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Halogenation is a term that refers to a chemical process that involves the addition of one or more halogens to a substance or a compound. The stoichiometry and route of halogenation are determined by the functional groups and structural characteristics of the organic substrate and the individual halogen. During halogenation, generally, an addition of a halogen to the substance takes place. Inorganic substances, such as metals, are also halogenated. A new product with entirely different properties is formed as a result. In this article, we will discuss definitions, types, examples, and sample questions of halogenation and look at the mechanism of the reaction.
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Key Terms: Halogenation, Lewis acid, Benzene, halogen atom, Halogen, Atom, iodine, chlorine, fluorine, bromine
What is Halogenation?
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Halogenation is a type of chemical reaction in which a halogen atom is replaced with another substance and the halogen atom becomes a component of that substance or molecule. Halogens, on the other hand, are a group of elements that includes iodine, chlorine, fluorine, and bromine.The majority of the time, these elements display identical behavior and are thus grouped
- The end result of a halogenation reaction is generally the addition of one or more halogens to the substance.
- Halogenation is defined as the process of replacing any number of hydrogen atoms in a group with these specific halogen elements.
- The halogenated product will have new and distinct properties compared to the original component.
Example of a halogenation reaction
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| Decarboxylation Reaction | Hydrolysis | Friedel Crafts reaction |
| Ortho Effect | Properties of Alcohol | Decomposition Reaction |
| Catalysis | Rosenmund reduction reaction | Methyl Ethyl Ketone |
Types of Halogenation Reaction
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Halogenation can take place in a variety of methods for both organic and inorganic substances. Depending on the substrate, halogenation can take place in the following ways:
- A free radical mechanism causes the halogenation of saturated hydrocarbons.
- Through an addition reaction, the unsaturated organics halogenate.
- Electrophilic substitution halogenates aromatics.
Free Radical Halogenation or Halogen Substitution
Saturated hydrocarbons are those in which the hydrogen atoms are usually replaced by halogens. The hydrocarbons are essentially halogenated by free radicals. The relative weakness of the C–H bonds available determines the halogenation regiochemistry of alkanes.
The halogens react with alkanes to generate alkyl halides under the effect of heat. Food instance, the production of chlorinated methane in the industries takes place through free radical halogenation.
CH4 + Cl2 → CH3Cl + HCl
Halogen Addition Reaction
In unsaturated carbons, particularly alkenes and alkynes, this type of reaction is widespread. The addition of halogen to alkenes is accomplished using intermediate halonium ions. For instance, bromine can be added to ethylene.
Aromatic Compounds Electrophilic Substitution Reaction or Halogenation
Mainly, chlorine and bromine compounds are involved in an electrophilic aromatic substitution process. This reaction is also carried out in the presence of a Lewis acid, such as FeX3. The presence of a Lewis acid is necessary to polarise the halogen-halogen bond. The halogen molecule becomes more electrophilic as a result of this.
Only in the presence of a chloride or iron catalyst does benzene react with bromine or chlorine in an electrophilic substitution process. Iron, on the other hand, isn't a catalyst because it undergoes lasting modifications during the reaction. It forms iron 3 chloride (FeCl3) or iron 3 bromide (FeBr3) when it combines with bromine.
Other methods used for halogenation are:
- Hell–Volhard–Zelinsky halogenation: Here, the carboxylic acid in the element is alpha-halogenated.
- Hunsdiecker reaction: From carboxylic acids, chain-shortened halides are extracted in this reaction.
- Sandmeyer reaction: This reaction is used to produce diazonium salt aryl halides, which are produced from anilines.
Examples of Halogenation Reactions
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The halogens' reactivity decreases in the following order: F2 > Cl2 > Br2 > I2. Some examples of halogenation reactions are:
Reaction with Chlorine
In the presence of either iron or aluminum chloride, the interaction between chlorine and benzene produces chlorobenzene. The presence of oxygen (a radical trap) in gas phase chlorinations slows the process.
Reaction with fluorine
Any organic substance that interacts quickly with fluorine is called fluorine (usually explosive). However, in the case of elemental fluorine (F2), we'll need certain special equipment and meet specific requirements. Fluorinating reagents like xenon difluoride and cobalt (III) fluoride are occasionally utilized in the reaction.
Reaction with bromine
In the presence of either iron or aluminum bromide, the reaction between bromine and benzene produces bromobenzene. Iron is commonly utilized since it is widely available and less expensive.
