What is Halogenation?


Core Concepts

This article serves as a guide to what halogenation reactions are in organic chemistry and what makes them essential. After reading this article, you will be able to understand and describe what halogenation is, the different types of reactions and reaction mechanisms, how the reaction works in different organic molecules, its environmental impact, and notable real-world applications.

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Introduction to Halogenation

Halogenation is a versatile tool and reaction in organic chemistry that involves the addition of one or more halogen atoms (X), such as fluorine (F), chlorine (Cl), bromine, (Br), and iodine (I), into organic molecules. Understanding its reaction mechanisms allows for the selective modification of organic compounds to create a large variety of products with diverse applications.

Halogenation reactions can occur through different mechanisms and result in substituting hydrogen atoms with halogen atoms in organic molecules. In addition to this, they can be referred to differently depending on the specific halogen being added. To put that into perspective, the addition of fluorine is known as fluorination, the addition of chlorine is known as chlorination, the addition of bromine is known as bromination, and the addition of bromine is known as iodination.

Also worth noting is that halogen atoms are highly electronegative which means that they have a strong tendency to attract electrons. This allows them to be introduced effectively into organic molecules and alter their chemical properties. Furthermore, halogens have different reactivities, determining how strongly they will react with organic molecules. This is a trend that decreases in order of decreasing electronegativity (F2 > Cl2 > Br2 > I2). Chlorination and bromination reactions are the most common while fluorination and iodination are much less commonly observed. This is because iodine normally isn’t very reactive while fluorine is difficult to control due to its aggressive reactivity.

Types of Halogenation and Reaction Mechanisms

Notable types of halogenation reactions include free radical, electrophilic substitution, and addition. The mechanisms of each one are different as they involve the addition of halogen atoms to different types of hydrocarbon organic molecules.

Free Radical Halogenation

Free radical halogenation reactions are substitution reactions that involve the replacement of hydrogen atoms by halogen atoms in saturated hydrocarbons, otherwise known as alkanes. The reaction mechanism consists of three steps which are chain initiation, chain propagation, and chain termination.

radical chloride reaction
Steps of the free radical halogenation mechanism with the example of the chlorination of methane.

Chain initiation is not energetically favorable and requires energy input. However, the reaction is otherwise self-sustaining and does not require further energy input. This initial step involves breaking the bond of a halogen molecule (X2) which forms halogen radicals. The next step, propagation, is made up of two parts. The first part involves the combination of halogen radicals with alkane to form a halogen acid (H-X) and an alkane radical. The second part involves additional use of the halogen reactant resulting in more halogen radicals. Some of these will combine with alkane radicals which form haloalkane products. The final step, termination, involves combining the remaining radicals to form more product (R-X), more reactant (X2), and longer alkane chains (R-R) as a byproduct.

radical chloride coupling
Example products formed in free radical halogenation (methane chlorination).

Electrophilic Substitution

Electrophilic substitution reactions are the halogenation of aromatic compounds. These reactions typically involve compounds containing chlorine or bromine and are often conducted in the presence of a Lewis acid. This polarizes halogen-halogen bonds and results in a more electrophilic halogen-containing compound.

electrophilic halogenation
An example of an electrophilic halogenation reaction with benzene.

The reaction mechanism of electrophilic substitutions will begin with a halogen molecule reacting with a Lewis acid. This forms a complex that makes the halogen more electrophilic. The aromatic ring then acts as a nucleophile and attacks the electrophilic halogen. This breaks a C=C bond and forms a carbocation intermediate. A C-H bond is then deprotonated which reforms the C=C bond to restore the aromatic ring. In the end, a hydrogen atom is substituted for a halogen atom to form an aryl halide product and halogen acid (H-X) as a byproduct.

electrophilic halogenation benzene bromine
Reaction mechanism of electrophilic substitution with the example of benzene bromination.

Addition Reactions

Addition halogenation reactions are simple reactions that involve the addition of a halogen molecule to unsaturated hydrocarbons such as alkenes and alkynes.

chloride halogenation alkene
An example of an addition halogenation with ethylene (C2H4) as the alkene and chlorine (Cl2) as the halogen molecule.

The mechanism of reaction begins with the nucleophilic attack of a double bond from an unsaturated hydrocarbon on a halogen molecule. This breaks the double bond and the halogen will then attack the hydrocarbon in return, resulting in the formation of a halogen bridge and a second halide that breaks away. The bridged halogen has a +1 charge due to having two bonds and two lone pairs. Next, the second halide attacks from the opposite side of the molecule which breaks the halogen bridge. This form the product which will be a vicinal dihalide.

alkene bromination halogenation
Reaction mechanism of an alkene halogenation addition reaction with bromine.

Environmental Impact

Halogenated compounds have a harsh impact on the environment, causing issues such as the contamination of water, air, and soil. For example, they promote the surface formation of greenhouse gases such as ozone and methane. This degrades air quality, contributes to climate change, and damages crops. In addition to this, they also damage the ozone layer which contributes to their destructive impact on atmospheric composition. Finally, halogenated compounds are harmful to human health as prolonged exposure can lead to respiratory ailments such as asthma.

Real-world Applications of Halogenation

  1. Pharmaceuticals: Halogenated compounds are essential to pharmaceutical research and development. Adding halogens to drugs can alter their properties to increase potency or stability.
  2. Herbicides and pesticides: The formulation of pesticides and herbicides often uses halogenated compounds, enhancing their effectiveness in crop protection.
  3. Plastics and polymers: Manufacturers often use halogenation to produce plastics and polymers such as polyvinyl chloride (PVC) and fluorine-containing Teflons.
  4. Dyes and pigments: Halogenation compounds can be used to manufacture dyes and pigments used in textiles, paints, and coatings.
  5. Flame retardants: Some halogenated compounds are used to reduce the flammability of materials used in textiles, plastics, and electronics. They act directly on flames, interfering with their chemistry.

Further Reading