What is Halogenation Reaction?
Halogenation is the term which can be defined as a chemical reaction that involves the addition of either one or more halogens either to material or compound. The halogenation's stoichiometry and pathway depend on the functional groups and structural features of the organic substrate and the specific halogen. Also, inorganic compounds like metals undergo halogenation.
What is Halogenation of Alkanes?
Halogenation of an alkane produces a hydrocarbon derivative, where either one or more halogen atoms have been substituted for the hydrogen atoms.
Usually, alkanes are unreactive compounds only because they are non-polar and lack the functional groups, where the reactions can occur. Therefore, free-radical halogenation provides a method by which alkanes are functionalized.
However, a severe limitation of radical halogenation is the count of similar C-H bonds, present in all but the simplest alkanes, so the selective reactions are difficult to achieve.
General Reaction of Alkanes
Alkane halogenation is given as an example of a substitution reaction, which is a type of reaction that often takes place in organic chemistry. A substitution reaction is a chemical reaction, where a part of a small reacting molecule replaces either an atom or the atom's group on a hydrocarbon or its derivative.
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The general equation substituting a single halogen atom for one of the hydrogen atoms of an alkane can be given as follows.
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General Features of Halogenation of Alkanes
The features of halogenation of alkanes can be listed as follows:
The R-H notation is the alkane's general formula. In this case, 'R' represents an alkyl group. At the same time, the addition of a hydrogen atom to an alkyl group forms the alkyl group's parent hydrocarbon.
The R-X notation on the product side can be represented as the general formula for a halogenated alkane, where, in this case, 'X' is the general symbol for a halogen atom.
The reaction conditions can be noted by placing these conditions on the equation arrow, which separates the reactants from products. Halogenation of an alkane needs the presence of light or heat.
Chlorination of Methane by Substitution
In the halogenation of an alkane, the alkane is stated to undergo either chlorination, fluorination, iodination, or bromination, depending on the identity of the halogen reactant. Bromination and Chlorination are the alkane halogenation reactions that are used widely. In general, the fluorination reactions proceed too quickly to be useful, whereas iodination reactions go too slowly.
The 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, which is unlike the complex transformation of combustion. A simple example of this reaction of chlorination of Methane has been shown below:
CH4 + Cl2 + Energy → CH3Cl + HCl
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The general equation substituting a single halogen atom for one of the hydrogen atoms of an alkane can be given as follows.
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Mechanism of Halogenation of Alkanes
1. Initiation Step- The Cl-Cl bond of the elemental chlorine undergoes hemolysis when irradiated with UV light. This process yields two chlorine atoms, which are also called chlorine radicals.
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2. Propagation Step- One of the chlorine radicals abstracts a hydrogen atom from methane to form the methyl radical. In turn, the methyl radical abstracts a chlorine atom from one of the chlorine molecules, and then, the formation of chloromethane takes place. Also, the second step of the propagation regenerates a chlorine atom, and these steps repeat several times until the termination happens.
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3. Termination Step- The termination happens when a chlorine atom either reacts with the other chlorine atom to generate Cl2, or a chlorine atom reacts with a methyl radical to produce chloromethane, which constitutes a minor pathway, where the product is made. Also, two methyl radicals can combine to form ethane, which is a minor by-product of this reaction.
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At this step, this reaction does not stop. However, the chlorinated methane product can be allowed to react with additional chlorine to form polychlorinated products.
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By controlling the reaction conditions, including the ratio of chlorine to methane, it can be possible to favour the formation of either one or another possible chlorinated methane product.
Organic Compounds Halogenation
Halogenation by Reaction Type
There exist many pathways to halogenate organic compounds, like ketone halogenation, free radical halogenation, electrophilic halogenation, and the halogen addition reaction. These enzymatic halogenation reactions on the organic molecule are also popular, which either follow electrophilic, nucleophilic, or free radical mechanisms. The structure substrate is one factor, which determines the pathway.
Halogenation Reactions (Substitution Type)
Alkenes can react with halogens to form alkyl halides (or haloalkanes) in the presence of heat or light , which is called as a substitution reaction, because on the alkenes structure, the halogen atom is taking the place of or is substituting one of the hydrogen atoms.
The reaction between fluorine and alkenes: This is an explosive reaction even in the dark and cold as you tend to get hydrogen and carbon fluoride produced instead of the desired substitution reaction. The reaction can be very dangerous as it does not yield the desired product and this also has no particular interest to organic chemists.
For example, the desired product is:
CH4 + F2 → CH3F + HF
But this is the result as the reaction goes so fast:
CH4 + 2F2 → C + 4HF
The reaction between iodine and alkenes: Under normal lab conditions, iodine does not react with alkenes to any extent. So this reaction is also not useful.
The reactions between bromine and chlorine with alkenes: The reaction will produce the desired alkyl halides in the presence of heat and light, but there is no reaction in the dark. So we shall focus our discussion on the halogenation reactions with bromine and chlorine.
Hydrogen atom in the Methane is replaced by a chlorine atom in the substitution reaction. Until all the hydrogens are replaced, this reaction can happen multiple times. Ultimately, more hydrogens in the alkene are replaced as long as the reaction proceeds. Thus, we end up with a mixture of chloromethane(CH3Cl), dichloromethane(CH2Cl2), trichloromethane(CHCl3), and tetrachloromethane (CCl4).
