Sheet Familiarity
- Know common m/z values for alkyl and
unsaturated fragments.
- Practice annotating spectra with fragment
origins.
Practice questions based
on the mass spectrum of 2-methylbut-1-ene
Three varied, technically sound
multiple-choice questions on the
mass spectrum of 2-methylbut-1-ene (C5H10),
designed for advanced pre-university chemistry students across AQA, Edexcel,
OCR, WJEC, CCEA, CIE, IB, and US AP/Honors curricula.
These questions explore fragmentation logic, isotopic neutrality, and
structural comparison with isomers — not just ion identification.
Question 1:
Fragmentation Pathway and Base Peak
based on the mass spectrum of 2-methylbut-1-ene
In the mass spectrum of 2-methylbut-1-ene, the base peak
appears at m/z 41.
Which of the following best explains the formation of
this fragment?
- Loss of a methyl radical to form a primary carbocation
- Cleavage of the double bond to form an allyl cation
- Formation of a tert-butyl cation after rearrangement
- Loss of ethene to form a stable acylium ion
Correct Answer: B
Explanation:
- 2-methylbut-1-ene contains a terminal alkene
and a branched methyl group
- Fragmentation often yields the allyl cation (CH2=CH–CH2⁺),
which is resonance-stabilized
- Allyl cation has m/z 41, and is commonly the
base peak in alkenes due to its stability
Distractor Analysis:
| Option |
Why It’s
Incorrect |
| A |
Primary carbocations are less
stable and typically appear at m/z 43 or lower |
| C |
No tert-butyl group is present in
the structure |
| D |
Acylium ions are characteristic of
carbonyl compounds, not alkenes |
Question 2:
Isotopic Pattern Recognition
based on the mass spectrum of 2-methylbut-1-ene
Why does the molecular ion peak of 2-methylbut-1-ene
appear as a single peak rather than a close cluster of isotopic
peaks?
- The molecule contains only carbon and hydrogen, which have
negligible isotope effects in mass spectrometry
- The molecule contains oxygen, which has only one stable isotope
- The molecule contains chlorine, but its isotopes are not
detected in mass spectrometry
- The molecule contains bromine, which produces a 1:1 isotope
pattern that overlaps
Correct Answer: A
Explanation:
- 2-methylbut-1-ene contains only C and H atoms
- ¹³C and ²H exist but are
low in abundance, so the M+1 peak is weak
- No halogens or elements with strong isotope patterns are
present, so the molecular ion appears as a single peak
Distractor Analysis:
| Option |
Why It’s
Incorrect |
| B |
Oxygen is not present in this
molecule |
| C |
Chlorine is not present; if it
were, it would produce a 3:1 M/M+2 pattern |
| D |
Bromine is not present; it would
produce a 1:1 M/M+2 pattern if it were |
Question 3: Isomer
Differentiation by Fragmentation
based on the mass spectrum of 2-methylbut-1-ene
Which of the following best explains why the mass
spectrum of 2-methylbut-1-ene differs from that of pent-1-ene, even
though both have the same molecular formula (C5H10)?
- 2-methylbut-1-ene has a higher molecular ion peak due to its
branched structure
- Pent-1-ene contains a chlorine atom, which alters its
fragmentation pattern
- 2-methylbut-1-ene forms a more stable allyl cation, leading to a
stronger base peak at m/z 41
- Pent-1-ene undergoes rearrangement to form a tertiary
carbocation, while 2-methylbut-1-ene does not
Correct Answer: C
Explanation:
- Both molecules have the same molecular ion (m/z 70), but
fragment differently
- 2-methylbut-1-ene fragments to form a
resonance-stabilized allyl cation (m/z 41)
- Pent-1-ene tends to form primary or
secondary carbocations, which are less stable
- This leads to different base peaks in their
spectra
Distractor Analysis:
| Option |
Why It’s
Incorrect |
| A |
Branching does not change the
molecular ion mass |
| B |
Neither molecule contains chlorine |
| D |
Pent-1-ene does not readily form
tertiary carbocations due to its linear structure |
Key words & phrases: 2-methyl-1-butene
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interpretation the mass spectrum of 2-methylbut-1-ene
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2-methylbut-1-ene (2-methyl-1-butene) molecule in
terms of its molecular structure.
Listing data of the prominent main peaks in the mass spectrum of
2-methylbut-1-ene (2-methyl-1-butene). How to explain the mass spectrum of
2-methylbut-1-ene (2-methyl-1-butene). The m/z value of the
molecular ion peak in the mass spectrum of 2-methylbut-1-ene
(2-methyl-1-butene). Identifying 2-methylbut-1-ene (2-methyl-1-butene) from
its mass spectrum pattern. The m/z m/e peak analysis of the mass
spectrum of the 2-methylbut-1-ene (2-methyl-1-butene) molecule. The uses of the mass spectrum of the
2-methylbut-1-ene (2-methyl-1-butene) molecule. The distinctive features of the mass spectrum of
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2-methylbut-1-ene (2-methyl-1-butene) equations showing the
formation of the ionised fragments in the mass spectrum of
2-methylbut-1-ene (2-methyl-1-butene)
what does the mass spectrum tell you about the structure and
properties of the 2-methylbut-1-ene (2-methyl-1-butene) molecule?
Data table of ionised fragments in the mass spectrum of 2-methylbut-1-ene
(2-methyl-1-butene) and equations for their formation in the fragmentation of
2-methylbut-1-ene (2-methyl-1-butene) molecules
Links associated
with
2-methylbut-1-ene
The Infrared spectrum of
2-methylbut-1ene
The H-1 NMR spectrum of
2-methylbut-1ene
The C-13 NMR spectrum of
2-methylbut-1ene
The chemistry of ALKENES
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- introduction and spectra index
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