Aromatic Chemistry - Halogenation Reactions in Benzene (A-Level Chemistry)
Halogenation Reactions in Benzene
Halogenation of Benzene
The replacement of one of the H atoms in benzene for a halogen atom is known as halogenation and it takes place via an electrophilic substitution reaction mechanism.
Halogenation takes place at room temperature, by reacting benzene with a halogen in the presence of a halogen carrier, such as aluminum chloride or iron (III) bromide.
Example: Bromination of Benzene
We will look at the substitution of bromine into benzene as an example of a halogenation reaction. The ruction mechanism is the following:
1. Electrophile formation. An electrophile is needed to attack the benzene ring. The electrophile in this case is a polarised bromine molecule. The halogen molecule becomes polarised by the halogen carrier catalyst iron(III) bromide.
2. Formation of a C-Br bond. The partial positive charge on one of atoms in the polarised bromine molecule attracts electrons in the benzene ring. Two electrons from the benzene ring are used to form a C-Br bond. This allows the other bromine atom leave with the FeBr₃ molecule in the form of FeBr₄-. The benzene ring has lost electrons, so is now positively charged.
3. Removal of a hydrogen atom. The FeBr₄- ion is attracted to the +ve benzene ring. The FeBr₄- removes a hydrogen atom, forming bromobenzene, HBr and FeBr₃. The catalyst FeBr₃ is therefore reformed.
Halogenation of alkylarenes
Alkylarenes are benzene rings with an alkyl side chain.
In alkylarenes, halogenation can occur in the alkyl side chain or in the benzene ring, depending on the reaction conditions.
- Halogenation of benzene ring – Halogenation takes place at positions 2 or 4 to form a mixture of products. This electrophilic substitution reaction takes place room temperature, by reacting the alkylarene with a halogen in the presence of a halogen carrier.
- Halogenation of the side chain – If a gaseous halogen gets passed over a boiling alkylarene in the presence of UV light or strong sunlight, the hydrogen atoms in the alkyl side chain get replaced one by one by halogen atoms by a free-radical mechanism.
Aromatic chemistry is a branch of organic chemistry that deals with the study of aromatic compounds. Aromatic compounds are a type of organic molecule that contain a ring structure known as an aromatic ring, which is characterized by its distinctive aroma and high stability. In A-level Chemistry, students learn about the basic concepts of aromatic chemistry, including the properties and structures of aromatic compounds.
Halogenation reactions are a type of chemical reaction that involve the addition of a halogen (elements from Group 17 of the periodic table, such as chlorine, bromine, or iodine) to an organic molecule. In A-level Chemistry, students learn about halogenation reactions as they pertain to benzene, which is an important aromatic compound.
In A-level Chemistry, students learn that halogenation reactions in benzene occur through a free radical substitution reaction. During this reaction, a halogen molecule is added to the benzene ring, replacing one of its hydrogen atoms. This reaction is usually carried out in the presence of a radical initiator, such as heat or light, which helps to generate the free radicals needed to initiate the reaction.
Studying halogenation reactions in benzene is an important concept in A-level Chemistry as it provides insight into the reactivity of aromatic compounds and the mechanism of chemical reactions. By studying this topic, students gain a deeper understanding of the properties and structures of aromatic compounds, as well as the factors that influence their reactivity. This knowledge is useful in fields such as organic chemistry, chemical engineering, and pharmaceuticals, where the ability to modify and manipulate the properties of organic molecules is important.
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