Sign In

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 °C, it undergoes allylic chlorination, forming 3‐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‐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.

Tags

Radical SubstitutionAllylic ChlorinationAlkenesHalogensAddition ReactionResonance StabilizationChain MechanismInitiation StepPropagation StepTermination StepChlorine MoleculeAllylic Radical IntermediateAllyl Chloride

From Chapter 20:

article

Now Playing

20.19 : Radical Substitution: Allylic Chlorination

Radical Chemistry

2.1K Views

article

20.1 : الجذار: الهيكل الإلكتروني والهندسة

Radical Chemistry

3.8K Views

article

20.2 : التحليل الطيفي للرنين المغناطيسي الإلكتروني (EPR): الجذور العضوية

Radical Chemistry

2.3K Views

article

20.3 : التشكيل الجذري: نظرة عامة

Radical Chemistry

2.0K Views

article

20.4 : التكوين الجذري: التجانس

Radical Chemistry

3.4K Views

article

20.5 : التشكيل الجذري: التجريد

Radical Chemistry

3.3K Views

article

20.6 : التشكيل الجذري: الإضافة

Radical Chemistry

1.6K Views

article

20.7 : التشكيل الجذري: القضاء

Radical Chemistry

1.6K Views

article

20.8 : التفاعل الجذري: نظرة عامة

Radical Chemistry

1.9K Views

article

20.9 : التفاعل الجذري: التأثيرات الفراغية

Radical Chemistry

1.8K Views

article

20.10 : التفاعل الجذري: تأثيرات التركيز

Radical Chemistry

1.5K Views

article

20.11 : التفاعل الجذري: الجذور المحبة للكهرباء

Radical Chemistry

1.8K Views

article

20.12 : التفاعل الجذري: الجذور المحبة للنواة

Radical Chemistry

2.0K Views

article

20.13 : التفاعل الجذري: داخل الجزيئات مقابل الجزيئات

Radical Chemistry

1.7K Views

article

20.14 : الأكسدة الجذرية

Radical Chemistry

2.1K Views

See More

JoVE Logo

Privacy

Terms of Use

Policies

Research

Education

ABOUT JoVE

Copyright © 2025 MyJoVE Corporation. All rights reserved