15.8
Spectrométrie de masse: fragmentation des composés aromatiques
Lors de l'ionisation, les composés aromatiques génèrent un ion moléculaire qui est observé comme un pic important dans leurs spectres de masse. Par ex…
Les composés aromatiques, lors de l’ionisation, forment des ions moléculaires.
L'ion moléculaire du benzène ne se fragmente pas beaucoup, ce qui nécessite une énergie énorme.
Au contraire, les ions moléculaires des benzènes substitués par des alkyles, comme le toluène, se fragmentent au carbone benzylique, perdant un atome d’hydrogène pour générer un carbocation benzyle stabilisé par résonance.
Le cation benzyle se réorganise ensuite en un ion tropylium plus stable, qui présente un fort pic dans les spectres de masse.
Alternativement, les ions moléculaires des alkylbenzènes avec des groupes alkyle plus grands se fragmentent par clivage de la chaîne latérale pour former initialement un cation benzyle, qui se réorganise ensuite en un ion tropylium.
Si la chaîne latérale a trois carbones ou plus et au moins un hydrogène sur le carbone γ, un réarrangement de McLafferty se produit, détecté à un rapport masse/charge de 92.
Les composés polyalkylés se fragmentent et perdent un atome d’hydrogène pour former un ion méthyltropylium, ce qui donne un pic moyen.
La perte d’un groupe méthyle forme l’ion tropylium.
Notamment, les isomères de ces cycles disubstitués ont des spectres de masse identiques. Ainsi, la spectrométrie de masse ne peut pas déterminer les modèles de substitution des benzènes polyalkylés.
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Q1: Why does benzene's molecular ion not fragment in mass spectrometry?
Benzene's molecular ion is highly stable due to its aromatic ring structure and requires enormous energy to disrupt. The aromatic stability of the benzene ring prevents extensive fragmentation, resulting in a prominent molecular ion peak at mass-to-charge ratio 78 without significant loss of fragments.
Q2: What happens when toluene undergoes fragmentation in mass spectrometry?
Toluene's molecular ion fragments by losing a hydrogen atom at the benzylic carbon, forming a benzylic carbocation. This carbocation rearranges to a resonance-stabilized tropylium ion, which appears as a strong peak at mass-to-charge ratio 91 in the mass spectra of alkyl-substituted benzenes.
Q3: How does the McLafferty rearrangement occur in alkylbenzene fragmentation?
The McLafferty rearrangement occurs when alkylbenzenes have side chains with three or more carbons and at least one hydrogen on the gamma carbon. This rearrangement is detected at a mass-to-charge ratio of 92 and represents a characteristic fragmentation pathway for these larger alkyl-substituted aromatic compounds.
Q4: What is the difference between benzyl cation and tropylium ion formation?
When alkylbenzenes fragment, they initially form a benzylic carbocation through cleavage at the benzylic carbon or side chain. This carbocation then rearranges to a more stable tropylium ion, which exhibits greater resonance stabilization and produces a strong peak in mass spectra.
Q5: Why can't mass spectrometry determine substitution patterns on polyalkylated benzenes?
Isomers of polyalkylated benzene rings produce identical mass spectra because they fragment through the same pathways, forming methyltropylium ions and tropylium ions regardless of substitution position. This loss of structural information makes positional isomers indistinguishable by mass spectrometry alone, limiting structural determination.
Q6: What fragmentation products form from polyalkylated benzenes?
Polyalkylated benzenes fragment and lose a hydrogen atom to form a methyltropylium ion, which gives a medium peak in mass spectra. Further loss of one methyl group produces the tropylium ion, representing sequential fragmentation of the alkyl substituents on the aromatic ring.
Q7: How do aromatic compounds compare to other functional groups in fragmentation behavior?
Aromatic compounds like benzene show remarkable stability upon ionization, contrasting sharply with other functional groups. While alkyl-substituted aromatics fragment predictably through benzylic cleavage and tropylium formation, this behavior differs from mass spectrometry aldehyde and ketone fragmentation patterns and other organic compound classes.