Name: mass spectra of linear alkane chains
Uploaded: 2024-12-05T18:32:19.000Z
Duration: 1 min 18 s
Channel: JoVE

В линейных алканах фрагментация молекулярных ионов, которые представляют собой радикальные катионы, образующиеся путем удаления одного электрона из родительской молекулы, преимущественно происходит вдали от конца цепи с образованием стабильных карбокатионов и радикалов.
Например, рассмотрим массовый спектр гексана. Выступающие пики разделены 14 u, что указывает на возможность начальной фрагментации на разных соседних углерод-углеродных связях в гексане.
Фрагментация между концевым углеродом и соседним углеродом менее вероятна из-за нестабильного метилкарбокатиона, или образующегося метильного радикала. Если происходит такая фрагментация, образование метилового радикала имеет приоритет над метилкарбокатионом.
Логика подсказывает, что наиболее возможная фрагментация приводит к наиболее стабильному карбокатиону, вытесняя наиболее устойчивый радикал. В масс-спектрах гексана бутилкарбокатион наиболее распространен и становится пиком основания.
Последующая фрагментация карбокатионов с потерей водорода ответственна за небольшие пики вблизи каждого выступающего пика.

Масс-спектрометрия: фрагментация длинноцепочечных алканов

Mass Spectrometry: Long-Chain Alkane Fragmentation

Молекулярные ионы линейных алканов предпочитают фрагментироваться по связи углерод-углерод вдали от конца цепи, поскольку разрыв внутренней связи создает стабильный карбокатион и стабильный радикал. Следовательно, массовые сигналы линейных алканов имеют интенсивные пики в середине графика отношения массы к заряду с более слабыми пиками на обоих концах. Фрагментация каждой связи углерод-углерод с высвобождением метильной группы при каждом расщеплении приводит к появлению выраженных пиков в масс-спектрах, разделенных 14 а.е.м. Меньшие массовые сигналы вблизи каждого выраженного пика возникают из-за вторичной фрагментации по связи углерод-водород.

Например, рассмотрим картину фрагментации н-гексана, показанную на рисунке 1, и соответствующие масс-спектры на рисунке 2.

<img alt="Figure1" src="/files/ftp_upload/12299/12299-PT_Figure_1.svg" style="text-align: center; height: 500px;">

Рисунок 1: Фрагментация молекулярного иона н-гексана.

<img alt="Figure2" src="/files/ftp_upload/12299/12299-PT_Figure_2.svg" style="text-align: center; height: 275px;">

Рисунок 2: Масс-спектр н-гексана

Сигналы при соотношении массы к заряду 71, 57, 43, 29 и 15 указывают на то, что расщепление может происходить по всем связям углерод-углерод в молекуле. Однако, относительное содержание этих пиков варьируется. Разница между сигналом молекулярного иона и сигналом при 71 указывает на отделение метильных групп (М.В. = 15 а.е.м.) от молекулярных ионов. Дальнейшее высвобождение группы CH_2 из этого фрагмента (М.В. = 14 а.е.м.) приводит к сигналу при значении 57. Сигналы при 43, 29 и 15 обусловлены последующим высвобождением групп CH_2. Содержание сигналов при 15 и 71 минимально из-за наименьшей вероятности образования нестабильного метилкарбокатиона или метильного радикала, соответственно. Базовый пик при 43 указывает на наиболее предпочтительную фрагментацию, которая дает пару стабильного карбокатиона и радикала.

Молекулярные ионы линейных алканов предпочитают фрагментироваться в углерод-углеродной связи вдали от конца цепи, поскольку расщепление внутренней связи ...

mass-spectrometry-long-chain-alkane-fragmentation

Education

JoVE Core

Analytical Chemistry

Аналитическая химия

Unit 1

Раздел 1

Mass Spectrometry Fragmentation Methods

Методы фрагментации в масс-спектрометрии

KEY TERMS AND CONCEPTS

ОСНОВНЫЕ ТЕРМИНЫ И ПОНЯТИЯ

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 fragmentation at a carbon-hydrogen bond.

n-Hexane shows this pattern well. Its spectrum has signals at mass-to-charge ratios of 71, 57, 43, 29, and 15. These peaks show that cleavage can happen at every carbon-carbon bond in the molecule, but the peak heights are not the same.

The difference between the molecular ion signal and the signal at 71 shows loss of a methyl group with a mass of 15 u. Loss of another CH2 group with a mass of 14 u gives the signal at 57. The signals at 43, 29, and 15 come from later CH2 losses.

The peaks at 15 and 71 are small because forming a methyl carbocation or a methyl radical is least likely. The base peak at 43 is the most favored fragmentation. It gives the most stable pair of carbocation and radical.

Mass Spectra of Linear Alkane Chains

mass spectrometry: long-chain alkane fragmentation

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

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 linear alkanes fragment in mass spectrometry, why peaks appear 14 u apart, and how n-hexane shows the pattern in its spectrum.

Mass Spectra of Linear Alkane Chains | JoVE K12

analytical chemistry

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Mass Spectra of Linear Alkane Chains

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