Halogenation

Introduction:  Halogenation Meaning

Halogenation is a chemical reaction involving the addition of one or more halogens to a substance or product. The halogenation mechanism and stoichiometry depend on the structural characteristics and functional groups of the organic substrate, as well as the actual halogen. Unlike metals, inorganic compounds also undergo halogenation.


Types of Halogenation


Halogenation by Various Reaction :

There are several mechanisms for organic compound halogenation including free radical halogenation, ketone halogenation, electrophilic halogenation, and halogen supplement reaction. Substratal structure is one factor determining the pathway.

  • Free radical halogenation

  • Addition of halogens to alkenes and alkynes

  • Halogenation of aromatic compounds

  • Other halogenation methods


Free Radical Halogenation

Typically, saturated hydrocarbons do not add halogens but undergo free radical halogenation, which involves halogen substitution of hydrogen atoms. The halogenation regiochemistry of alkanes is usually determined by the relative weakness of the C –H bonds available.

The preference for reaction in tertiary and secondary positions is the result of the greater stability of the resulting free radicals and their intermediate condition. For the industrial production of chlorinated methanes, free-radical halogenation is used.

CH4 + Cl2 → CH3Cl + HCl


Addition of Halogens to Alkenes and Alkynes

Unsaturated compounds, and in particular alkenes and alkynes, add halogens:

RCH=CHR′ + X2 → RCHX–CHXR′

Halogen addition to alkenes is achieved through intermediate halonium ions. These intermediates had been removed in special cases


Electrophilic Aromatic Substitution

An electrophilic halogenation of the aromatic compounds

RC6H5 + X2 → HX + RC6H4X

This reaction only works for chlorine and bromine and is performed in the presence of a Lewis acid like FeX3 (laboratory method). Lewis acid's role is to polarize the halogen-halogen bond, which makes the halogen molecule more electrophilic.

Industrially this is done in the presence of iron metal, by treating the aromatic compound with X2. When the halogen is pumped into the vessel reaction, it reacts with iron, generating catalytic amounts of FeX3. The mechanism of reaction can be as follows

Since fluorine is highly reactive, the above-mentioned procedure would not be efficient, as the aromatic molecule would react destructively with F2. For this purpose, other processes, such as the Balz–Schiemann reaction, must be used to prepare aromatic fluorinated compounds.

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Moreover, to achieve iodination, oxidizing conditions need to be used for iodination.

This can be done by performing the reaction in the presence of an oxidizing agent which oxidizes HI to I2, thus removing HI from the reaction and producing further iodine which can respond further. The iodination based reaction steps are as follows:

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Other Halogenation Methods

In the Hunsdiecker reaction, the chain-shortened halide is extracted from carboxylic acids. First the carboxylic acid is converted to its silver salt, then oxidized with halogen:

RCO2Ag + Br2 → RBr + CO2 + AgBr

The Sandmeyer reaction is used to give diazonium salt aryl halides, which are obtained from anilines.

The carboxylic acids are alpha-halogenated in the Hell–Volhard–Zelinsky halogenation.

In oxychlorination, as demonstrated by this path to dichloroethane, the combination of hydrogen chloride and oxygen serves as the chlorine equivalent:

2 HCl + CH2=CH2 + ​12 O2 → ClCH2CH2Cl + H2O


Halogenation Reaction By Other Halogens 

The halogenation system is affected by that halogen. Highly electrophilic, fluorine, and chlorine are more active halogenating chemicals. Bromine is a slower halogenating agent than both fluorine and chlorine, while the least resistant of all is iodine.

Iodine is easy to extract from organic compounds, including highly stable organofluorine substances.


Fluorination

Organic compounds, both saturated and unsaturated, react readily with fluorine, typically explosively. Elemental fluorine fluorination (F2) requires highly specialized environments and apparatuses. Many of the commercially important organic compounds are electrochemically fluorinated using hydrogen fluoride as the fluorine supply.

Organic compounds, saturated and unsaturated alike, react readily, typically explosively, with fluorine. Fluorination with elemental fluorine (F2) requires highly specialized environments and equipment. Many commercially important organic compounds are fluorinated electrochemically using hydrogen fluoride as the basis of fluorine.


Chlorination

Usually the chlorination is strongly exothermic. Both saturated and unsaturated compounds directly react with chlorine, with the former generally needing UV light to start chlorine homolysis. Industrially, chlorination is carried out on a large scale; major processes involve routes to 1,2-dichloroethane (a precursor to PVC), as well as different chlorinated ethans as solvents.


Bromination

Bromination is more effective than chlorination, because there is less exothermic reaction. Bromination is most commonly done through the application of Br2 to alkenes. Another source of bromination is the organic synthesis of the trichloroethylene anesthetic halothane.

The most popular organohalides in nature are the organobromine compounds. Their formation is catalyzed by the Bromoperoxidase enzyme which uses bromide as an oxidant in combination with oxygen. It is projected that the oceans emit 1–2 million tons of bromoform and 56,000 tons of bromomethane per annum.

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FAQ (Frequently Asked Questions)

1. What is a Halogenation Reaction?

Halogenation is a reaction that occurs with the addition of one or more halogens to a substance. In the periodic table, Halogens occupy the seventh column which contains fluorine, chlorine, bromine, iodine and astatine. The substance resulting from a halogenation reaction is called a halogenated compound.

2. What is the Mechanism of Halogenation?

The reaction of a halogen with alkane results in the formation of a haloalkane (alkyl halide) in the presence of ultraviolet (UV) light or heat. The Reaction Mechanism explains the phenomenon. Halogenating Mechanism. In the methane molecule, the carbon-hydrogen bonds are covalent bonds with low polarity.

3. Why is Halogenation Important?

One such reaction is halogenation, or the replacement on the alkane of single hydrogen for a single halogen to create a haloalkane.

4. Is Halogenation an Addition Reaction?

A halogen addition reaction is a simple organic reaction where the carbon-carbon double bond of an alkene functional group is attached to a halogen molecule. This type of reaction is an electrophilic add-on and a halogenation.