15.4
Spettrometria di Massa: Frammentazione degli Alcheni
Gli alcheni perdono un elettrone dal legame π insaturo durante la ionizzazione e formano ioni molecolari stabili. Un'ulteriore frammentazione degli al…
Dopo la ionizzazione, un elettrone del legame pi greco degli alcheni viene spostato per formare ioni molecolari.
Come nel caso del 2-metil-1-pentene, il segnale di massa dello ione molecolare è prominente per gli alcheni.
La popolare via di frammentazione negli ioni molecolari alchenici è la scissione in posizione allilica poiché il frammento allilico è stabilizzato dalla risonanza.
Una frammentazione alternata si verifica in corrispondenza del legame carbonio-carbonio adiacente al legame insaturo, con conseguente carbocatione alchilico e i corrispondenti radicali alchilici.
Gli alcheni che hanno un γ idrogeno rispetto al doppio legame subiscono un riarrangiamento di McLafferty per produrre un catione radicale alchene a basso peso molecolare e un alchene.
È difficile distinguere gli isomeri posizionali e geometrici degli alcheni utilizzando i segnali di massa quasi identici.
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Q1: What happens to alkenes when they undergo ionization in mass spectrometry?
Upon ionization, one electron from the pi bond of alkenes is displaced to form stable molecular ions. Alkenes typically produce prominent mass signals from their molecular ions. This ionization step initiates fragmentation pathways that generate characteristic fragment ions used for structural identification.
Q2: Why is allylic cleavage the most common fragmentation pathway for alkenes?
Allylic cleavage is the dominant fragmentation pathway because the resulting allylic carbocation is stabilized by resonance. In terminal alkenes, this fragment appears at a mass-to-charge ratio of 41. For internal alkenes with two possible allylic cleavage sites, the pathway preferentially generates the more substituted alkyl radical as a co-product.
Q3: What fragmentation occurs at the carbon-carbon bond adjacent to the unsaturated bond?
Fragmentation at the carbon-carbon bond adjacent to the double bond produces an alkenyl carbocation and a corresponding alkyl radical. This pathway preferentially occurs to generate the more stable alkyl radical. For example, in 2-methyl-1-pentene, this fragmentation favors formation of the propenyl cation and propyl radicals.
Q4: How does McLafferty rearrangement differ from other alkene fragmentation pathways?
McLafferty rearrangement occurs when alkenes possess a gamma hydrogen with respect to the double bond. The gamma hydrogen transfers to the double bond in a cyclic six-membered ring, cleaving bonds between the alpha and beta carbons. This produces a lower molecular weight alkene radical cation and a neutral alkene, generating specific fragment ions for structural identification.
Q5: What are the three main fragmentation pathways for alkenes in mass spectrometry?
Alkenes fragment through three pathways: allylic cleavage producing resonance-stabilized allylic carbocations, cleavage at the carbon-carbon bond adjacent to the unsaturated bond yielding alkenyl carbocations, and McLafferty rearrangement for alkenes with gamma hydrogens. Each pathway generates distinct fragment ions that aid in structural analysis and compound identification.
Q6: Why is it difficult to distinguish alkene isomers using mass spectrometry?
Positional and geometrical isomers of alkenes produce almost identical mass signals, making them challenging to differentiate by mass spectrometry alone. The fragmentation patterns and molecular ion peaks are similar across isomers, requiring complementary analytical techniques or detailed spectroscopic analysis for definitive structural determination.
Q7: How does the stability of alkyl radicals influence alkene fragmentation selectivity?
Alkene fragmentation preferentially occurs through pathways that generate more stable alkyl radicals as co-products. For internal alkenes undergoing allylic cleavage, the more substituted alkyl radical forms preferentially. Similarly, fragmentation at the adjacent carbon-carbon bond favors production of the more stable alkyl radical, directing the fragmentation pathway.