Question

When ethylene gas is passed into a mixture of an aqueous solution of bromine and sodium chloride, the products formed are \(\mathrm{BrCH}_{2} \mathrm{CH}_{2} \mathrm{Br}, \mathrm{BrCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) and \(\mathrm{BrCH}_{2} \mathrm{CH}_{2} \mathrm{Cl}\). Justify the formation of these products by giving a suitable mechanism.

Short answer

Question: Explain how 1,2-dibromoethane, 2-bromoethanol, and 2-bromochloroethane can be formed from the reaction of ethylene gas with a mixture of an aqueous solution of bromine and sodium chloride. Answer: The formation of these products occurs through the initial formation of a bromonium ion intermediate. This positively charged intermediate is attacked by various nucleophiles in the solution, such as bromide ions (forming 1,2-dibromoethane), water molecules (forming 2-bromoethanol), and chloride ions (forming 2-bromochloroethane).

Step-by-step solution

Reaction of ethylene with bromine

When ethylene gas (C2H)_4) is passed into a mixture of an aqueous solution of bromine (Br2), the double bond in ethylene attacks the bromine molecule, causing it to form a cyclic bromonium ion. This intermediate has a positive charge and is highly reactive. The intermediate can be represented as follows: C2H4 + Br2 -> [Br-CH2-CH2-Br]+

Attack of the bromide ion on the bromonium ion

In the solution, there are bromide ions (Br-) present. These bromide ions can attack the bromonium ion, opening the cyclic structure and forming the product 1,2-dibromoethane (BrCH2CH2Br). This is an example of a nucleophilic attack on an electrophilic intermediate. [Br-CH2-CH2-Br]+ + Br- -> BrCH2CH2Br

Formation of 2-bromoethanol

The bromonium ion can also be attacked by water molecules present in the solution. This would lead to the formation of 2-bromoethanol (BrCH2CH2OH). This is another example of a nucleophilic attack on an electrophilic intermediate. [Br-CH2-CH2-Br]+ + H2O -> BrCH2CH2OH

Formation of 2-bromochloroethane

In the presence of sodium chloride (NaCl), there are chloride ions (Cl-) in the solution. These chloride ions can also attack the bromonium ion, forming the product 2-bromochloroethane (BrCH2CH2Cl). This reaction occurs less frequently, as bromide ions are better nucleophiles than chloride ions. [Br-CH2-CH2-Br]+ + Cl- -> BrCH2CH2Cl In conclusion, the formation of 1,2-dibromoethane, 2-bromoethanol, and 2-bromochloroethane occurs through the initial formation of a bromonium ion intermediate, which is a positively charged species that can be attacked by various nucleophiles in the solution, such as bromide ions, water molecules, and chloride ions.

Key concepts

Bromonium Ion

The concept of a bromonium ion is central to understanding the reaction of ethylene with bromine. When ethylene, which is a simple alkene, reacts with bromine, it forms a reactive intermediate known as a bromonium ion. This occurs because the double bond in ethylene attacks the bromine molecule, leading to the temporary formation of a three-membered ring. This ring includes the two carbon atoms from ethylene and a bromine atom.
The structure of the bromonium ion is quite unique. It carries a positive charge and has a significant amount of strain due to the small size of the ring. This makes the bromonium ion very reactive and prone to further chemical reactions.

Nucleophilic Attack

Nucleophilic attack is a critical step that explains the transformation of the bromonium ion into various products. In chemistry, a nucleophile is an electron-rich species that seeks to donate electrons to electron-deficient areas, like a bromonium ion.
In the described reaction, several nucleophiles are present in the solution, including bromide ions, water molecules, and chloride ions. Each of these nucleophiles can attack different parts of the bromonium ion.
- **Bromide ions**: These negatively charged ions can attack the bromonium ion, leading to the formation of 1,2-dibromoethane.
- **Water molecules**: Despite being a weaker nucleophile than bromide ions, water can also attack the bromonium ion, forming 2-bromoethanol.
- **Chloride ions**: Present in lower concentrations compared to bromide ions, chloride can still attack the bromonium ion, resulting in the production of 2-bromochloroethane. This shows that multiple nucleophiles can attack a given electrophilic intermediate.

Electrophilic Intermediate

An electrophilic intermediate plays a vital role in organic reaction mechanisms. In this context, the bromonium ion acts as the electrophilic intermediate. Electrophiles are species that are attracted to electron-rich areas because they can accept electrons to form new bonds.
The formation of the bromonium ion from ethylene and bromine is a classic example. Here, the bromonium ion is a positively charged, electron-poor species that is ready to accept electrons from nucleophiles. Its positive charge and structural strain make it a highly reactive intermediate that facilitates further transformations.
Understanding the nature of electrophilic intermediates helps explain why certain products form in a reaction. It highlights how molecular interactions and electronic demands can dictate the pathway and outcomes of chemical reactions.

Halogen Addition Reaction

Halogen addition reactions are a type of electrophilic addition reaction where halogens are added across the double bonds of alkenes. The process begins when a halogen molecule, like bromine, approaches the double bond and forms a bromonium ion. This cyclic structure serves as an intermediate that directs the subsequent nucleophilic attack.
The addition of halogens is a common method to transform alkenes into more complex molecules. In this reaction, bromine is added to ethylene, and due to the presence of different nucleophiles in the solution, multiple products can form.

• 1,2-dibromoethane is produced by bromide ion attack.
• 2-bromoethanol arises from water attack.
• 2-bromochloroethane results from chloride ion attack.

This versatility makes halogen addition reactions a powerful tool in organic synthesis, allowing the generation of various products from a single type of intermediate.