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

Lors de l’ionisation, un électron de la liaison pi des alcènes est déplacé pour former des ions moléculaires.
Comme dans le cas du 2-méthyl-1-pentène, le signal de masse de l’ion moléculaire est proéminent pour les alcènes.
La voie de fragmentation populaire dans les ions moléculaires d’alcène est le clivage en position allylique, car le fragment allylique est stabilisé par résonance.
Une fragmentation alternée se produit au niveau de la liaison carbone-carbone adjacente à la liaison insaturée, ce qui donne des carbocations alcényles et les radicaux alkyles correspondants.
Les alcènes ayant un hydrogène γ par rapport à la double liaison subissent un réarrangement de McLafferty pour produire un cation radical alcène de faible poids moléculaire et un alcène.
Il est difficile de distinguer les isomères de position et géométriques des alcènes à l’aide de signaux de masse presque identiques.

spectrométrie de masse: fragmentation des alcènes

Mass Spectrometry: Alkene Fragmentation

Les alcènes perdent un électron de la liaison π insaturée lors de l'ionisation et forment des ions moléculaires stables. Une fragmentation supplémentaire des alcènes se produit par trois voies de réaction différentes. La fragmentation la plus importante est le clivage en position allylique. Le carbocation allylique résultant est stabilisé par résonance. Dans les spectres de masse des alcènes terminaux, ce fragment apparaît à un rapport masse/charge de 41. Dans les alcènes internes, où il existe deux choix de clivage allylique, le clivage génère préférentiellement un radical alkyle plus substitué comme coproduit. La figure 1 montre la fragmentation du 2-méthyl-1-pentène en position allylique.

<img alt="Figure1" src="/files/ftp_upload/13043/13043_PT_Figure_1.svg" style=" height: 80px;">

Figure 1. Fragmentation du 2-méthyl-1-pentène en position allylique.

Une autre voie de fragmentation se produit au niveau de la liaison carbone-carbone adjacente à la liaison insaturée, produisant un carbocation alcényle et un radical alkyle correspondant. La fragmentation se produit de préférence pour générer un radical alkyle plus stable. Par exemple, la fragmentation du 2-méthyl-1-pentène peut se produire de chaque côté de la liaison insaturée, comme illustré dans la Figure 2. Les signaux de masse des carbocations alcényles résultants sont présentés dans la Figure 3. L'intensité relative montre que la fragmentation conduit au cation propényle et que les radicaux propyles sont favorisés.

<img alt="Figure2" src="/files/ftp_upload/13043/13043_PT_Figure_2.svg" style=" height: 165px;">

Figure 2. Fragmentation du 2-méthyl-1-pentène au niveau du carbone adjacent à la liaison insaturée.

<img alt="Figure3" src="/files/ftp_upload/13043/13043_PT_Figure_3.svg" style=" height: 135px;">

Figure 3. Spectre de masse du 2-méthyl-1-pentène.

Les alcènes possédant un hydrogène γ par rapport à la liaison insaturée subissent un réarrangement de McLafferty. Le réarrangement de McLafferty se produit lorsque l'hydrogène γ est transféré à la double liaison dans un cycle à six chaînons. Cela conduit au clivage des liaisons entre les carbones α et β et produit ainsi deux fragments : un cation radical alcène de poids moléculaire inférieur et un alcène neutre. Ce réarrangement, important en spectrométrie de masse, permet de générer des ions fragments spécifiques, aidant ainsi à identifier la structure des alcènes. La figure 4 montre la fragmentation du 2-méthyl-1-pentène via le réarrangement de McLafferty.

<img alt="Figure4" src="/files/ftp_upload/13043/13043_PT_Figure_4.svg" style=" height: 92px;">

Figure 4. Fragmentation du 2-méthyl-1-pentène via le réarrangement de McLafferty.

Spectrométrie de masse: fragmentation des alcènes

Les alcènes perdent un électron de la π insaturée se lient lors de l’ionisation et forment des ions moléculaires stables. Une fragmentation supplémentaire des alcènes se produit par ...

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Analytical Chemistry

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Mass Spectrometry Fragmentation Methods

Méthodes de fragmentation en spectrométrie de masse

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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...

mass spectrometry: branched alkane fragmentation

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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,...

mass spectrometry: alkene fragmentation

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

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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.

mass spectrometry: alkyne fragmentation

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 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

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Carbonyl Fragment Patterns in Mass Spectra

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mass spectrometry: carboxylic acid, ester, and amide fragmentation

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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

analytical chemistry

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Alkene Ion Fragmentation Patterns

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