20.19
Sostituzione radicale: clorurazione allilica
Tipicamente, quando gli alcheni reagiscono con gli alogeni a basse temperature, si verifica una reazione di addizione. Tuttavia, aumentando la tempera…
Generalmente, gli alcheni subiscono una reazione di addizione elettrofila quando vengono trattati con alogeni a basse temperature.
Tuttavia, le reazioni di sostituzione allilica si verificano a basse concentrazioni di alogeni e in condizioni radicaliche come alte temperature o in presenza di iniziatori radicalici.
Ad esempio, il propene subisce una clorurazione allilica quando reagisce con il cloro a 400 oC in fase gassosa.
L'idrogeno allilico viene sostituito perché il legame C-H allilico sp3 è più debole del legame C-H vinilico sp2.
Inoltre, l'intermedio radicale allilico formato è stabilizzato per risonanza.
La clorurazione allilica segue un meccanismo a catena, che comprende le fasi di inizio, propagazione e terminazione.
Nella fase di iniziazione, la molecola di cloro si dissocia in due atomi di cloro altamente reattivi.
Nella prima fase di propagazione, l'atomo di cloro astrae l'idrogeno allilico, portando alla formazione di un radicale allilico intermedio.
Nella seconda fase di propagazione, il radicale allilico reagisce con una molecola di cloro per formare un cloruro di allile e un atomo di cloro, che propaga ulteriormente la reazione.
Infine, la reazione viene terminata accoppiando due radicali qualsiasi, esaurendo così gli intermedi reattivi.
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Q1: Why does allylic chlorination occur instead of addition when propene reacts with chlorine?
Allylic chlorination occurs under radical conditions (high temperature or radical initiators) at low halogen concentrations, favoring substitution over addition. The allylic hydrogen is more reactive because the sp3 allylic C–H bond is weaker than the sp2 vinylic C–H bond. Additionally, the formed allylic radical intermediate is resonance stabilized, making substitution thermodynamically favorable.
Q2: What is the initiation step in the allylic chlorination chain mechanism?
In the initiation step, the chlorine molecule dissociates into two highly reactive chlorine atoms. This step generates the radical species needed to begin the chain reaction. High temperatures or radical initiators provide the energy required for this homolytic cleavage of the Cl–Cl bond.
Q3: How do the propagation steps work in allylic chlorination?
The first propagation step occurs when a chlorine atom abstracts an allylic hydrogen, forming a resonance-stabilized allylic radical intermediate. In the second propagation step, this allylic radical reacts with a chlorine molecule to form allyl chloride and regenerate a chlorine atom, which continues the chain reaction.
Q4: What conditions favor allylic substitution over electrophilic addition in alkene halogenation?
Allylic substitution is favored at high temperatures, in the presence of radical initiators, and at low halogen concentrations. These conditions promote radical formation and propagation. In contrast, electrophilic addition occurs at low temperatures with high halogen concentrations, following a different reaction pathway.
Q5: Why is the allylic radical intermediate stable in this reaction?
The allylic radical intermediate is resonance stabilized because the unpaired electron can delocalize across the allylic carbon and the adjacent pi system. This resonance stabilization lowers the energy of the intermediate, making its formation favorable and contributing to the overall reactivity of allylic hydrogens in radical substitution reactions.
Q6: How does the termination step stop the allylic chlorination reaction?
The termination step occurs when any two radicals couple together to form non-radical products, depleting the reactive intermediates and stopping the chain reaction. Common termination reactions include chlorine atom coupling or recombination of allylic radicals with chlorine atoms, effectively ending the propagation cycle.
Q7: What product forms when propene undergoes allylic chlorination at 400 °C?
When propene is treated with chlorine in the gas phase at 400 °C, it undergoes allylic chlorination to form 3-chloropropene, also called allyl chloride. The chlorine atom substitutes the allylic hydrogen on propene, producing this characteristic product of radical substitution reactions under high temperature conditions.