Name: alkene ion fragmentation patterns
Uploaded: 2024-12-05T18:32:19.000Z
Duration: 59 s
Channel: JoVE

Upon ionization, one electron from the pi bond of alkenes is displaced to form molecular ions.
As in the case of 2-methyl-1-pentene, the mass signal of the molecular ion is prominent for alkenes.
The popular fragmentation pathway in alkene molecular ions is cleavage at the allylic position since the allylic fragment is stabilized by resonance.
An alternate fragmentation occurs at the carbon-carbon bond adjacent to the unsaturated bond, resulting in alkenyl carbocations and the corresponding alkyl radicals.
Alkenes having a &#947; hydrogen with respect to the double bond undergo McLafferty rearrangement to yield a low molecular weight alkene radical cation and an alkene.
It is challenging to distinguish the positional and geometrical isomers of alkenes using the almost identical mass signals.

mass spectrometry: alkene fragmentation

Mass Spectrometry: Alkene Fragmentation

Alkenes lose one electron from the unsaturated &#960; bond upon ionization and form stable molecular ions. Further fragmentation of alkenes occurs through three different reaction pathways. The most prominent fragmentation is the cleavage at the allylic position. The resultant allylic carbocation is resonance stabilized. In the mass spectra of terminal alkenes, this fragment appears at a mass-to-charge ratio of 41. In the internal alkenes, where there are two choices of allylic cleavage, the cleavage preferentially generates a more substituted alkyl radical as a co-product. Figure 1 shows the fragmentation of 2-methyl-1-pentene at the allylic position. 
<img alt="Chemical reaction diagram; fragmentation pattern; mass spectrometry; m/z=84 to m/z=55 transition." src="/files/ftp_upload/13043/13043_PT_Figure_1.svg" style="text-align: center; height: 80px;" data-altupdatedat="1761830439000" data-alt="Figure1" />
Figure 1. Fragmentation of 2-methyl-1-pentene at the allylic position.
Another fragmentation pathway occurs at the carbon-carbon bond adjacent to the unsaturated bond, yielding an alkenyl carbocation and a corresponding alkyl radical. The fragmentation preferably occurs to generate a more stable alkyl radical. For example, the fragmentation of 2-methyl-1-pentene can occur on either side of the unsaturated bond, as shown in Figure 2. The mass signals of resultant alkenyl carbocations are shown in Figure 3. The relative intensity shows fragmentation leads to propenyl cation and propyl radicals are favored.
<img alt="Mass spectrometry ion fragmentation diagram, showing m/z ratios for structural analysis." src="/files/ftp_upload/13043/13043_PT_Figure_2.svg" style="text-align: center; height: 165px;" data-altupdatedat="1761830439000" data-alt="Figure2" />
Figure 2. Fragmentation of 2-methyl-1-pentene at the adjacent carbon of unsaturated bond.
<img alt="Mass spectrometry result; peak intensity vs. mass-to-charge ratio; molecular analysis chart." src="/files/ftp_upload/13043/13043_PT_Figure_3.svg" style="text-align: center; height: 135px;" data-altupdatedat="1761830439000" data-alt="Figure3" />
Figure 3. Mass spectrum of 2-methyl-1-pentene.
The alkenes possessing &#947; hydrogen with respect to the unsaturated bond undergo McLafferty rearrangement. The McLafferty rearrangement occurs when the &#947; hydrogen transfers to the double bond in a cyclic six-membered ring. This leads to the cleavage of bonds between the &#945;- and &#946;-carbons and thereby produces two fragments: a lower molecular weight alkene radical cation and a neutral alkene. This rearrangement, significant in mass spectrometry, helps generate specific fragment ions, aiding in identifying the structure of alkenes. Figure 4 shows the fragmentation of 2-methyl-1-pentene via McLafferty rearrangement.
<img alt="Chemical reaction diagram; mass spectrometry fragmentation; m/z ratios 84 to 56; structural formula." src="/files/ftp_upload/13043/13043_PT_Figure_4.svg" style="text-align: center; height: 92px;" data-altupdatedat="1761830439000" data-alt="Figure4" />
Figure 4. Fragmentation of 2-methyl-1-pentene via McLafferty rearrangement.

Alkenes lose one electron from the unsaturated &#960; bond upon ionization and form stable molecular ions. Further fragmentation of alkenes occurs through ...

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Analytical Chemistry

Unit 1

Mass Spectrometry Fragmentation Methods

KEY TERMS AND CONCEPTS

alkene ion fragmentation patterns

Alkene ion fragmentation patterns in mass spectrometry begin when the molecule loses one electron from the unsaturated π bond. This ionization forms a stable molecular ion. The ion can then break apart by three main reaction pathways.

