20.10
Réactivité radicalaire : effets de concentration
Dans une réaction radicalaire, la concentration des matières premières détermine la sélectivité d’un radical. Par exemple, la réaction entre un halogé…
La concentration des matériaux de départ affecte considérablement la sélectivité des réactions radicalaires.
Considérons la réaction entre un halogénure d’alkyle et un alcène. En présence d’hydrure de tributylétain et d’AIBN, un initiateur radicalaire, le groupe alkyle est ajouté à l’alcène.
En règle générale, la réaction commence par la génération d’un radical d’étain, qui fait ensuite abstraction de l’halogène de l’halogénure d’alkyle, produisant un radical alkyle.
Le radical alkyle a maintenant le choix. Il peut soit extraire de l’hydrogène de l’hydrure d’étain produisant un alcane, soit l’ajouter à l’alcène en formant un radical stabilisé au nitrile, qui réagit davantage pour former le produit d’addition.
En effet, les constantes de vitesse des deux réactions sont à peu près les mêmes.
Pour stimuler la réaction vers le produit d’ajout, la concentration d’alcène doit toujours être au moins 10 fois supérieure à celle de l’hydrure d’étain.
Cela augmenterait la vitesse à laquelle le radical alkyle s’ajoute à l’alcène.
En conclusion, la concentration des matériaux de départ régit la sélectivité des réactions radicalaires en favorisant une réaction par rapport à l’autre.
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Q1: How does alkene concentration affect radical reaction selectivity?
Alkene concentration directly governs which product forms in radical reactions. When alkene concentration is at least 10 times higher than tin hydride, the alkyl radical preferentially adds to the alkene rather than abstracting hydrogen from the tin hydride. This concentration ratio increases the reaction rate for alkene addition, favoring formation of the addition product over the alkane.
Q2: What happens when an alkyl radical encounters both tin hydride and an alkene?
The alkyl radical faces two competing pathways: it can abstract hydrogen from tin hydride to form an alkane, or add to the alkene to form a nitrile-stabilized radical that yields the addition product. Both reactions have similar rate constants, making the outcome dependent on the relative concentrations of these reagents rather than inherent reactivity differences.
Q3: Why are rate constants important in controlling radical reaction outcomes?
Rate constants determine how quickly competing reactions proceed. In this system, the rate constants for alkyl radical addition to alkene and hydrogen abstraction from tin hydride are approximately equal. This means concentration becomes the controlling factor—adjusting reagent ratios shifts which reaction dominates without changing the intrinsic reactivity of either pathway.
Q4: What is the role of tributyltin hydride in radical reactions with alkyl halides?
Tributyltin hydride serves dual roles: it generates the initial tin radical that abstracts halogen from the alkyl halide, and it provides a hydrogen source for competing side reactions. The tin radical initiates the reaction sequence, while the hydride itself can trap alkyl radicals, reducing selectivity toward the desired addition product unless its concentration is carefully controlled.
Q5: How does AIBN function in this radical reaction system?
AIBN acts as a radical initiator by undergoing homolysis to generate free radicals that activate the tin hydride. Once activated, the tin radical begins the catalytic cycle by abstracting halogen from the alkyl halide. AIBN's role is to jumpstart the reaction sequence, enabling the subsequent radical transformations.
Q6: What is a nitrile-stabilized radical and how does it form?
A nitrile-stabilized radical forms when an alkyl radical adds to an alkene. The resulting intermediate radical is stabilized by electron-withdrawing groups or resonance effects. This stabilized radical then undergoes further reaction to generate the final addition product, representing the desired outcome when alkene concentration is optimized.
Q7: What concentration ratio maximizes addition product formation in radical reactions?
Maintaining alkene concentration at least 10 times higher than tin hydride concentration maximizes addition product formation. This ratio increases the probability that alkyl radicals encounter alkene molecules before encountering tin hydride, thereby shifting the reaction selectivity toward the desired addition product and away from unwanted alkane formation.