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

直鎖状アルカンでは、親分子から単一の電子が除去されることによって形成されるラジカルカチオンである分子イオンの断片化が、主に鎖の末端から離れて起こり、安定したカルボカチオンとラジカルを形成します。
たとえば、ヘキサンの質量スペクトルについて考えてみます。顕著なピークは14 u離れており、ヘキサンの異なる隣接する炭素-炭素結合で初期断片化の可能性を示しています。
末端炭素とその隣接炭素との間の断片化は、不安定なメチルカルボカチオンまたは生成されるメチルラジカルのために、実現可能性が低くなります。この断片化が発生すると、メチルカルボカチオンよりもメチルラジカルの形成が優先されます。
論理的には、最も実現可能な断片化は最も安定したカルボカチオンをもたらし、最も安定したラジカルを排出します。ヘキサン質量スペクトルでは、ブチルカルボカチオンが最も豊富で、ベースピークになります。
その後の水素損失を伴うカルボカチオンの断片化は、各顕著なピークの近くの小さなピークの原因です。

質量分析: 長鎖アルカンのフラグメンテーション

Mass Spectrometry: Long-Chain Alkane Fragmentation

直鎖アルカンの分子イオンは、鎖の末端から離れた炭素-炭素結合でフラグメンテーションが発生する傾向があります。これは、内部結合の切断によって安定したカルボカチオンと安定したラジカルが生成されるためです。その結果、直鎖アルカンの質量信号は、質量電荷比プロットの中央に強いピークが見られ、両端には弱いピークが見られます。各炭素-炭素結合のフラグメンテーションでは、各分裂でメチル基が放出されるため、質量スペクトルに 14 u 離れた顕著なピークが見られます。顕著なピークの近くにある質量信号が小さいのは、炭素-水素結合での二次フラグメンテーションの結果です。

たとえば、図 1 に示す n-ヘキサンのフラグメンテーションパターンと、図 2 の対応する質量スペクトルを考えてみましょう。

<img alt="Figure1" src="/files/ftp_upload/12299/12299-PT_Figure_1.svg" style="height: 500px;">

図 1: n-ヘキサン分子イオンのフラグメンテーション。

<img alt="Figure2" src="/files/ftp_upload/12299/12299-PT_Figure_2.svg" style="height: 275px;">

図 2: n-ヘキサンの質量スペクトル

質量電荷比 71、57、43、29、15 の信号は、分子内のすべての炭素-炭素結合で切断が発生する可能性があることを示しています。ただし、これらのピークの相対的な存在量は異なります。分子イオン信号と 71 の信号の違いは、メチル基 (M.W. = 15 u) が分子イオンから分離していることを示しています。そのフラグメント (M.W. = 14 u) からさらに CH_2 基が放出されると、57 の信号が発生します。43、29、15 の信号は、その後の CH_2 基の放出によるものです。15 と 71 の信号の存在量は、それぞれ不安定なメチルカルボカチオンまたはメチルラジカルの形成の可能性が最も低いため、最小限です。 43 のベースピークは、最も優先的なフラグメンテーションを示しており、安定したカルボカチオンとラジカルのペアが生成されます。

線状アルカンの分子イオンは、内部結合の切断以来、鎖の端から離れて炭素-炭素結合で断片化することを好む...

mass-spectrometry-long-chain-alkane-fragmentation

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

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

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。

Mass Spectra of Linear Alkane Chains

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