The most common pathway is cleavage at the allylic position, which is the carbon next to the double bond. This produces an allylic carbocation that is stabilized by resonance. In terminal alkenes, this fragment appears at a mass-to-charge ratio of 41. In internal alkenes, there are two possible allylic cleavages, and the preferred one gives the more substituted alkyl radical as the other product.

A second pathway breaks the carbon-carbon bond adjacent to the unsaturated bond. This forms an alkenyl carbocation and a matching alkyl radical. The cleavage usually happens in the direction that gives the more stable alkyl radical. For 2-methyl-1-pentene, this can occur on either side of the double bond, and the spectrum shows signals that favor the propenyl cation and propyl radical products.

A third pathway is the McLafferty rearrangement, which occurs in alkenes that have a γ hydrogen relative to the unsaturated bond. In this step, the γ hydrogen transfers to the double bond through a cyclic six-membered ring. The rearrangement then breaks the bonds between the α- and β-carbons. It produces a lower molecular weight alkene radical cation and a neutral alkene, which helps identify alkene structure in mass spectrometry.

Alkene Ion Fragmentation Patterns

mass spectrometry: long-chain alkane fragmentation

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Mass Spectra of Linear Alkane Chains

Mass spectra of linear alkanes show a clear fragmentation pattern. The molecular ion often breaks at a carbon-carbon bond away from the end of the chain. This cleavage forms a stable carbocation and a stable radical, so the middle of the mass-to-charge plot usually has the strongest signals while the ends are weaker. As each bond breaks, a methyl group is lost and the main peaks in the spectrum appear 14 u apart. Smaller peaks near each main signal can also appear. These come from secondary...

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Branched Alkanes in Mass Spectra

Branched alkanes in mass spectrometry fragment quickly because branching can create stable carbocations. When cleavage happens at a branching point, a secondary or tertiary carbocation may form. That stability helps explain why the molecular ion peak is often very weak or missing compared with a linear alkane. The lesson uses 2-methylbutane, neopentane, and n-pentane to show how this pattern appears in mass spectra. 2-methylbutane and neopentane break apart to form secondary and tertiary...

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Cycloalkane Ions and Mass Peaks

Cycloalkane ions in mass spectrometry often give strong peaks because the ring structure helps stabilize the molecular ion. Compared with linear or branched alkanes, this added stability changes how the molecule breaks apart. As a result, cycloalkanes can produce prominent signals in the mass spectrum. Cyclohexane is a clear example. Its molecular ion has an m/z of 84, and this peak is often stronger than the signal from hexane, a linear alkane. The closed ring helps hold the ion together,...

Alkene ion fragmentation patterns in mass spectrometry begin when the molecule loses one electron from the unsaturated π bond. This ionization forms a stable molecular ion. The ion can then break apart by three main reaction pathways. The most common pathway is cleavage at the allylic position, which is the carbon next to the double bond. This produces an allylic carbocation that is stabilized by resonance. In terminal alkenes, this fragment appears at a mass-to-charge ratio of 41. In internal...

mass spectrometry: cycloalkene fragmentation

mass-spectrometry-cycloalkene-fragmentation

Cycloalkene Side-Chain Cleavage in Mass Spectra

Cycloalkenes can break apart in a mass spectrometer through retro-Diels–Alder fragmentation. In this process, two carbon-carbon bonds in the ring split, and ethene is released. The remaining fragment is a dienyl radical cation that is 28 u less than the molecular ion. This pathway is similar to the fragmentation of cycloalkanes that also releases ethene. The difference is the type of charged fragment that remains. Cycloalkanes form an alkyl species, while cycloalkenes form a dienyl species.

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Alkyne Mass Spectra and Key Peaks

Alkyne mass spectra often show strong peaks from two main fragmentation paths. One path breaks the carbon-carbon bond between the alpha and beta carbons of the alkyne bond. This produces a 3-propynyl cation, also called a propargyl cation. In terminal alkynes, this is the only fragmentation path that gives the 3-propynyl cation. The unsubstituted 3-propynyl cation appears at a mass-to-charge ratio of 39. In internal alkynes, the 3-propynyl cation is substituted. For example, 2-pentyne forms a...

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Alcohol Fragmentation Patterns in Mass Spectra

Alcohols fragment in mass spectrometry in predictable ways. When an alcohol, written as R-OH, ionizes, it can lose one non-bonded electron from the oxygen atom. That forms a molecular ion, but the molecular ion peak is often weak or may not appear because alcohols break apart quickly. Two main fragmentation routes are important for alcohols: β-cleavage and dehydration. In β-cleavage, the bond at the β-position next to the hydroxyl group breaks. This gives a resonance-stabilized cation and a...

