Profile
Settings
Refer your friends
Sign out
Substitution reactions are the swapping of one atom with another in a compound. This kind of swapping helps to form new compounds of practical importance. In halogenations, atoms are swapped with halogens like chlorine, fluorine, iodine, and bromine. Free radical halogenations are a kind of halogenations, where one free radical is formed with the product, which undergoes further halogenations or is used in the recombination of free radicals. Let us learn about halogen substitution, free radicals, and free radical halogenations.
Halogen Substitution
Halogenations or halogen substitution is merely a substitution of an atom with halogen. They can be free radical halogenations, ketone halogenations, or halogen addition reactions.
Free Radicals
Free radicals are atoms or molecules with unpaired electrons. They react with other atoms or molecules with unpaired electrons to form compounds. Halogen-free radicals’ addition to saturated hydrocarbons is known as free radical halogenations.
Free Radical Halogenations
To perform halogen substitution in saturated hydrocarbons requires an accessible radical pathway. Saturated hydrocarbons are hydrocarbons with single bonds between carbon atoms like propane.
Steps of Free Radical Halogenations
Free radical halogenation has three steps to proceed :-
- Initiation
- propagation
- Termination.
Let’s understand the concept with the classical example itself- chlorination of methane.
Free radical halogenations of saturated hydrocarbons move under UV light. There must be an external source of energy to start initiation.
Initiation
Due to an external energy source, chlorine splits and forms two chlorine atoms during initiation.
Cl-Cl →Cl ● + Cl●
Propagation
In propagation, one chlorine substitutes one hydrogen for methane. The other chlorine is bound to expel hydrogen and form hydrochloric acid.
CH₄ + Cl● → CH₃ ⁻ + HCl
CH₃⁻ + Cl-Cl → CH₃Cl + Cl●
Termination
All remaining radicals combine to form either more product or more reactant. A combination of two methyl radicals produces another side product.
Cl● + Cl● → Cl-Cl
CH₃⁻ + Cl-Cl →CH₃Cl + Cl●
CH₃⁻ + CH₃⁻ →CH₃-CH₃
- The main problem with the chlorination of methane is once it starts to propagate, it is difficult to control. We may obtain dichloroethane, trichloroethane, and tetrachloroethane more than simple chloroethane. Using large methane concentrations can help to manage this problem.
Reactivity of Halogens in Halogenations
- Fluorine is highly reactive.
- Bromination of methane forms the primary product when a large bromine concentration is added.
- Iodine doesn’t form anything even if we use large quantities of iodine.
- Bromination is less endothermic than chlorination. Therefore it is more selective to halogenations. Iodine reactions are completely endothermic. Thus, no reaction occurs.
Reactivity
F2 > Cl2 > Br2 > I2.
Free radical halogenation usually takes place in the gaseous or liquid phase. In the liquid phase, peroxides facilitate reactions as a radical initiator. In the gaseous phase, oxygen behaves as a radial trap and inhibits the reaction.
Free radical halogenation reactivity based on compounds
Carbons with one or more aryl substituents > Carbons with three alkyl substituents > Carbons with three alkyl substituents > Carbons with two alkyl substituents > Carbons with one or zero substituents.
Conclusion
Substitution reactions are the swapping of one atom with another in a compound. Atoms are swapped with halogens like chlorine, fluorine iodine, and bromine in halogenations. Free radicals are atoms or molecules with unpaired electrons. They react with other atoms or molecules with unpaired electrons to form compounds. Free radical’s addition to saturated hydrocarbons is known as free radical halogenations. Free radical halogenation has three steps to proceed- initiation, propagation, and termination. Initiation requires a source of energy to initiate. Free radical halogenation normally takes place in the gaseous or liquid phase. In the liquid phase, peroxides facilitate reactions as a radical initiator. In the gaseous phase, oxygen behaves as a radial trap and inhibits the reaction.
