Question

The correct statement during following propagation step of photohalogenation is/are (A) If $\mathrm{X}=\mathrm{Cl}$, transition state is almost planar in nature (B) If $\mathrm{X}=\mathrm{Br}$, transition state is almost tetrahedral in nature (C) If $\mathrm{X}=\mathrm{Br}$, transition state is almost planar in nature (D) If $\mathrm{X}=\mathrm{Cl}$, transition state is almost tetrahedral in nature

Short answer

The correct statements during the following propagation step of photohalogenation are: (A) If X=Cl, the transition state is almost planar in nature (B) If X=Br, the transition state is almost tetrahedral in nature

Step-by-step solution

Understanding of photohalogenation and the transition state

Photohalogenation is a chemical reaction in which a halogen (X) reacts with an organic compound in the presence of light or ultraviolet radiation to replace one or more hydrogen atoms. The formation of the reaction intermediate from the reactants is the first step, followed by the propagation steps and termination steps. Here we want to specifically analyze the propagation step and the nature of the transition state, when the halogen is chlorine (Cl) or bromine (Br). The transition state is a high-energy structure formed during a chemical reaction, at which the reactants are about to convert into the products. It is essential to know whether the transition state of the propagation step is planar (flat) or tetrahedral (pyramidal) when X=Cl and X=Br.

Nature of transition state when X=Cl

When X=Cl, the transition state during the propagation step of photohalogenation is known to be almost planar. This is because of the low steric requirements of chlorine and its ability to form a delocalized π-transition state due to the overlapping of its 3p orbitals with the 2p orbitals of the reacting species. Thus, statement (A) is correct.

Nature of transition state when X=Br

When X=Br, the transition state during the propagation step of photohalogenation is known to be almost tetrahedral. This is due to the larger atomic size and relatively lesser ability to form a delocalized π-transition state as compared to chlorine. The larger size of bromine results in a preference for the tetrahedral transition state. Thus, statement (B) is correct.

Analyze the remaining statements

Statement (C) says that when X=Br, the transition state is almost planar. This is incorrect, as we have established that, for bromine, the transition state is almost tetrahedral. Statement (D) says that when X=Cl, the transition state is almost tetrahedral. This is also incorrect, as we have established that, for chlorine, the transition state is almost planar.

Conclusion

Based on our analysis of the transition states during the propagation step of photohalogenation for X=Cl and X=Br, the correct statements are: (A) If X=Cl, the transition state is almost planar in nature (B) If X=Br, the transition state is almost tetrahedral in nature

Key concepts

Transition State in Photohalogenation

During photohalogenation, the transition state is a pivotal stage that occurs between the reactants and products. It represents a high-energy configuration where bonds are partially broken and formed.
The nature of the transition state varies depending on the halogen involved. When considering chlorine (Cl) during this process, the transition state tends to be almost planar. This happens because chlorine's smaller size allows overlapping of its 3p orbitals with the 2p orbitals of adjacent atoms, creating a more delocalized and planar state.

• The overlapping π-transition state, characteristic of smaller and lighter halogens like chlorine, reduces steric hindrance.
• A planar transition state is due to chlorine's ability to reduce overall structural tension and allow smoother bond transformation.

Understanding this concept is crucial in predicting the outcome and rate of reactions involving different halogens.

Propagation Step in Photohalogenation

The propagation step in photohalogenation is where the main chain of reactions occurs. This step involves radical intermediates that react with the surrounding molecules to perpetuate the reaction cycle.
Essentially, during this stage, a halogen radical reacts with hydrogen atoms in an organic compound, forming a new radical. This ongoing cycle proceeds until stable products form or the radicals are terminated.

• Propagation is characterized by two sub-steps: breaking of a C-H bond and formation of a C-X bond.
• During this step, the transition state's nature significantly impacts reaction speed and mechanism.

In the context of chlorine or bromine, chlorine typically reacts faster in the propagation step due to its higher reactivity, while bromine is more selective.

Chlorine and Bromine Reactivity

Chlorine and bromine, both halogens, exhibit distinct behaviors in photohalogenation due to their different atomic sizes and electron configurations.
Chlorine is widely recognized for its high reactivity. Its relatively small atomic size allows it to easily form radicals and engage in rapid reaction processes with a variety of substrates. This high reactivity often results in less selectivity, meaning it can form multiple products.
Bromine, on the other hand, is less reactive compared to chlorine but is more selective. Due to its larger atomic size, bromine radicals are more stable and tend to react primarily with more active sites.

• Chlorine's smaller size leads to higher reactivity but lower selectivity.
• Bromine's larger size offers more stability and selectivity in product formation.

Knowing the differences between chlorine and bromine reactivity helps in better planning and prediction of outcomes in photohalogenation reactions.