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

Tras la ionización, un electrón del enlace pi de los alquenos se desplaza para formar iones moleculares.
Al igual que en el caso del 2-metil-1-penteno, la señal de masa del ion molecular es prominente para los alquenos.
La vía de fragmentación popular en los iones moleculares de alqueno es la escisión en la posición alílica, ya que el fragmento alílico se estabiliza por resonancia.
Se produce una fragmentación alterna en el enlace carbono-carbono adyacente al enlace insaturado, lo que da lugar a carbocationes de alquenilo y los radicales alquilo correspondientes.
Los alquenos que tienen un γ hidrógeno con respecto al doble enlace se someten a un reordenamiento de McLafferty para producir un catión radical alqueno de bajo peso molecular y un alqueno.
Es difícil distinguir los isómeros posicionales y geométricos de los alquenos utilizando señales de masa casi idénticas.

espectrometría de masas: fragmentación de alquenos.

Mass Spectrometry: Alkene Fragmentation

Los alquenos pierden un electrón del enlace π insaturado al ionizarse y forman iones moleculares estables. La fragmentación posterior de los alquenos se produce a través de tres vías de reacción diferentes. La fragmentación más destacada es la escisión en la posición alílica. El carbocatión alílico resultante se estabiliza por resonancia. En los espectros de masas de los alquenos terminales, este fragmento aparece con una relación masa-carga de 41. En los alquenos internos, donde hay dos opciones de escisión alílica, la escisión genera preferentemente un radical alquilo más sustituido como coproducto. La figura 1 muestra la fragmentación del 2-metil-1-penteno en la posición alílica.

<img alt="Figure1" src="/files/ftp_upload/13043/13043_PT_Figure_1.svg" style=" height: 80px;">

Figura 1. Fragmentación del 2-metil-1-penteno en la posición alílica.

Otra vía de fragmentación ocurre en el enlace carbono-carbono adyacente al enlace insaturado, lo que produce un carbocatión de alquenilo y un radical alquilo correspondiente. La fragmentación ocurre preferiblemente para generar un radical alquilo más estable. Por ejemplo, la fragmentación de 2-metil-1-penteno puede ocurrir en cualquier lado del enlace insaturado, como se muestra en la Figura 2. Las señales de masa de los carbocationes de alquenilo resultantes se muestran en la Figura 3. La intensidad relativa muestra que la fragmentación conduce al catión propenilo y se favorecen los radicales propilo.

<img alt="Figure2" src="/files/ftp_upload/13043/13043_PT_Figure_2.svg" style=" height: 165px;">

Figura 2. Fragmentación de 2-metil-1-penteno en el carbono adyacente del enlace insaturado.

<img alt="Figure3" src="/files/ftp_upload/13043/13043_PT_Figure_3.svg" style=" height: 135px;">

Figura 3. Espectro de masas de 2-metil-1-penteno.

Los alquenos que poseen hidrógeno γ con respecto al enlace insaturado experimentan una reorganización de McLafferty. El reordenamiento de McLafferty ocurre cuando el hidrógeno γ se transfiere al doble enlace en un anillo cíclico de seis miembros. Esto conduce a la ruptura de los enlaces entre los carbonos α y β y, por lo tanto, produce dos fragmentos: un catión radical alqueno de menor peso molecular y un alqueno neutro. Este reordenamiento, significativo en la espectrometría de masas, ayuda a generar iones de fragmentos específicos, lo que ayuda a identificar la estructura de los alquenos. La Figura 4 muestra la fragmentación del 2-metil-1-penteno mediante el reordenamiento de McLafferty.

<img alt="Figure4" src="/files/ftp_upload/13043/13043_PT_Figure_4.svg" style=" height: 92px;">

Figura 4. Fragmentación del 2-metil-1-penteno mediante el reordenamiento de McLafferty.

Espectrometría de Masas: fragmentación de Alquenos.

Los alquenos pierden un electrón del enlace π insaturado al ionizarse y forman iones moleculares estables. Una mayor fragmentación de los alquenos se produce a través de ...

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Analytical Chemistry

Química Analítica

Unit 1

Unidad 1

Mass Spectrometry Fragmentation Methods

Métodos de fragmentación en espectrometría de masas

KEY TERMS AND CONCEPTS

TÉRMINOS Y CONCEPTOS CLAVE

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 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,...

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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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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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

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Carbonyl Fragment Patterns in Mass Spectra

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Mass Spectra of Carboxylic Compounds

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chemical ionization (ci) mass spectrometry

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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...

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How MALDI Turns Samples Into Ions

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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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