Mechanism of Halogenation of Alkanes

• The reaction doesn’t stop after one chlorination. 
• It is difficult to get monosubstituted chloromethane. Rather di-, tri- and even tetra- chloromethane are formed. 
• To avoid this difficulty, one must use a higher concentration of methane as compared to chloride.
• Initially, this free-radical chain reaction contains few free radicals and many molecules of reactants.
• As the reaction progresses, the number of free radicals increases with a decrease in the number of reactants.
• Nearing the end, we’ve many more free radicals than reactant molecules.
• Overall, the termination step becomes the predominant reaction.
• Altogether, halogenation mechanism reactions occur swiftly, and the formation of the products takes only microseconds.

How is halogenation controlled?

Halogenation of alkanes never stops at mono-substitution.

The chlorination of methane gives rise to dichloromethane, chloroform, and carbon tetrachloride but depending on the reaction.

By Controlling the concentration of halogen, mono , di , tri or tetra halogenated products are possible. 

There exists more than one possible product of hydrocarbons depending on the replacement of hydrogen. For example, Butane (CH3-CH2-CH2-CH2Cl) can be chlorinated at position 1 giving 1-chloro-butane (CH3-CH2-CH2-CH2Cl) or at position 2 giving 2- chloro-butane (CH3-CH2-CHCl-CH3). The product depends on reactive rate- position 2 of butane reacts swiftly giving the major product 2-chloro-butane.

Bromination>Chlorination>Fluorination (> means ‘less selective’).

Some other important points 

• Fluorination is least selective yet is highly exothermic and special attention is needed to avoid explosion or a runaway reaction. This relationship can be elucidated with the help of the Hammond postulate and is seen as a demonstration of the relativity-selectivity principle.
• The transition state of hydrogen abstraction has a radical character and is reached late while a bromine radical is not very reactive.
• The transition state of reactive chlorine radical resembles the reactant with radical character.
• The alkyl radical can benefit from any resonance stabilization and is fully formed in a transition state thereby maximizing selectivity.
• The selectivity of bromination is understood through Bond Dissociation Energies(BDEs). The BDE is the energy of a bond required to break it with the help of homolytic cleavage. It further determines if the reaction is exothermic or endothermic.
• No reaction between alkanes and iodine takes place as iodine does not react with alkane.
• No reaction between alkane and chlorine or bromine in the dark but if the light/heat is present then the required alkyl halides can be produced.

For the other halogens, free-radical halogenation takes place in the following order:

(> reacts faster than)

Carbons with one or more aryl substituents (benzylic positions) > Carbons with three alkyl substituents (tertiary positions) > Carbons with two alkyl substituents (secondary positions)>Carbons with one or zero substituents (primary positions).

Halogenation inhibitor- OXYGEN.

Halogenation of aromatic compounds typically works well for chlorine and bromine.

Aromatic compounds are subject to electrophilic halogenation.

RC6H5+X2 → HX+RC6H4X

Balz-Schiemann reaction-To prepares fluorinated aromatic compounds.

Conclusion

Alkanes undergo very few reactions. One is being halogenated by single hydrogen substituted for a single halogen forming haloalkane. The halogenation of alkanes takes place in the presence of photochemical conditions. The halogenation of alkanes mechanism is divided into 3 steps: Initiation, Propagation and Termination. The selectivity of bromination is understood through Bond Dissociation Energies(BDEs). The BDE is the energy of a bond required to break it with the help of homolytic cleavage. It further determines if the reaction is exothermic or endothermic.