Halogenation refers to a type of chemical reaction that involves the replacement of a halogen atom with another substance, wherein it ends up as a part of that substance or a compound. In general, during the halogenation reaction, there is usually an addition of one or more halogens to the substance.
Halogenation can be further described as the process of replacing any number of hydrogen atoms with these given elements in the group. The product that is formed after halogenation will possess more new and unique properties than the initial substance. Meanwhile, halogens are a group of elements, such as iodine, chlorine, fluorine and bromine. Most of the time, these elements exhibit similar behaviour, and are therefore categorised under the same group.
Types of Halogenation
Halogenation can occur in several ways for organic compounds as well as for inorganic compounds. Halogenations can occur in a particular way, depending on the substrate.
- Saturated hydrocarbons halogenate via a free radical process.
- Unsaturated organics halogenate via an addition reaction.
- Aromatics halogenate via electrophilic substitution.
We will briefly discuss the types of halogenation below.
Halogen Addition
This type of reaction is common in unsaturated carbons. Typically, alkynes and alkenes follow this reaction where they add halogens. For example, the addition of bromine to ethane.
Halogen addition to alkenes is achieved through intermediate halonium ions.
Halogen Substitution or Free Radical Halogenation
Here, we consider saturated hydrocarbons, where the hydrogen atoms are usually replaced by halogens. The hydrocarbons basically undergo free radical halogenation. In this, saturated hydrocarbons do not add halogens. The halogenation regiochemistry of alkanes is usually determined by the relative weakness of the C-H bonds available.
We will take the example of halogens reacting with alkanes under the influence of heat to form alkyl halides.
As you can see, the halogen atom replaces the hydrogen atom into an alkane hence called a substitution reaction.
Meanwhile, aromatic compounds also go through substitution reactions or electrophilic halogenation in the presence of Lewis acids.
Electrophilic Substitution Reaction or Halogenation of Aromatic Compounds
An electrophilic aromatic substitution reaction mainly involves chlorine and bromine compounds. This reaction is further conducted in the presence of a Lewis acid such as FeX3 (laboratory method), which is used mainly to polarise the halogen-halogen bond. This results in the halogen molecule being more electrophilic.
Benzene reacts with bromine or chlorine in an electrophilic substitution reaction only in the presence of a catalyst which is either chloride or iron. However, iron is not entirely a catalyst because it changes permanently during the reaction. It reacts with some bromine to form iron 3 chloride, FeCl3 or iron 3 bromide, FeBr3.
Other Important Halogenation Methods
If we look at the Hunsdiecker reaction, carboxylic acids are changed to the chain-shortened halide. The carboxylic acid converts into silver salt. This is then oxidised with halogen:
- RCO2Ag + Br2 → RBr + CO2 + AgBr
Usually, the Sandmeyer reaction is used to produce diazonium salt aryl halides. These are mostly obtained from anilines. Another reaction is known as Hell–Volhard–Zelinsky halogenation, where the carboxylic acids are alpha-halogenated.
Inorganic Chemistry
Elements apart from argon, neon, and helium can form fluorides when they react with fluorine. In the case of chlorine, it is quite selective but still can react with heavier nonmetals and metals. Bromine is less reactive, and if we take iodine, it is the least reactive of them all.
Nonetheless, chlorination of metals is usually considered not essential industrially. Chlorides can easily be made from oxides and hydrogen halides. On the contrary, chlorination of inorganic compounds is done on a large scale, mainly for the production of sulfur monochloride and phosphorus trichloride.
Halogenation of Benzene – Mechanism
Step 1: The bromine reacts with Lewis acid to create a complex that makes bromine more electrophilic.
Step 2: The π electrons of aromatic C=C behave as a nucleophile which attacks the electrophilic Br and displaces iron tetrabromide.
Step 3: The proton is removed from sp3 C and bears the Bromo group that reforms C=C, and the aromatic system generates HBr and regulates the active catalyst.
The compounds act as a catalyst and behave similarly to aluminium chloride in the following chemical reactions.
We will further look at how halogenation is influenced by each halogen or the halogenating agents.
Halogenation Reactions
Just to recap, a halogenation reaction mainly occurs when one or more chlorine fluorine, chlorine, bromine, or iodine atoms replace hydrogen atoms in an organic compound. As for the order of reactivity, it is as follows:
Fluorine > Chlorine > Bromine > Iodine
As for fluorine, it is slightly aggressive and reacts violently with organic materials. However, it tends to make the most stable of the organohalogens. A fluorine atom, once added, is difficult to remove. On the other hand, iodine is difficult to add to an organic molecule, but after the formation of iodoorganic forms, the iodine atom is easily removed. From this, we can say that halogenation reactions depend upon,
- The electronegativity of the halogen atom.
- The nature of the substrate molecule that is being halogenated.
Reaction with Chlorine
The reaction between chlorine and benzene gives chlorobenzene in the presence of either iron or aluminium chloride.
Reaction with Bromine
The reaction between bromine and benzene gives bromobenzene in the presence of either iron or aluminium bromide. Iron is used normally because it is readily available and cheaper.
Reaction with Fluorine
If we take any organic compounds, it readily reacts with fluorine (usually explosive in nature). However, in the case of elemental fluorine (F2), we will need some proper apparatus and fulfil certain conditions. A variety of fluorinating reagents, such as xenon difluoride and cobalt (III) fluoride, are also used sometimes for the reaction.
Reaction with Iodine
In comparison to the above three halogens, iodine is the least reactive halogen. It does not readily react with organic substances.
Simultaneously, when we study inorganic chemistry, almost every element, excluding helium, argon and neon, forms fluorides when it is reacted with fluorine.
Importance of Halogenation Reactions
Halogenations reactions are very useful and have a broad scope of use in synthetic chemistry. Halogenation reactions are important in chemical synthesis, and the intermediates generated via this process are widely found in products, such as polymers and plastics, refrigerants, fire retardants, fuel additives, agro products, etc. In pharmaceuticals, fluorine or chlorine atoms are added to a molecule to increase the effectiveness of its therapeutic aspects.
Additionally, important commercial chemicals are produced from halogenation reactions. For instance, chloroform is fluorinated to produce chlorodifluoromethane. This is then converted to fluoroethylene and polymerised to yield PTFE. Another common example that we can look at is the addition halogenation of ethylene with chlorine. This is done to form dichloroethane which is further polymerised to yield PVC.
Know more about applications of halogenation of benzene, along with chemical reactions involved in every application, at BYJU’S.
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