20.7
Formação de Radicais: Eliminação
Outro método de formação de radicais é o processo de eliminação. É o oposto da via de adição e é impulsionado pela instabilidade do radical. Por exemp…
A formação de radicais através do processo de eliminação é o reverso mecanicista da reação de adição radical. Ocorre devido a um radical instável.
Por exemplo, o peróxido de dibenzoíla sofre homólise para formar um radical, que é instável. Assim, ele sofre eliminação para gerar um radical fenil e dióxido de carbono.
O local do elétron desemparelhado no radical é a posição α. Durante a reação de eliminação, o radical instável cliva a ligação carbono-carbono σ na posição β para produzir um radical estável.
Assim, forma-se uma ligação dupla entre as posições α e β, resultando em dois fragmentos de produto - uma espécie radical e uma molécula insaturada.
View the full transcript and gain access to JoVE Core videos
Q1: How does radical formation via elimination differ from radical addition?
Radical elimination is the mechanistic reverse of radical addition. While addition builds new bonds, elimination cleaves existing bonds to form radicals. Elimination occurs when an unstable radical breaks a carbon-carbon σ bond at the β position relative to the unpaired electron, generating a stable radical and an unsaturated molecule as products.
Q2: What role does radical instability play in the elimination process?
Radical instability drives the elimination reaction. When an unstable radical forms, it spontaneously undergoes elimination to achieve greater stability. For example, dibenzoyl peroxide generates unstable radicals upon homolysis, which then eliminate via C-C bond cleavage to form a more stable phenyl radical and carbon dioxide.
Q3: Where does the unpaired electron sit during radical elimination?
The unpaired electron occupies the α position of the radical. During elimination, the radical cleaves the carbon-carbon σ bond at the β position relative to this α site. This cleavage creates a double bond between the α and β positions, producing a stable radical and an unsaturated molecule.
Q4: What products form when dibenzoyl peroxide undergoes radical elimination?
Dibenzoyl peroxide homolysis produces unstable radicals that eliminate to yield a phenyl radical and carbon dioxide. The phenyl radical represents the stable radical product, while carbon dioxide is the unsaturated byproduct. This transformation demonstrates how radical instability triggers bond cleavage and product formation.
Q5: How does the α-β position relationship determine radical elimination outcomes?
The α position holds the unpaired electron, while the β position contains the carbon-carbon σ bond targeted for cleavage. Breaking the α-β bond generates a double bond between these positions and releases two fragments: a stable radical and an unsaturated molecule. This positional relationship is fundamental to the elimination mechanism.
Q6: What is the relationship between radical elimination and unsaturated molecule formation?
Radical elimination creates unsaturated molecules as byproducts through α-β bond cleavage. When the carbon-carbon σ bond breaks between the α and β positions, a π bond forms between these carbons, generating an alkene or similar unsaturated species alongside the stable radical product.
Q7: Why is radical elimination considered the reverse of radical anti-markovnikov addition to alkenes?
Elimination and radical anti-markovnikov addition to alkenes are mechanistic opposites. Addition combines a radical with an unsaturated molecule to form new bonds, while elimination breaks bonds to regenerate radicals and unsaturated molecules. Both processes involve the same intermediates but proceed in opposite directions.