Halogenation of Alkanes: Reactions, Features and Mechanisms

Halogenation of alkanes produces a hydrocarbon derivative wherein one or more halogen atoms have been substituted for hydrogen atoms. Alkanes are considered to be notoriously unreactive compounds as they are non-polar and lack functional groups at which reactions can take place. A chemical reaction in which one or more halogens are added to a material or molecule is known as halogenation. The functional groups and structural properties of the organic substrate and the specific halogen determine the stoichiometry and pathway of halogenation. Metals and other inorganic compounds can also be halogenated. Let’s have a closer look at the topic and discuss some important questions.

Key takeaways: Halogenation of alkanes, Alkanes, Hydrogen, Atoms, Hydrocarbon, Halogenation reactions, Chemical reaction, Organic substrate, Halogen

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What is Halogenation of Alkanes?

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When an alkane is halogenated, one or more halogen atoms are swapped for hydrogen atoms, resulting in a hydrocarbon derivative. Alkanes are often unreactive chemicals because they are non-polar and lack functional groups where reactions can take place. As a result, free-radical halogenation is a method for functionalizing alkanes.

The count of identical C-H bonds, which is present in all but the simplest alkanes, is a major constraint of radical halogenation, making selective reactions difficult to execute.

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General Reaction of Alkanes

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A substitution reaction, which is a typical type of reaction in organic chemistry, involves the halogenation of alkanes. When a tiny component of a responsive molecule replaces an atom or the atom's group on a hydrocarbon or its derivative, it is called a substitution reaction.

The general equation for replacing one of an alkane's hydrogen atoms with a single halogen atom is as follows.

General Reaction of Alkanes

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General Features of Halogenation of Alkanes

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Some of the properties of alkane halogenation are as follows:

• The R-H notation is used to write the alkane's general formula. In this case, the letter 'R' stands for an alkyl group.
• The R-X notation can be represented as the general formula for a halogenated alkane on the product side, where 'X' is the universal symbol for a halogen atom in this case.

Make a note of the reaction conditions by writing them on the equation arrow that divides the reactants from the products. Halogenation of an alkane requires the presence of light or heat.

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Chlorination of Methane by Substitution

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An alkane is said to undergo chlorination, fluorination, iodination, or bromination during halogenation depending on the identity of the halogen reactant. Bromination and chlorination of alkanes are extensively used in halogenation reactions. Iodination reactions are too slow to be practical, whereas fluorination reactions are too quick.

In contrast to combustion's complex transformation, halogenation of an alkene appears to be a simple substitution reaction in which a C-H bond is broken and a new C-X bond is formed. The chlorination of Methane process is depicted in the diagram below:

Chlorination of Methane by Substitution

The general equation for substituting a single halogen atom for one of an alkane's hydrogen atoms.

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Mechanism of Halogenation of Alkanes

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1. Initiation Step- When elemental chlorine is exposed to UV light, the Cl-Cl bond undergoes haemolysis. Two chlorine atoms, commonly known as chlorine radicals, are produced as a result of this reaction.
2. Propagation Step- To produce the methyl radical, one of the chlorine radicals takes a hydrogen atom from methane. The methyl radical, in turn, removes a chlorine atom from one of the chlorine molecules, resulting in the production of chloromethane. In addition, the propagation's second phase regenerates a chlorine atom, and these stages are repeated multiple times until the termination occurs.
3. Termination Step- The product is formed when a chlorine atom either interacts with another chlorine atom to make Cl2, or when a chlorine atom reacts with a methyl radical to produce chloromethane, which is a secondary pathway. A minor by-product of this reaction is ethane, which is formed when two methyl radicals interact.

This reaction continues at this point. The chlorinated methane product, on the other hand, can be allowed to react with more chlorine to produce polychlorinated compounds. It is feasible to favour the creation of one or more chlorinated methane products by adjusting the reaction conditions, including the chlorine-to-methane ratio.

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Things to Remember

• Halogenation is a chemical reaction in which one or more halogens are added to a substance or molecule.
• Electrophiles weaken the double bond of alkenes by attaching to it. In contrast to alkene hydrogenation, catalysts do not allow for the generation of adjacent dichalcogenides by adding chlorine or molecular bromine.
• Halogenation of organic compounds can be accomplished through a variety of methods, including ketone halogenation, free radical halogenation, electrophilic halogenation, and the halogen addition reaction.
• These enzymatic halogenation reactions on organic molecules, which follow electrophilic, nucleophilic, or free radical processes, are very popular.

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