Name: cycloalkane ions and mass peaks
Uploaded: 2024-12-05T18:32:19.000Z
Duration: 1 min 5 s
Channel: JoVE

环烷烃分子离子比直链或支链烷烃分子离子更稳定。
例如，环己烷在质荷比为84处的分子离子信号强度高于正己烷的分子离子峰。
环己烷的分子离子裂解生成一个乙烯分子和一个新的自由基阳离子，其质荷比为56。该自由基阳离子相对稳定，是质谱中的基峰。
在支链环烷烃中，除了裂解生成乙烯和丁基自由基阳离子外，另一种裂解方式导致分子离子失去侧链。例如，甲基环戊烷分子离子失去其甲基侧链，生成环戊基阳离子。
环戊基正离子进一步发生断裂，生成一个乙烯分子和一个丙基自由基正离子。

质谱法：环烷烃裂解

Mass Spectrometry: Cycloalkane Fragmentation

在质谱法中，环烷烃表现出了独特的裂解模式，这是由于其分子离子与线性烷烃或支链烷烃相比具有固有的稳定性。环烷烃的环状结构为其分子离子提供了额外的稳定性，通常会在质谱中形成明显的离子峰。


 例如，环己烷分子离子的质荷比（m/z）为 84，这往往会产生比线性烷烃（例如正己烷）更加强烈的信号。这种稳定性通常来自闭环结构，它使得离子更加稳定。环己烷的常见裂解途径包括损失一个乙烯分子（C_2H_4），以此来留下质荷比为 56 的自由基阳离子。这种阳离子是非常稳定的，经常能够形成基峰，即在质谱中最为强烈的信号。


 <img alt="Figure1" src="/files/ftp_upload/13042/13042-PT_Figure_1.svg" style="height: 80px;">


 图 1. 环己烷分子离子的裂解。


 支链环烷烃（例如甲基环戊烷）由于存在侧链，表现出额外的裂解途径。除了典型的乙烯损失外，分子离子还可能失去侧链，如甲基（CH_3）。例如，分子离子可能失去甲基自由基（CH_3•），从而产生环戊基阳离子。这种环戊基阳离子进一步进行裂解，通常会导致乙烯损失，从而产生稳定的丙基自由基阳离子。


 这些裂解模式展示了环结构和分支如何对质谱仪中的离子稳定性产生影响。乙烯和侧链等小分子的特征损耗会产生高度稳定的阳离子，在光谱中可以观察到这些阳离子的突出的峰值。

<img alt="Figure2" src="/files/ftp_upload/13042/13042-PT_Figure_2_.svg" style="height: 180px;">

 图 2. 甲基环戊烷的裂解会产生：（顶部）乙烯和丁基自由基阳离子；（底部）甲基自由基和环戊基碳阳离子。随后其中的环戊基碳阳离子会进一步发生裂解。

在质谱分析中，环烷烃由于其分子离子相较于直链或支链烷烃具有更高的内在稳定性，因而表现出独特的碎裂模式。

mass-spectrometry-cycloalkane-fragmentation

Education

教育

JoVE Core

Analytical Chemistry

分析化学

Unit 1

第一单元

Mass Spectrometry Fragmentation Methods

质谱碎片化方法

KEY TERMS AND CONCEPTS

关键术语与概念

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, which makes the molecular ion more stable.

A common fragmentation path for cyclohexane is the loss of ethylene, C2H4. After this loss, a radical cation with an m/z of 56 remains. This ion is highly stable and often becomes the base peak, which is the most intense peak in the spectrum.

Branched cycloalkanes, such as methyl cyclopentane, can fragment in more than one way because of the side chain. Along with the usual loss of ethylene, the molecular ion may also lose a methyl group, CH3. This can form a cyclopentyl cation.

The cyclopentyl cation can fragment again. It usually loses ethylene and forms a stable propyl radical cation. These patterns show how ring size and branching affect ion stability. The loss of small molecules like ethylene and side chains often produces the most prominent peaks.

Cycloalkane Ions and Mass Peaks

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

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

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

See how cycloalkanes fragment in mass spectrometry, from the stable cyclohexane molecular ion to ethylene loss and peaks from branched rings.

Cycloalkane Ions and Mass Peaks | JoVE K12

analytical chemistry

JoVE Core Analytical Chemistry simplifies complex chemical principles by breaking them down into clear, engaging animations, making topics from SI Units to NMR spectroscopy more accessible and straightforward. Additionally, the scientist-in-action videos highlight the application of these concepts in real-world research conducted in laboratories globally.

Cycloalkane Ions and Mass Peaks

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