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Mass Spectra of Benzene and Toluene

Mass spectra of aromatic compounds show a strong molecular ion peak. In ionization, the molecule becomes a molecular ion, which appears as a prominent peak in the spectrum. Benzene gives a molecular ion at m/z 78, and toluene gives one at m/z 92. Benzene’s molecular ion is especially stable. It does not fragment readily because breaking the aromatic benzene ring requires a large amount of energy. Alkyl-substituted benzenes behave differently in mass spectrometry. Toluene can lose a hydrogen...

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Amines in Mass Spectra: Nitrogen Rule

Amines can be identified in mass spectrometry by their fragmentation patterns and by the nitrogen rule. The molecular ions of amines often break apart through β-cleavage, which cuts a carbon-carbon bond next to the nitrogen. This process forms an alkyl radical and a resonance-stabilized nitrogen-containing cation. The nitrogen rule helps predict the mass of the molecular ion. A compound with one nitrogen atom, or any odd number of nitrogen atoms, gives a molecular ion with an odd molecular...

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Mass Spectra of Alkyl Halides

Mass spectra of alkyl halides show clear isotope patterns from the halogen atom. Chlorine has two common isotopes, 35Cl and 37Cl, in a 3:1 ratio. Bromine has two common isotopes, 79Br and 81Br, in a 1:1 ratio. These isotope ratios create two molecular ion peaks, [M] and [M + 2], with relative heights that match the halide abundance. This pattern is easy to see in examples such as 2-chloropropane and 1-bromopropane. 2-Chloropropane gives peaks in a 3:1 ratio. 1-Bromopropane gives peaks in a 1:1...

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Carbonyl Fragment Patterns in Mass Spectra

Aliphatic aldehydes and ketones break apart in mass spectrometry through a few key carbonyl fragment patterns. The main pathways are alpha-cleavage, inductive cleavage, and the McLafferty rearrangement. These patterns help chemists read the mass spectrum and spot a carbonyl group. Alpha-cleavage is a common break next to the carbonyl group. The bond beside the carbonyl splits, and this forms an alkyl radical and an acylium cation. The acylium cation is often seen in the spectrum because it is...

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Mass Spectra of Carboxylic Compounds

Mass spectra of carboxylic acids, esters, and amides often show two key fragmentation routes: β-cleavage and McLafferty rearrangement. In β-cleavage, the carbon-carbon bond at the β-position next to the carboxylic group breaks. This produces a neutral radical and a cation. Long-chain compounds can also fragment by McLafferty rearrangement if they have a hydrogen on the γ-carbon. The γ-carbon is the third carbon away from the carboxylic group. This rearrangement gives a radical cation and a...

chemical ionization (ci) mass spectrometry

Using Reagent Gas in CI Mass Spectrometry

Chemical ionization mass spectrometry uses a reagent gas to help detect molecules that break apart too quickly in electron impact ionization. It is a milder ionization method that can extend the life of the ionized analyte, which makes molecular identification easier. In chemical ionization, the sample is mixed with an excess reagent gas. This setup makes the electron impact occur mainly on the reagent gas instead of on the analyte. The charged species formed from the reagent then protonates...

electrospray ionization (esi) mass spectrometry

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How ESI Turns Biomolecules Into Ions

Electrospray ionization, or ESI, is a mass spectrometry method used to detect biomolecules that are too large or nonvolatile for conventional electron impact ionization. These molecules may break down before they can ionize or evaporate. ESI helps prevent rapid fragmentation and lets scientists record a mass signal for the whole biomolecule. ESI uses electrical energy to move ions from the liquid phase into the gas phase. The analyte biomolecule is mixed with an ionic liquid, which is a...

matrix-assisted laser desorption ionization (maldi)

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How MALDI Turns Samples Into Ions

Matrix-assisted laser desorption ionization (MALDI) is a mass spectrometry technique that turns biomolecules into ions for analysis. It is used to identify and characterize proteins, peptides, nucleic acids, and carbohydrates. Because of this, it is common in biological and medical research, as well as pharmacology and biochemistry. In MALDI, the analyte of interest is mixed with a matrix material. The matrix is usually an organic compound, such as a small organic acid or a crystalline...

key terms and concepts

Learn how alkene ions break apart in mass spectrometry through allylic cleavage, adjacent bond cleavage, and McLafferty rearrangement for structure clues.

Alkene Ion Fragmentation Patterns | JoVE K12

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Alkene Ion Fragmentation Patterns

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