Mass spectrometry - spectra index

Introductory note on the mass spectrum of 3-methylbut-1-ene

Students and teachers please note my explanation of the mass spectrum of 3-methylbut-1-ene is designed for advanced, but pre-university, chemistry courses.

If M represents the 3-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 3-methylbut-1-ene and only the formation of singly charged positive are considered for the mass spectrum of 3-methylbut-1-ene.

I've included a stick diagram and table of m/z ions for the mass spectrum of 3-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 3-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!

The molecular structure and naming of alkenes

Interpreting the fragmentation pattern of the mass spectrum of 3-methylbut-1-ene (3-methyl-1-butene)

[M]⁺ is the molecular ion peak (M) with an m/z of 70 corresponding to [C₅H₁₀]⁺, the original 3-methylbut-1-ene (3-methyl-1-butene) molecule minus an electron, [H₂C=CHCH(CH₃)₂]⁺.

The small M+1 peak at m/z 71, corresponds to an ionised 3-methylbut-1-ene (3-methyl-1-butene) molecule with one ¹³C atom in it i.e. an ionised 3-methylbut-1-ene (3-methyl-1-butene) molecule of formula ¹³C¹²C₄H₁₀

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of 3-methylbut-1-ene (3-methyl-1-butene).

The m/z ion 55 [C₄H₇]⁺ is the base ion peak and arbitrarily given a relative intensity value of 100.

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 3-methylbut-1-ene (3-methyl-1-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.

Equations to explain the most abundant ion peaks of 3-methylbut-1-ene (3-methyl-1-butene)

Formation of m/z 69 ion

[H₂C=CHCH(CH₃)₂]⁺  ===>  [C₅H₉]⁺  +  H

Loss of a proton from various sites on the molecule, C-H bond broken.

mass change 70-1 = 69 (M-1 ion peak)

Formation of m/z 55 and 56 ions

[H₂C=CHCH(CH₃)₂]⁺  ===>  [C₄H₇]⁺  +  CH₃

C-C bond scission in the parent molecular 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.

mass change 70 - 15 = 55 (M-15 ion peak)

The m/z 56 ion could be formed in the same way, but containing a ¹³C carbon isotope atom i.e. [¹³C¹²C₃H₇]⁺ rather than the [C₄H₈]⁺ ion.

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:

[¹³C¹²C₃H₇]⁺ = 56.0580, [C₄H₈]⁺ = 56.0624, a difference of 0.0044 in relative ion mass.

Formation of m/z 43 ion

[C₅H₁₀]⁺  or  [H₂C=CHCH(CH₃)₂]⁺  ===>  [C₃H₇]⁺  +  C₂H₃

C-C bond scission in the parent molecular ion.

mass change 70 - 27 = 43 (M-27 ion peak)

Formation of m/z 42 ion

[C₅H₁₀]⁺  or  [H₂C=CHCH(CH₃)₂]⁺  ===>  [C₃H₇]⁺  +  C₂H₄

mass change 70 - 28 = 42 (M-28 ion peak)

Formation of m/z 27 ion

[C₅H₁₀]⁺  or  [H₂C=CHCH(CH₃)₂]⁺  ===>  [C₂H₃]⁺  +  C₃H₇

C-C bond scission in the parent molecular ion.

mass change 70 - 43 = 27 (M-43 ion peak)

Formation of other smaller fragments

These arise directly from other fragmentations of the original molecular ion or larger fragment ions such as the m/z ions 69, 55 and 42 above.

Overview: 3-Methylbut-1-ene Structure

Molecular formula: C₅H₁₀
Molecular ion (M⁺): m/z = 70
Structure:
This is a branched alkene with a terminal double bond and two methyl groups.

Key Fragment Ions in the Mass Spectrum

Fragmentation Pathways

• α-cleavage near the double bond yields allylic ions (e.g., m/z 41).
• Loss of methyl group from the branched carbon gives m/z 55.
• Rearrangements may yield m/z 43 or 29 depending on stability.

Common Misconceptions

1. Assuming the molecular ion is always the base peak
   • In alkenes, the M⁺ peak (m/z 70) is often weak or absent due to instability.
2. Confusing m/z 41 with aromatic fragments
   • m/z 41 is common in alkenes due to the allyl cation, not aromatic systems.
3. Misidentifying m/z 43
   • This ion can arise from multiple sources (propyl, acylium, rearrangement). Context matters.
4. Overlooking rearrangements
   • Some fragments (e.g., m/z 43 or 29) may result from hydride shifts or methyl migrations.

Exam Revision Tips

•  Start with M⁺: Identify the molecular ion peak to determine the molar mass.
•  Look for m/z 41: Allyl cation is a strong indicator of alkene presence.
•  Use fragment differences: Subtract m/z values to deduce lost groups (e.g., 70 → 55 = loss of CH₃).
•  Sketch fragmentation: Draw the molecule and break bonds near functional groups.
•  Compare isomers: Practice with spectra of pent-1-ene, 2-methylbut-2-ene, etc., to spot differences.
•  Check base peak: The tallest peak is the most stable ion, often m/z 41 in alkenes.

The infrared spectrum of 3-methylbut-1-ene

The H-1 NMR spectrum of 3-methylbut-1-ene

The C-13 NMR spectrum of 3-methylbut-1-ene

The chemistry of ALKENES revision notes INDEX

Mass spectrometry index

ALL SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

Use My Google search site box

Email doc b: chem55555@hotmail.com