Name: allylic chlorination by radical chain reaction
Uploaded: 2023-04-30T11:22:13.000Z
Duration: 1 min 31 s
Channel: JoVE

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&#8211;H bond is weaker than the sp2 vinylic C&#8211;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&#160;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.

radical substitution: allylic chlorination

Radical Substitution: Allylic Chlorination

Typically, when alkenes react with halogens at low temperatures, an addition reaction occurs. However, upon increasing the temperature or under reaction conditions that form radicals, providing a low but steady concentration of halogen radicals, allylic substitution reaction is favored. This is because allylic hydrogens are very reactive as the formed intermediate is resonance stabilized. For example, when propene is treated with chlorine in the gas phase at 400 &#176;C, it undergoes allylic chlorination, forming 3&#8208;chloropropene.
The radical substitution reaction of allylic chlorination follows a chain mechanism similar to alkane halogenations and involves initiation, propagation, and termination steps. The initiation step involves the dissociation of the chlorine molecule into two chlorine atoms. The propagation step involves two chain-propagation steps. In the first chain-propagation step, the chlorine atom abstracts the allylic hydrogen atom, forming a resonance stabilized allylic&#8208;radical intermediate. In the second chain-propagation step, the allyl radical intermediate reacts with a chlorine molecule, forming an allyl chloride and a chlorine atom. The chlorine atom formed in the second chain-propagation step further abstracts allylic hydrogen and propagates the reaction. In the termination step, the radicals react with each other to form non-radical products and stop the reaction.&#160;

Typically, when alkenes react with halogens at low temperatures, an addition reaction occurs. However, upon increasing the temperature or under reaction ...

radical-substitution-allylic-chlorination

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JoVE Core

Organic Chemistry

Unit 1

Radical Chemistry

KEY TERMS AND CONCEPTS

allylic chlorination by radical chain reaction

Allylic chlorination is a radical chain reaction in alkenes. It happens when halogen reactions move away from simple addition and toward substitution at the allylic position, which is the carbon next to a double bond.

At low temperatures, alkenes usually react with halogens by addition. When the temperature increases, or when the reaction conditions create radicals, allylic substitution becomes favored. This works because allylic hydrogens are very reactive and the intermediate formed is resonance stabilized.

Propene gives a clear example. When propene reacts with chlorine in the gas phase at 400 °C, it forms 3-chloropropene. The reaction follows the same chain pattern seen in alkane halogenation.

The chain mechanism has three parts: initiation, propagation, and termination. In initiation, the chlorine molecule splits into two chlorine atoms. In propagation, one chlorine atom removes an allylic hydrogen and forms a resonance-stabilized allyl radical. That allyl radical then reacts with another chlorine molecule to make allyl chloride and a new chlorine atom.

The new chlorine atom keeps the chain going by abstracting another allylic hydrogen. In termination, radicals react with each other and form non-radical products. This stops the reaction.

Allylic Chlorination by Radical Chain Reaction

allylic-chlorination-by-radical-chain-reaction

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Allylic chlorination is a radical chain reaction in alkenes. It happens when halogen reactions move away from simple addition and toward substitution at the allylic position, which is the carbon next to a double bond. At low temperatures, alkenes usually react with halogens by addition. When the temperature increases, or when the reaction conditions create radicals, allylic substitution becomes favored. This works because allylic hydrogens are very reactive and the intermediate formed is...

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HBr Selectivity in Alkene Addition Thermodynamics

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key terms and concepts

Explore allylic chlorination in alkenes, including radical chain steps, resonance-stabilized intermediates, and the example of propene forming 3-chloropropene.

Allylic Chlorination by Radical Chain Reaction | JoVE K12

organic chemistry

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Allylic Chlorination by Radical Chain Reaction

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