15.8
Espectrometría de masas: fragmentación de compuestos aromáticos
Al ionizarse, los compuestos aromáticos generan un ion molecular que se observa como un pico destacado en sus espectros de masas. Por ejemplo, el pico…
Los compuestos aromáticos, al ionizarse, forman iones moleculares.
El ion molecular del benceno no se fragmenta extensamente, lo que requiere una enorme energía.
Por el contrario, los iones moleculares de los bencenos alquilsustituidos, como el tolueno, se fragmentan en el carbono bencílico, perdiendo un átomo de hidrógeno para generar un carbocatión de bencilo estabilizado por resonancia.
El catión bencilo se reorganiza aún más a un ion tropilio más estable, que exhibe un fuerte pico en los espectros de masas.
Alternativamente, los iones moleculares de los alquilbencenos con grupos alquilo más grandes se fragmentan a través de la escisión de la cadena lateral para formar inicialmente un catión bencilo, que luego se reorganiza a un ion tropilio.
Si la cadena lateral tiene tres o más carbonos y al menos un hidrógeno en el carbono γ, se produce un reordenamiento de McLafferty, detectado en una relación masa-carga de 92.
Los compuestos polialquilados se fragmentan y pierden un átomo de hidrógeno para formar un ion de metiltropilio, que da un pico medio.
La pérdida de un grupo metilo forma el ion tropilio.
En particular, los isómeros de dichos anillos disustituidos tienen espectros de masas idénticos. Por lo tanto, la espectrometría de masas no puede determinar los patrones de sustitución de bencenos polialquilados.
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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.