Name: alcohol fragmentation patterns in mass spectra
Uploaded: 2024-12-05T18:32:20.000Z
Duration: 1 min 3 s
Channel: JoVE

Les alcools, lors de l’ionisation, perdent un électron non lié de l’oxygène pour former des ions moléculaires.
Cependant, comme le montre le spectre de masse du 1-butanol, les ions moléculaires des alcools présentent une intensité de pic très faible car ils se fragmentent facilement.
Les modèles de fragmentation de l’alcool couramment observés sont le clivage et la déshydratation.
Dans le clivage ⍺, une liaison C-C au carbone ⍺ à côté du groupe hydroxyle se clive pour former un cation stabilisé par résonance et un radical.
Alternativement, lors de la déshydratation, les alcools subissent une élimination intramoléculaire d’une molécule d’eau, produisant un cation radical alcène qui génère un pic à M−18.
Comme on le voit dans le spectre de masse du 1-butanol, le pic proéminent avec un rapport masse/charge de 56 correspond à la déshydratation.
Le pic de base à un rapport masse/charge de 31 correspond à ⍺ clivage.

spectrométrie de masse: fragmentation de l'alcool

Mass Spectrometry: Alcohol Fragmentation

Les alcools (R-OH) s'ionisent pour perdre un électron non lié de l'atome d'oxygène, formant ainsi des ions moléculaires. En raison de leur tendance à se fragmenter rapidement, l'intensité du pic d'ion moléculaire dans le spectre de masse est faible, voire absente. Les schémas de fragmentation des alcools se produisent de deux manières, à savoir le clivage ⍺ et la déshydratation. Au cours du clivage ⍺, la liaison en position ⍺ adjacente au groupe hydroxyle se brise pour donner un cation stabilisé par résonance et un radical. Cependant, la déshydratation intramoléculaire implique la perte d'une molécule d'eau d'un alcool, formant un cation alcène, qui présente un pic à M−18. Considérez la fragmentation du butanol comme indiqué ci-dessous. Le clivage ⍺ produit un pic de base avec un rapport masse/charge de 31. Dans le cas d'une déshydratation, la fragmentation forme un cation radical alcène avec un rapport masse/charge de 56.

<img alt="Figure1" src="/files/ftp_upload/13046/13046-PT_Figure_1.svg" style="height: 175px;">

Spectrométrie de masse: fragmentation de l'alcool

Les alcools (R-OH) s’ionisent pour perdre un électron non lié de l’atome d’oxygène, formant des ions moléculaires. En raison de leur tendance à se fragmenter rapidement, les ...

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Analytical Chemistry

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Mass Spectrometry Fragmentation Methods

Méthodes de fragmentation en spectrométrie de masse

KEY TERMS AND CONCEPTS

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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 radical.

Alcohols can also undergo intramolecular dehydration. In this process, the molecule loses water and forms an alkene cation. This appears as a peak at M−18 in the mass spectrum.

Butanol shows both of these patterns. β-cleavage gives a base peak at m/z 31. Dehydration produces an alkene radical cation at m/z 56.

Alcohol Fragmentation Patterns in Mass Spectra

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mass spectrometry: long-chain alkane fragmentation

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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

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Cycloalkane Ions and Mass Peaks

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Cycloalkene Side-Chain Cleavage in Mass Spectra

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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

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

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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

Using Reagent Gas in CI Mass Spectrometry

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electrospray ionization (esi) mass spectrometry

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key terms and concepts

Learn how alcohols fragment in mass spectrometry through β-cleavage and dehydration, and see why their molecular ion peaks are often weak or absent.

Alcohol Fragmentation Patterns in Mass Spectra | JoVE K12

analytical chemistry

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Alcohol Fragmentation Patterns in Mass Spectra

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