The original mixture of a green gas( Cl2) And a colourless gas(CH4) would produce a mist of organic liquids (mixture of the chlorinated methane) and steamy fumes of hydrogen chloride(HCl). Except for chloromethane(CH3Cl) all the other organic products are liquid at room temperature.
Halogenation by the Halogen Type
The halogen influences the halogenation facility. Chlorine and Fluorine are more electrophilic and also aggressive halogenating agents. In comparison, bromine is a weaker halogenating agent compared to both chlorine and fluorine. At the same time, iodine can be given as the least reactive of them all. The dehydrohalogenation facility follows the reverse trend, where the iodine can be removed most easily from organic compounds, and the organofluorine compounds are highly stable.
Nature of the Mechanism of Alkanes' Halogenation
In the presence of either heat or ultraviolet light (UV), the halogen reaction with an alkane results in a haloalkane formation (which is an alkyl halide). This phenomenon can be explained using the reaction mechanism - A mechanism to halogenate. In the methane molecule, the carbon‐hydrogen bonds are the low-polarity covalent bonds.
Did You Know?
Electrophiles, which attach to the double bond of alkenes, weaken the ÿ bond. In contrast to the alkene hydrogenation, catalysts do not allow adding chlorine or molecular bromine to generate nearby dichalcogenides.
FAQs on Halogenation of Alkanes
1. What is the halogenation of alkanes, with a specific example?
Halogenation of alkanes is a type of substitution reaction where one or more hydrogen atoms in an alkane are replaced by halogen atoms (like chlorine, bromine). This reaction is typically initiated by ultraviolet (UV) light or high temperatures. For example, when methane (CH₄) reacts with chlorine (Cl₂), it forms chloromethane (CH₃Cl) and hydrogen chloride (HCl). The overall reaction is: CH₄ + Cl₂ → CH₃Cl + HCl.
2. What are the three main steps in the free-radical mechanism of alkane halogenation?
The halogenation of alkanes follows a free-radical chain mechanism, which consists of three distinct steps:
- Initiation: The reaction starts when the halogen molecule (e.g., Cl₂) absorbs energy from UV light or heat, causing it to split into two highly reactive halogen free radicals (Cl•).
- Propagation: These free radicals attack the alkane, creating an alkyl radical and hydrogen halide. This alkyl radical then reacts with another halogen molecule to form the haloalkane and a new halogen radical, continuing the chain.
- Termination: The reaction stops when two free radicals combine to form a stable, non-reactive molecule, ending the chain. This can happen in several ways, such as two halogen radicals combining or an alkyl radical combining with a halogen radical.
3. What are the essential conditions required to initiate the halogenation of an alkane?
The reaction between an alkane and a halogen does not occur under normal conditions in the dark. It requires a significant input of energy to start. The essential conditions are either exposure to ultraviolet (UV) light or heating the mixture to high temperatures (typically 520-670 K). This energy is necessary to break the diatomic halogen bond (e.g., Cl-Cl) and generate the initial free radicals that start the chain reaction.
4. Why is the halogenation of alkanes considered a free-radical substitution, not an SN1 or SN2 reaction?
The halogenation of alkanes is a free-radical substitution because alkanes are non-polar and lack a good leaving group, making them unreactive towards the nucleophiles required for SN1 and SN2 reactions. The reaction is initiated by UV light, which generates neutral but highly reactive free radicals, not the ions (carbocations or charged nucleophiles) characteristic of SN1/SN2 pathways. The mechanism proceeds via a chain reaction involving these radical intermediates.
5. How does the reactivity of halogens (F₂, Cl₂, Br₂, I₂) differ in the halogenation of alkanes?
The reactivity of halogens with alkanes varies significantly and follows the order: F₂ > Cl₂ > Br₂ > I₂.
- Fluorination is extremely violent and explosive, making it difficult to control.
- Chlorination and bromination are moderately fast and are the most synthetically useful reactions.
- Iodination is very slow and is a reversible reaction because hydrogen iodide (HI), a product, is a strong reducing agent that can convert the iodoalkane back to the alkane.
This trend is determined by the activation energy of the rate-determining step in the propagation phase.
6. Why is a tertiary C-H bond more reactive than a primary C-H bond during halogenation?
A tertiary C-H bond is more reactive because the abstraction of a hydrogen atom from it leads to the formation of a more stable tertiary (3°) free radical. The stability of alkyl free radicals follows the order: tertiary > secondary > primary. This increased stability is due to hyperconjugation and the electron-donating inductive effect of the alkyl groups, which delocalise the unpaired electron. Because the tertiary radical intermediate is more stable, it forms faster, making the corresponding C-H bond more reactive.
7. What is a major limitation of free-radical halogenation for synthesising a specific haloalkane?
A major limitation, especially with chlorine, is the lack of selectivity, which leads to the formation of a mixture of products. For instance, reacting methane with excess chlorine can produce chloromethane, dichloromethane, trichloromethane, and carbon tetrachloride. For higher alkanes, a mixture of isomeric products can also form (e.g., chlorinating propane gives both 1-chloropropane and 2-chloropropane). This makes it difficult to obtain a high yield of a single desired product, complicating its use in fine chemical synthesis.
