Question

What product is formed when ${\mathrm{X}}_2(\mathrm{X}=\mathrm{Br},\mathrm{Cl})$ is added to alkenes in the dark? Explain the mechanism of the reaction.

Short answer

When diatomic halogens, such as Bromine (Br) or Chlorine (Cl), are added to alkenes in the dark, they react through an electrophilic addition mechanism to form a vicinal dihalide. The mechanism involves two key steps: 1) Formation of a cyclic halonium ion, and 2) Attack by the second halogen atom, leading to the formation of the vicinal dihalide product.

Step-by-step solution

Understand the given information

We are given that a diatomic halogen molecule, either Bromine (Br) or Chlorine (Cl), is added to alkenes in the dark.

Determine the general product of the reaction

When diatomic halogens (Br2 or Cl2) are added to alkenes in the dark, they react through an electrophilic addition mechanism, resulting in the formation of a vicinal dihalide. For example, when ethene reacts with Br2 in the dark, they form the vicinal dihalide 1,2-dibromoethane.

Understanding the general reaction mechanism

The reaction mechanism for the electrophilic addition of halogens (X2, where X=Br, Cl) can be broken down into two key steps: 1. Formation of a cyclic halonium ion. 2. Attack by the second halogen atom and formation of vicinal dihalide.

Elaborate on the formation of the cyclic halonium ion

In the first step, the pi electrons in the double bond of the alkene interact with the halogen molecule, forming a bond with one of the halogen atoms and creating a cyclic halonium ion in the process. The electron cloud in the remaining halogen atom becomes negatively charged, resulting in a halide ion (X-).

Explain the attack by the second halogen atom and the formation of the vicinal dihalide

In the second step, the negatively charged halide ion (X-) attacks the positively charged cyclic halonium ion at the more substituted carbon (if applicable, depends on the alkene used). This bond formation leads to the opening of the cyclic halonium ion and results in the formation of a vicinal dihalide as the final product. Overall, when a diatomic halogen molecule (Br2 or Cl2) is added to alkenes in the dark, a vicinal dihalide is formed through an electrophilic addition mechanism involving the formation of a cyclic halonium ion and subsequent attack by a halide ion.

Key concepts

Vicinal Dihalide Formation

When alkenes react with diatomic halogens such as bromine (Br${}_2$) or chlorine (Cl${}_2$) in the dark, the result is the formation of a vicinal dihalide. A vicinal dihalide is a type of molecule where two halogen atoms are added to adjacent carbon atoms across a carbon-carbon double bond. This addition reaction is not random; it is a step-by-step process that's predictable.

This electrophilic addition mechanism leads to a specific product structure. For example, when ethene (H${}_2$C=CH${}_2$) reacts with bromine, the product is 1,2-dibromoethane (H${}_2$CBr-CH${}_2$Br). Understanding this product formation requires insight into the mechanism, specifically the role of the cyclic halonium ion and how the halide ions interact.

Cyclic Halonium Ion

The formation of a cyclic halonium ion is a crucial step in the electrophilic addition of halogens to alkenes. In this process, the double bond electrons are used to initially bond with one of the halogen atoms, leading to the formation of this cyclic structure.

The halonium ion is a positively charged three-membered ring where the halogen atom forms bonds with both carbon atoms from the original double bond. This cyclic structure is quite stable due to the way the electrons are distributed in the three-membered ring.

• The halogen atom temporarily carries a positive charge.
• This positive charge facilitates the next step of the reaction.

The halonium ion structure is critical because it governs how the rest of the reaction will proceed, particularly regarding the placement and addition of the second halogen atom.

Alkene Reaction Mechanism

The alkene reaction mechanism involves several steps that result in the transformation of simple alkenes into vicinal dihalides. At the heart of this mechanism is electrophilic addition, a type of reaction where a pi bond is broken and sigma bonds are formed, thereby adding atoms across the double bond.

It all begins when the alkene's pi bond electrons interact with a diatomic halogen molecule, forming a temporary bond with one of the halogen atoms. This leads to the formation of a cyclic halonium ion. The halonium ion captures the electrons more strongly committed toward forming two temporary bonds, preparing the molecule for the next step.

• Electrophilic attack because of positive cyclic ion.
• Directs the negative halide ion to attack the more substituted carbon, if applicable.

Both steps are essential for the conversion of the alkene to the dihalide, precisely dictating how the product is shaped.

Diatomic Halogen Addition

The addition of diatomic halogens such as Br${}_2$ and Cl${}_2$ to alkenes is characterized by the breaking of the strong diatomic bond and the formation of new bonds across the alkene's double bond. This addition is notably carried out in the dark to prevent side reactions initiated by light.

Each halogen molecule is composed of two atoms held together by a covalent bond. During the reaction, this bond is temporarily broken as the halogen approaches the electron-rich double bond of the alkene.

• One halogen atom participates in forming the cyclic halonium ion.
• The other atom becomes a halide ion, ready to complete the reaction.

Understanding this process is essential as it highlights the balanced energetic and electron shifts required to form stable vicinal dihalides.