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

イオン化すると、アルケンのπ結合から1つの電子が置換されて分子イオンが形成されます。
2-メチル-1-ペンテンの場合と同様に、分子イオンの質量シグナルはアルケンで顕著です。
アルケン分子イオンの一般的なフラグメンテーション経路は、アリルフラグメントが共鳴によって安定化されるため、アリリック位置での切断です。
不飽和結合に隣接する炭素-炭素結合で代替の断片化が起こり、アルケニルカルボカチオンとそれに対応するアルキルラジカルが生じます。
二重結合に対してγ水素を有するアルケンは、マクラファティ転位を受けて低分子量のアルケンラジカルカチオンおよびアルケンを生成する。
アルケンの位置異性体と幾何学的異性体を、ほぼ同一の質量シグナルで区別することは困難です。

質量分析：アルケンのフラグメンテーション

Mass Spectrometry: Alkene Fragmentation

アルケンはイオン化時に不飽和 π 結合から電子を 1 つ失い、安定した分子イオンを形成します。アルケンのさらなるフラグメンテーションは、3 つの異なる反応経路を通じて発生します。最も顕著なフラグメンテーションは、アリル位での開裂です。結果として生じるアリルカルボカチオンは共鳴安定化されます。末端アルケンの質量スペクトルでは、このフラグメントは質量電荷比 41 で現れます。アリル開裂の選択肢が 2 つある内部アルケンでは、開裂により副産物としてより置換されたアルキルラジカルが優先的に生成されます。図1は、アリル位での 2 -メチル- 1 - ペンテンのフラグメンテーションを示しています。

<img alt="Figure1" src="/files/ftp_upload/13043/13043_PT_Figure_1.svg" style=" height: 80px;">

図1.アリル位での2-メチル-1-ペンテンのフラグメンテーション。

もう 1 つのフラグメンテーション経路は、不飽和結合に隣接する炭素-炭素結合で発生し、アルケニルカルボカチオンと対応するアルキルラジカルを生成します。フラグメンテーションは、より安定したアルキルラジカルを生成するために発生するのが望ましいです。たとえば、2 -メチル- 1 - ペンテンのフラグメンテーションは、図 2 に示すように、不飽和結合のどちら側でも発生する可能性があります。結果として生じるアルケニルカルボカチオンの質量信号は、図 3 に示されています。相対的な強度から、フラグメンテーションによってプロペニルカチオンが生成され、プロピルラジカルが優先されることがわかります。

<img alt="Figure2" src="/files/ftp_upload/13043/13043_PT_Figure_2.svg" style=" height: 165px;">

図 2. 不飽和結合の隣接炭素での 2-メチル-1-ペンテンのフラグメンテーション。

<img alt="Figure3" src="/files/ftp_upload/13043/13043_PT_Figure_3.svg" style=" height: 135px;">

図 3. 2-メチル-1-ペンテンの質量スペクトル。

不飽和結合に関して γ 水素を持つアルケンは、マクラファティ転位を受けます。マクラファティ転位は、環状 6 員環の二重結合に γ 水素が転移するときに起こります。これにより、α 炭素と β 炭素の間の結合が切断され、2 つのフラグメント (低分子量アルケンラジカルカチオンと中性アルケン) が生成されます。質量分析において重要なこの転位は、特定のフラグメントイオンを生成するのに役立ち、アルケンの構造の特定に役立ちます。図 4 は、マクラファティ転位による 2 -メチル- 1 - ペンテンのフラグメンテーションを示しています。

<img alt="Figure4" src="/files/ftp_upload/13043/13043_PT_Figure_4.svg" style=" height: 92px;">

Figure 4. Fragmentation of 2-methyl-1-pentene via McLafferty rearrangement.

図 4. マクラファティ転位による 2-メチル-1-ペンテンのフラグメンテーション。

アルケンは、イオン化時に不飽和π結合から電子を1つ失い、安定した分子イオンを形成します。アルケンのさらなる断片化は、...

Education

JoVE Core

Analytical Chemistry

分析化学

Unit 1

ユニット1

Mass Spectrometry Fragmentation Methods

質量分析におけるフラグメンテーション法

KEY TERMS AND CONCEPTS

主要用語と概念

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

mass-spectrometry-long-chain-alkane-fragmentation

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

mass-spectrometry-branched-alkane-fragmentation

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...

mass spectrometry: cycloalkane fragmentation

mass-spectrometry-cycloalkane-fragmentation

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

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.

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...

mass spectrometry: alcohol fragmentation

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...

mass spectrometry: aromatic compound fragmentation

mass-spectrometry-aromatic-compound-fragmentation

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...

mass spectrometry: amine fragmentation

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...

mass spectrometry: alkyl halide fragmentation

mass-spectrometry-alkyl-halide-fragmentation

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...

mass spectrometry: aldehyde and ketone fragmentation

mass-spectrometry-aldehyde-and-ketone-fragmentation

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...

mass spectrometry: carboxylic acid, ester, and amide fragmentation

mass-spectrometry-carboxylic-acid-ester-and-amide-fragmentation

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

electrospray-ionization-esi-mass-spectrometry

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)

matrix-assisted-laser-desorption-ionization-maldi

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

JoVE Core 分析化学 は、複雑な化学原理をアニメーションで視覚化することで、SI 単位から NMR 分光法を含むトピックをよりわかりやすくします。実験を撮影した実写動画では、これらの概念が世界中の研究所でどのように応用されているかを取り上げています。

Alkene Ion Fragmentation Patterns

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