20.19
Radical Substitution: Allylic Chlorination
Typically, when alkenes react with halogens at low temperatures, an addition reaction occurs. However, upon increasing the temperature or under reacti…
Generally, alkenes undergo an electrophilic addition reaction when treated with halogens at low temperatures.
However, allylic substitution reactions occur at low halogen concentrations, and under radical conditions such as high temperatures or in the presence of radical initiators.
For example, propene undergoes allylic chlorination when reacted with chlorine at 400 oC in the gas phase.
Allylic hydrogen gets substituted 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.
Allylic chlorination follows a chain mechanism, including initiation, propagation, and termination steps.
In the initiation step, the chlorine molecule dissociates into two highly reactive chlorine atoms.
In the first propagation step, the chlorine atom abstracts allylic hydrogen, leading to the formation of an allylic radical intermediate.
In the second propagation step, the allylic radical reacts with a chlorine molecule to form an allyl chloride and a chlorine atom, which further propagates the reaction.
Finally, the reaction gets terminated by coupling any two radicals, thereby depleting the reactive intermediates.
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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.