Question

Geometry of transition state during chlorination and bromination in Ist step of propagation respectively is: (A) Tetrahedral, planar (B) Planar, tetrahedral (C) Planar, Planar (D) Tetrahedral, Tetrahedral

Short answer

The geometry of transition states during chlorination and bromination in the first step of propagation is (A) Tetrahedral, planar.

Step-by-step solution

1. Understand the mechanism of the halogenation process

: In a radical halogenation process, the reaction mechanism consists of three main stages: initiation, propagation, and termination. Our focus is on the first step of the propagation stage.

2. Determine the first step of propagation for chlorination and bromination

: For chlorination (Cl₂) and bromination (Br₂), the first step of propagation involves the reaction between a halogen molecule with a hydrocarbon (R-H) to create a new radical (R•) and a hydrogen halide (H-X). This process can be represented as follows: R-H + X₂ -> R• + H-X, where X can either be Cl or Br.

3. Analyze transition state geometries

: For both chlorination and bromination, we have a carbon center in the hydrocarbon molecule, which is initially sp³ hybridized, indicating a tetrahedral geometry. When reacting with halogen molecules, the hydrocarbon goes through a transition state where the bond between carbon and hydrogen is partially broken, and the bond between carbon and halogen is forming. This transition state is the key point of the first step of propagation where we need to analyze the geometries. So let's look at the geometries for chlorination and bromination: - Chlorination (Cl₂): In this case, due to the small size of the chlorine atom, it can form a transition state close to the tetrahedral arrangement. So carbon goes from sp³ to a partial tetrahedral geometry with some planar character (≈sp³ with C-Cl bond in formation). - Bromination (Br₂): Due to the larger size of the bromine atom, the approach of the atom to the carbon center is less effective which results in a more planar arrangement. So, carbon goes from sp³ to planar (sp²) hybridization with a partial C-Br bond in formation.

4. Choose the correct answer option

: Now let's relate the geometries analyzed in step 3 to the answer options: (A) Tetrahedral, planar: This option indicates that the geometry of transition states during chlorination is tetrahedral and during bromination is planar. This choice is in line with the conclusions made in step 3. (B) Planar, tetrahedral (C) Planar, Planar (D) Tetrahedral, Tetrahedral Therefore, the correct answer is (A) Tetrahedral, planar.

Key concepts

Transition State Geometry

Understanding the geometry of the transition state is crucial in the halogenation reaction mechanism. During a chemical reaction, the transition state represents a high-energy, unstable arrangement of atoms where old bonds are breaking, and new bonds are forming. It is like a molecular 'tightrope walker' balancing between the reactants and the products.

In halogenation, the transition state geometry varies depending on the size of the halogen atom involved. For chlorination, where chlorine is smaller, the transition state retains more of the original tetrahedral shape of the carbon atom involved, which indicates that it closely resembles the sp³ hybridized state. The smaller size of chlorine allows it to approach the carbon atom more effectively, forming a bond without significantly disrupting the original geometry.

Conversely, bromination involves a larger bromine atom, leading to a transition state that is more planar, akin to sp² hybridization. This happens because the larger bromine atom cannot form a bond without pushing other substituents away, thus flattening the molecule's structure during the bond formation.

The correct understanding of these geometries can be the difference in predicting the correct outcome of the reaction and is essential for students learning about reaction mechanisms.

Propagation Stage in Halogenation

The propagation stage is where the heart of the radical halogenation process beats. After the initiation step, which generates the reactive halogen radicals, propagation carries the reaction forward through a chain mechanism. This stage involves a sequence of steps that sustain the reaction until the reactants are consumed.

During the propagation stage of chlorination or bromination, a halogen radical reacts with a hydrocarbon, abstracting a hydrogen atom to form a hydrogen halide and a new carbon-centered radical. This new radical can then react with another halogen molecule, creating another halogen radical and a halogenated hydrocarbon - the desired product. This cycle can repeat many times, propagating the chain reaction.

The propagation stage is crucial because it determines the rate at which the reaction proceeds and the specificity in terms of which hydrogen atoms are abstracted, which directly affects the product distribution. Understanding this stage helps in controlling the reaction to achieve the desired halogenation with minimal side products.

Radical Halogenation Process

Radical halogenation is a multi-step process involving the generation and consumption of radicals, which are atoms or molecules with an unpaired electron. These radicals are highly reactive, seeking to pair up their single electron by forming new bonds - a characteristic that drives the halogenation reaction.

The process begins with the initiation step where halogen molecules break into radicals under the influence of heat or light. Following initiation, the propagation stage takes over, where these halogen radicals react with hydrocarbons, systematically replacing hydrogen atoms with halogen atoms. Each step produces a new radical, which sustains the reaction, hence the term 'chain reaction'.

The final step is the termination stage where radicals combine with each other, forming stable, non-radical species and effectively quenching the reaction. A complete radical halogenation reaction is an interplay of these steps resulting in the substitution of hydrogen for halogen in an organic compound. It is a fundamental reaction in organic chemistry and a valuable tool in the synthesis of various halogenated compounds.