Importance of Halogenation Reactions
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- In synthetic chemistry, halogenation reactions are extremely valuable and have a wide range of applications.
- Halogenation reactions are significant in chemical synthesis, and the intermediates produced by this method can be found in a wide range of goods, including polymers and plastics, refrigerants, fire retardants, fuel additives, agro products, and so on.
- Fluorine or chlorine atoms are added to a molecule in pharmaceuticals to improve its medicinal features.
- In addition, halogenation processes yield essential commercial compounds. Chloroform, for example, is fluorinated to make chlorodifluoromethane. The fluoroethylene is subsequently transformed to PTFE and polymerized.
- The halogenation of ethylene with chlorine is another common example. This produces dichloroethane, which is then polymerized to produce PV.
Halogenation of Benzene
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Step 1: Bromine combines with Lewis acid to form a compound that increases the electrophilicity of bromine.
Step 2: The aromatic C=C electrons act as a nucleophile, attacking the electrophilic Br and dislodging iron tetrabromide.
Step 3: The proton is removed from sp3 C and replaced with a Bromo group, which repairs C=C and activates the active catalyst by generating HBr and regulating the aromatic system.
In the following chemical processes, the compounds act as a catalyst and behave similarly to aluminum chloride.
Mechanism of Halogenation of Alkanes
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The mechanism of halogenation of alkanes is broken down into three phases, as shown below.
Step 1: Initiation
When exposed to UV light, the Cl-Cl bond of elemental chlorine hemolyzes. Two chlorine atoms, commonly known as chlorine radicals, are produced as a result of this reaction.
Step 2: Propagation
The methyl radical is formed when one of the chlorine radicals removes 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.
Step 3: Termination
The termination occurs when a chlorine atom interacts with another chlorine atom to make Cl2, or when a chlorine atom reacts with a methyl radical to produce chloromethane, which is a minor pathway in the production of the product. A minor by-product of this reaction is ethane, which is formed when two methyl radicals interact.
This reaction continues at this point. However, because the chlorinated methane product can be allowed to react with more chlorine to generate polychlorinated compounds, this is a risk.
It is feasible to favor the creation of one or more chlorinated methane products by adjusting the reaction conditions, including the chlorine-to-methane ratio.
General features of Halogenation of Alkanes
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The following are the characteristics of alkane halogenation.
- The alkane's general formula is written in R-H notation. The letter 'R' stands for an alkyl group in this example. The addition of a hydrogen atom to an alkyl group creates the alkyl group's parent hydrocarbon at the same time.
- On the product side, the R-X notation can be interpreted as the generic formula for a halogenated alkane. Where 'X' is the general symbol for a halogen atom in this case.
- Place the reaction conditions on the equation arrow that separates the reactants from the products to make a note of them. The presence of light or heat is required for the halogenation of an alkane.
Things To Remember
- Halogenation is a sort of chemical reaction in which a halogen atom is replaced with another substance and the halogen atom becomes a component of that substance or molecule.
- Electrophiles weaken the double bond of alkenes by attaching to them. In contrast to alkene hydrogenation, catalysts do not allow for the generation of adjacent dichalcogenides by adding chlorine or molecular bromine.
- When a halogen reacts with an alkane in the presence of either heat or ultraviolet light (UV), a haloalkane is formed (which is an alkyl halide). The reaction mechanism, a mechanism to halogenate, can be used to explain this phenomenon. Carbonhydrogen bonds are low-polarity covalent links in the methane molecule.
- The addition of one or more halogens to the material is usually the result of the halogenation reaction
- The stoichiometry and route of halogenation are determined by the functional groups and structural characteristics of the organic substrate and the individual halogen. Inorganic substances, such as metals, are also halogenated.
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Sample Questions
Ques. What happens when alkanes are halogenated? (2 marks)
Ans. Halogenation is a chemical process that occurs when one or more halogens are introduced to a material. Halogens are the 17th group in the periodic table, and they include substances like chlorine, fluorine, bromine, astatine, and iodine. A halogenated compound is a chemical that results from the halogenation reaction of alkanes.
Ques. Can alkane halogenation be described as a rearrangement reaction? (2 marks)
Ans. The relative strength of the available C – H bonds determines the halogenation regiochemistry of alkanes in general. A free radical halogenation of alkanes can be exploited for the commercial processing of chlorinated methane.
