free radical mechanism of halogenation of alkyl halides, a vital concept in organic chemistry. This post simplifies the three-step mechanism: initiation, propagation, and termination. Learn how different halogens react, their selectivity patterns, and tips for controlling reaction outcomes. Whether you’re a student or an enthusiast, these tips and tricks will help you understand and apply this mechanism effectively.
Main Reaction
General Reaction:
R−H+X2→ R-X + HX (heat or UV)
Where:
- R−H: Alkane
- X2: Halogen (e.g., Cl2, Br2)
- R−X: Alkyl halide
Example: Chlorination of methane:
CH4+Cl2→ CH3Cl + HCl
Mechanism: Free Radical Halogenation
This reaction proceeds through a free radical chain mechanism in three key steps:
1. Initiation
- Halogen molecule absorbs energy (heat/UV), splitting into two halogen free radicals.
Cl2→Cl. + Cl. - The bond cleavage is homolytic, producing highly reactive radicals.
2. Propagation
- Step 1: A halogen radical abstracts a hydrogen atom from the alkane, forming an alkyl radical.
CH4+Cl⋅→CH3⋅+HCl - Step 2: The alkyl radical reacts with another halogen molecule, producing an alkyl halide and regenerating the halogen radical.
CH3⋅+Cl2→CH3Cl+Cl⋅
3. Termination
- Two radicals combine to terminate the chain reaction.
Cl⋅+Cl⋅→Cl2
CH3⋅+Cl⋅→CH3Cl
CH3⋅+CH3⋅→C2H6
Tips and Tricks for the Mechanism
- Radical Stability Order:
Tertiary > Secondary > Primary > Methyl radicals.- More substituted alkyl radicals are more stable, favoring halogenation at tertiary carbons.
- Reactivity of Halogens:
- Cl2: Reacts vigorously, less selective, forms multiple products.
- Br2: Slower and more selective, prefers substitution at more stable radicals.
- Controlling Products:
- Use excess alkane to minimize polyhalogenation.
- Use excess halogen for multiple substitutions.
- Temperature Effect:
- Higher temperatures favor radical formation and increase reaction rate.
- UV Light:
- Necessary for initiating the reaction by breaking the halogen bond.
- Selectivity Rule:
- Bromination is more selective than chlorination due to differences in activation energy for hydrogen abstraction.
Detailed Insights on Selectivity in Halogenation
The selectivity of halogenation depends on the halogen used and the type of hydrogen being replaced. Here’s a detailed breakdown:
1. Reactivity and Selectivity of Halogens
- Chlorination (Cl₂):
- Reactivity: High, leading to faster reactions.
- Selectivity: Poor; less discrimination between types of hydrogens (e.g., primary vs. secondary vs. tertiary).
- Tends to produce multiple products unless controlled conditions are applied.
- Bromination (Br₂):
- Reactivity: Moderate, slower than chlorination.
- Selectivity: High; bromine prefers hydrogen attached to the most stable carbon radical (tertiary > secondary > primary).
- Iodination (I₂):
- Generally not favored due to low reactivity and a highly endothermic nature.
- Needs special conditions, like oxidizing agents, to proceed.
- Fluorination (F₂):
- Extremely reactive and often explosive. Rarely used in laboratory settings.
2. Factors Influencing Selectivity
Radical Stability
- Tertiary radicals are most stable due to hyperconjugation and inductive effects, making them preferred sites for halogenation in bromination.
Energy Considerations
- Chlorination:
- Lower activation energy differences among primary, secondary, and tertiary hydrogens.
- Produces a mix of products.
- Bromination:
- Higher activation energy differences, favoring reactions at tertiary carbons due to stability.
4. Controlling Polyhalogenation
Polyhalogenation (multiple substitutions) can be controlled by:
- Using an excess of alkane relative to the halogen.
- Limiting the amount of halogen added.
- Carefully controlling reaction conditions like temperature and UV exposure.
5. Selectivity Ratio
For bromination, the relative reactivity of hydrogens is much higher for tertiary hydrogens:
- Tertiary:Secondary:Primary Reactivity Ratio (approx.):
- Chlorination: 5:4:1
- Bromination: 1700:80:1
Key Takeaways
- Chlorination is suitable for fast, broad reactions but often yields multiple products.
- Bromination is ideal when targeting specific products due to its high selectivity.
- Reaction conditions and halogen choice dictate product formation and yield.
