Intermediate Topic 770 of 1102 Organic Chemistry
Free Radical Halogenation
Alkanes react with halogens under light via initiation, propagation and termination steps.
In-depth explanation of Free Radical Halogenation
Free radical halogenation replaces hydrogen atoms in an alkane with halogen atoms, and it proceeds through three stages. Initiation uses light or heat to split the halogen molecule into two radicals, X2 giving 2X. Propagation then alternates between a halogen radical abstracting a hydrogen to form HX and an alkyl radical, and that alkyl radical reacting with X2 to form the haloalkane and a new halogen radical, which continues the chain. Termination occurs when two radicals combine. Bromination is more selective than chlorination because the bromine radical is less reactive and abstracts the most stable radical site preferentially.
Going deeper: Free Radical Halogenation
Key ideas: Chain steps: initiation (X2 to 2X by light), propagation (X plus RH gives HX plus R, then R plus X2 gives RX plus X), termination. Selectivity depends on halogen: bromine is highly selective (tertiary to primary about 1600 to 1) while chlorine is not (about 5 to 1 per hydrogen at room temperature). Product ratio equals statistical count times relative reactivity. Fluorine is violent, iodine does not react.
Common mistakes and connections: Students predict chlorination gives only the most stable radical product; it gives a mixture. Another error is expecting no multiple halogenation; use excess alkane for monosubstitution. Links to radical stability, the Hammond postulate (bromination has a late transition state) and allylic bromination with NBS.
Visualization of Free Radical Halogenation
Bromine radicals are far more selective than chlorine: tertiary C-H is attacked about 1600 times faster than primary versus only about 5 times for chlorine. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Methane reacts with chlorine under ultraviolet light to give a mixture including CH3Cl and CH2Cl2.
Frequently asked questions about Free Radical Halogenation
What is Free Radical Halogenation?
Alkanes react with halogens under light via initiation, propagation and termination steps.
Is Free Radical Halogenation a beginner, intermediate or advanced topic?
Free Radical Halogenation is a intermediate topic in Organic Chemistry. It builds on the vocabulary of beginner topics in this area.
What should I learn before Free Radical Halogenation?
Study these earlier topics in Organic Chemistry first: Homolytic and Heterolytic Fission, Electrophiles and Nucleophiles, Reaction Types in Organic Chemistry.
What comes after Free Radical Halogenation?
The next topics in Organic Chemistry are Combustion of Hydrocarbons and Cracking.
Can I practise Free Radical Halogenation with quizzes?
Yes. The interactive lesson includes a self-check question on Free Radical Halogenation, and the Organic Chemistry quiz draws on every topic in this area.
Where Free Radical Halogenation fits in Organic Chemistry
Free Radical Halogenation is topic 42 of 148 in Organic Chemistry.