Question 1

Why does allylic chlorination occur instead of addition when propene reacts with chlorine?

Accepted Answer

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.

Question 2

What is the initiation step in the allylic chlorination chain mechanism?

Accepted Answer

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.

Question 3

How do the propagation steps work in allylic chlorination?

Accepted Answer

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.

Question 4

What conditions favor allylic substitution over electrophilic addition in alkene halogenation?

Accepted Answer

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.

Question 5

Why is the allylic radical intermediate stable in this reaction?

Accepted Answer

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.

Question 6

How does the termination step stop the allylic chlorination reaction?

Accepted Answer

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.

Question 7

What product forms when propene undergoes allylic chlorination at 400 °C?

Accepted Answer

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.

Radikalsubstitution: Allylische Chlorierung

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