20.6
Formation radicale : ajout
Les radicaux peuvent être formés en ajoutant un radical à une molécule à spin apparié. Ceci est généralement observé avec les espèces insaturées, où l…
Lors de l’ajout d’un radical à travers la liaison π de spin-appariement, il génère un nouveau radical.
Par exemple, le radical brome interagit avec un alcène pour produire un radical au centre du carbone.
Le mécanisme de la réaction est basé sur la conservation du spin, similaire à la conservation de la charge dans une réaction chimique. Ainsi, un réactif avec un électron non apparié forme un produit avec un électron non apparié.
La réaction d’addition radicalaire la plus simple est le processus de réduction, où un électron est ajouté à une molécule appariée en spin.
Un exemple est la réduction de bouleau, où les métaux contenus dans l’ammoniac liquide génèrent des électrons qui réagissent avec les cétones pour former des radicaux cétyliques.
View the full transcript and gain access to JoVE Core videos
Q1: What happens when a radical adds to an alkene?
When a radical adds across the π bond of an alkene, it generates a new radical at a carbon center. For example, a bromine radical interacting with an alkene produces a carbon-centered radical. This process dissolves the π bond while preserving the unpaired electron, following spin conservation principles similar to charge conservation in chemical reactions.
Q2: How does spin conservation apply to radical addition reactions?
Spin conservation in radical reactions parallels charge conservation in traditional chemistry. A reactant with an unpaired electron must form a product with an unpaired electron. This fundamental principle ensures that when a radical adds to a spin-paired molecule, the resulting product retains an unpaired electron, maintaining the total spin state throughout the reaction.
Q3: What is the Birch reduction and how does it form radicals?
Birch reduction is a radical formation process where group 1 metals dissolve in liquid ammonia to generate electrons. These electrons react with ketones to form ketyl radicals, which are stable radical intermediates. This represents the simplest category of radical formation via addition, where a single electron is added to a spin-paired molecule.
Q4: Why is radical addition considered a reduction process?
Radical addition functions as reduction because a single electron is added to a spin-paired molecule. In processes like Birch reduction, metals generate electrons that add to unsaturated species such as ketones. This electron addition converts the spin-paired molecule into a radical species, making it the most facile category of radical formation through addition mechanisms.
Q5: What role do group 1 metals play in generating radicals?
Group 1 metals dissolve in liquid ammonia to produce electrons that initiate radical formation. These electrons add to unsaturated organic compounds, generating radical intermediates. The metal is oxidized to a stable M+1 ion during this process, making it an effective electron source for radical addition reactions in synthetic organic chemistry.
Q6: How does radical addition differ from traditional ionic addition to alkenes?
Radical addition involves unpaired electrons and follows spin conservation rather than charge-driven mechanisms. When a radical adds to an alkene's π bond, it produces a new radical at a carbon center. Unlike ionic additions, radical additions preserve unpaired electrons throughout the reaction and do not rely on carbocation intermediates or charge stabilization.
Q7: What types of molecules can undergo radical addition reactions?
Unsaturated species with π bonds readily undergo radical addition. Alkenes are common substrates where radicals like bromine add across the double bond to form carbon-centered radicals. Ketones also participate in radical addition, particularly in reduction processes like Birch reduction, where electrons add to the carbonyl π bond to generate ketyl radicals.