15.8
Espectrometria de Massa: Fragmentação de Compostos Aromáticos
Após a ionização, os compostos aromáticos geram um íon molecular que é observado como um pico proeminente em seus espectros de massa. Por exemplo, o p…
Os compostos aromáticos, após a ionização, formam íons moleculares.
O íon molecular do benzeno não se fragmenta extensivamente, exigindo uma enorme energia.
Pelo contrário, íons moleculares de benzenos substituídos por alquil, como o tolueno, fragmentam-se no carbono benzílico, perdendo um átomo de hidrogênio para gerar um carbocátion benzílico estabilizado por ressonância.
O cátion benzila ainda se reorganiza para um íon tropílio mais estável, que exibe um forte pico nos espectros de massa.
Alternativamente, os íons moleculares de alquilbenzenos com grupos alquila maiores se fragmentam por clivagem da cadeia lateral para formar inicialmente um cátion benzila, que então se reorganiza em um íon tropílio.
Se a cadeia lateral tiver três ou mais carbonos e pelo menos um hidrogênio no carbono γ, ocorre um rearranjo de McLafferty, detectado em uma relação massa-carga de 92.
Os compostos polialquilados se fragmentam e perdem um átomo de hidrogênio para formar um íon metiltropílio, que dá um pico médio.
A perda de um grupo metil forma o íon tropílio.
Notavelmente, os isômeros de tais anéis dissubstituídos têm espectros de massa idênticos. Portanto, a espectrometria de massa não pode determinar os padrões de substituição de benzenos polialquilados.
View the full transcript and gain access to JoVE Core videos
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.