CH4 + Cl2 -> CH3Cl + HCl
These free radical reactions can also be accompanied by a rearrangement.
Ques. Describe the importance of halogenation reactions.(3 marks)
Ans. In synthetic chemistry, halogenation reactions are extremely valuable and have a wide range of applications. Halogenation reactions are significant in chemical synthesis, and the intermediates produced by this method can be found in a wide range of goods, including polymers and plastics, refrigerants, fire retardants, fuel additives, agro products, and so on. Fluorine or chlorine atoms are added to a molecule in pharmaceuticals to improve its medicinal features.
Ques. Explain the reactions of alkanes.(2 marks)
Ans. Alkanes (one of the most important organic molecules) are only subjected to a few reactions. Halogenation and combustion are two major alkane processes (which is, the substitution for a single halogen of single hydrogen on the alkane).
Ques. Give features of halogenation of alkanes.(5 marks)
Ans. The following are the characteristics of alkane halogenation.
- The alkane's general formula is written in R-H notation. The letter 'R' stands for an alkyl group in this example.
- The addition of a hydrogen atom to an alkyl group creates the alkyl group's parent hydrocarbon at the same time.
- On the product side, the R-X notation can be interpreted as the generic formula for a halogenated alkane. Where 'X' is the general symbol for a halogen atom in this case.
- Place the reaction conditions on the equation arrow that separates the reactants from the products to make a note of them.
- The presence of light or heat is required for the halogenation of an alkane.
Ques. Name 3 types of halogenation reactions.(3 marks)
Ans. Depending on the substrate, halogenation can take place in a variety of ways.
- A free radical mechanism causes saturated hydrocarbons to halogenate.
- Through an addition process, unsaturated organics halogenate.
- Electrophilic substitution halogenates aromatics.
Ques. Describe free radical halogenation.(2 marks)
Ans. Saturated hydrocarbons are those in which the hydrogen atoms are usually replaced by halogens. The hydrocarbons are essentially halogenated by free radicals. Add no halogens to these saturated hydrocarbons. The relative strength of the C–H bonds available determines the halogenation regiochemistry of alkanes.
Ques. Describe empirical considerations for a halogenation reaction to occur. (5 marks)
Ans. Any acceptable explanation for the halogenation reaction must account for the following facts.
- The halogens' reactivity decreases in the following order: F2 > Cl2 > Br2 > I2
- Because fluorine is so explosively reactive that it's impossible to manage, and iodine is normally unreactive, we'll focus on chlorine and bromine.
- Exothermic chlorinations and brominations are common.
- These halogenations require energy input in the form of heat or light to begin.
- Thousands of molecules respond to each photon of light absorbed when light is used to trigger halogenation.
- Halogenation reactions can take either in a gaseous or liquid state.
- The presence of oxygen (a radical trap) in gas phase chlorinations slows the process.
- Halogenation in the liquid phase is aided by radical initiators such as peroxides.
Ques. Below is the structure diagram of 2-methyl butane.(5 marks)
a) Draw the structures of all probable mono-chloro compounds formed by chlorinating 2-methyl butane using free radicals.
b) What do you think the main product will be based just on statistics? Is this the same structure as the major product that will be released? Explain.
c) How would using bromine instead of chlorine affect the relative yield of the products?
Ans.
a) Because 2-methyl butane has five different carbons, you should theoretically receive five separate products shown below: 20% A, 20% B, 20% C, 20% D, and 20% E.
But take note that A and B are the same thing! As a result, just four products are available.
b) Based on statistics alone, we would expect product A (or B) to be the most important product, accounting for 40% of all items (20% A + 20% B).
However, this is not what happens in practice because A is primary, and more substituted products are preferred in free-radical halogenation reactions. As a result, C, a tertiary product, will be the most important.
The reason for this is that the intermediate is a radical, and radical stability is 3° > 2° > 1°.
c) Because chlorine reacts faster than bromine, it is less selective.
We know, for example, that the 3° halide will be the most important product. However, when it comes to chlorine, the proportions can be 70 percent 3°, 20 percent 2°, and 10 percent 1°.
Bromine, on the other hand, reacts more slowly, so you'll have better selectivity. The product proportions may be 95 percent 3°, 4 percent 2°, and 1 percent 1°.
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