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

Bei der Ionisation wird ein Elektron aus der pi-Bindung von Alkenen verdrängt, um molekulare Ionen zu bilden.
Wie im Fall von 2-Methyl-1-penten ist auch bei Alkenen das Massensignal des Molekülions ausgeprägt.
Der beliebte Fragmentierungsweg bei Alken-Molekülionen ist die Spaltung an der allylischen Position, da das allylische Fragment durch Resonanz stabilisiert wird.
Eine abwechselnde Fragmentierung tritt an der Kohlenstoff-Kohlenstoff-Bindung auf, die an die ungesättigte Bindung angrenzt, was zu Alkenylcarbokationen und den entsprechenden Alkylresten führt.
Alkene, die einen γ Wasserstoff in Bezug auf die Doppelbindung aufweisen, durchlaufen eine McLafferty-Umlagerung, um ein Alkenradikalkation mit niedrigem Molekulargewicht und ein Alkene zu erhalten.
Es ist schwierig, die positionellen und geometrischen Isomere von Alkenen anhand der nahezu identischen Massensignale zu unterscheiden.

massenspektrometrie: alkenfragmentierung

Mass Spectrometry: Alkene Fragmentation

Alkene verlieren bei Ionisierung ein Elektron aus der ungesättigten π-Bindung und bilden stabile Molekülionen. Die weitere Fragmentierung von Alkenen erfolgt über drei verschiedene Reaktionswege. Die markanteste Fragmentierung ist die Spaltung an der Allylposition. Das resultierende allylische Carbokation ist resonanzstabilisiert. In den Massenspektren terminaler Alkene erscheint dieses Fragment bei einem Masse-zu-Ladung-Verhältnis von 41. Bei den internen Alkenen, bei denen es zwei Möglichkeiten der allylischen Spaltung gibt, erzeugt die Spaltung vorzugsweise ein stärker substituiertes Alkylradikal als Nebenprodukt. Abbildung 1 zeigt die Fragmentierung von 2-Methyl-1-penten an der Allylposition.

<img alt="Figure1" src="/files/ftp_upload/13043/13043_PT_Figure_1.svg" style=" height: 80px;">

Abbildung 1. Fragmentierung von 2-Methyl-1-penten an der Allylposition.

Ein weiterer Fragmentierungspfad findet an der Kohlenstoff-Kohlenstoff-Bindung neben der ungesättigten Bindung statt und führt zu einem Alkenylcarbokation und einem entsprechenden Alkylradikal. Die Fragmentierung erfolgt vorzugsweise, um ein stabileres Alkylradikal zu erzeugen. Beispielsweise kann die Fragmentierung von 2-Methyl-1-penten auf beiden Seiten der ungesättigten Bindung erfolgen, wie in Abbildung 2 gezeigt. Die Massensignale der resultierenden Alkenylcarbokationen sind in Abbildung 3 dargestellt. Die relative Intensität zeigt, dass die Fragmentierung zu einem Propenylkation führt und Propylradikale bevorzugt werden.

<img alt="Figure2" src="/files/ftp_upload/13043/13043_PT_Figure_2.svg" style=" height: 165px;">

Abbildung 2. Fragmentierung von 2-Methyl-1-penten am benachbarten Kohlenstoff der ungesättigten Bindung.

<img alt="Figure3" src="/files/ftp_upload/13043/13043_PT_Figure_3.svg" style=" height: 135px;">

Abbildung 3. Massenspektrum von 2-Methyl-1-penten.

Die Alkene, die in Bezug auf die ungesättigte Bindung γ-Wasserstoff besitzen, unterliegen einer McLafferty-Umlagerung. Die McLafferty-Umlagerung tritt auf, wenn der γ-Wasserstoff auf die Doppelbindung in einem zyklischen Sechsring übertragen wird. Dies führt zur Spaltung von Bindungen zwischen den α- und β-Kohlenstoffen und erzeugt dadurch zwei Fragmente: ein Alkenradikalkation mit niedrigerem Molekulargewicht und ein neutrales Alken. Diese Umlagerung, die in der Massenspektrometrie von Bedeutung ist, trägt zur Erzeugung spezifischer Fragmentionen bei und hilft bei der Identifizierung der Struktur von Alkenen. Abbildung 4 zeigt die Fragmentierung von 2-Methyl-1-penten durch McLafferty-Umlagerung.

<img alt="Figure4" src="/files/ftp_upload/13043/13043_PT_Figure_4.svg" style=" height: 92px;">

Abbildung 4. Fragmentierung von 2-Methyl-1-penten durch McLafferty-Umlagerung.

Massenspektrometrie: Alkenfragmentierung

Alkene verlieren bei der Ionisation ein Elektron aus der ungesättigten π Bindung und bilden stabile Molekülionen. Eine weitere Fragmentierung der Alkene erfolgt durch ...

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Mass Spectrometry Fragmentation Methods

Fragmentierungsmethoden der Massenspektrometrie

KEY TERMS AND CONCEPTS

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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

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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...

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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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