Question

(a) The bromination of benzene is catalyzed by small amounts of iodine. Consider a possible explanation for this catalytic effect. (b) The kinetic expression for the bromination of naphthalene in glacial acetic acid involves a term that is first order in naphthalene and second order in bromine. Consider how two molecules of bromine and one of naphthalene could be involved in the rate-determining step of bromination. Suggest a reason why the kinetic expression simplifies to first order in naphthalenes and first order in bromine in 50 per cent aqueous acetic acid.

Short answer

(a) Iodine catalyzes the bromination of benzene by generating a bromonium ion (Br+), which electrophilically attacks the benzene ring, increasing the overall reaction rate. (b) In glacial acetic acid, the rate-determining step may involve two bromine molecules and one naphthalene molecule, with one bromine forming an intermediate complex. In a 50% aqueous acetic acid, the formation of bromine hydrate (HOBr) changes the reaction pathway, leading to a simplified kinetic expression of first order in naphthalenes and first order in bromine.

Step-by-step solution

(a) Iodine's catalytic effect in bromination of benzene

Iodine can act as a catalyst by generating a bromonium ion (Br+), which can electrophilically attack the benzene ring. This can be done by forming a complex with bromine, which weakens the bond between the two bromine atoms, allowing one to act as an electrophilic bromine (Br+) and the other as a leaving group (Br-). This process increases the overall reaction rate and facilitates the bromination of benzene.

(b) Kinetic expression for bromination of naphthalene in glacial acetic acid

The reaction is first order in naphthalene and second order in bromine, which suggests that the rate-determining step (slowest step in the reaction mechanism) might involve two molecules of bromine and one molecule of naphthalene. A possible mechanism could be that one bromine molecule forms an intermediate complex with naphthalene, and then this complex reacts with another bromine molecule to form the final product.

(b) Simplification of the kinetic expression in 50% aqueous acetic acid

In a 50% aqueous solution of acetic acid, the solubility of bromine increases, leading to the formation of bromine hydrate (HOBr). This changes the reaction pathway and involves the reaction between naphthalene and bromine hydrate instead of two bromine molecules. The reaction rate would depend significantly on the concentration of naphthalene and bromine hydrate which leads to a kinetic expression of first order in naphthalenes and first order in bromine.

Key concepts

Kinetics

Kinetics is a branch of chemistry that deals with reaction rates. It's basically about how fast a chemical process occurs. In our context with bromination, kinetics gives us insights into the rate at which benzene and naphthalene react with bromine. By understanding the kinetic expression, which is a formula that correlates the rate of a reaction to the concentration of reactants, we can determine factors that influence reaction speed. For bromination of benzene, the kinetic expression identifies iodine's role as a catalyst.
Similarly, for naphthalene in glacial acetic acid, the kinetic expression appears first order in naphthalene and second order in bromine, indicating that the reaction rate depends heavily on bromine molecules reacting efficiently with naphthalene molecules in the rate-determining step. Studying kinetics helps predict how long it takes for a reaction to reach completion and understand the conditions needed to control those rates.

Catalysis

Catalysis is all about making a reaction happen faster without the catalyst itself being consumed in the process. It's like a helping hand, speeding things up. In the bromination of benzene, iodine acts as a catalyst. This occurs because iodine can facilitate the creation of a bromonium ion ( Br^+ ) from bromine.
How does it work? Iodine forms a complex with bromine, weakening the bond between bromine atoms, and thereby assisting one bromine atom to become an active electrophile. By introducing a reactive bromine species, the role of iodine accelerates the bromination process.
So, even if the catalyst doesn't get used up, it makes the job of bromine atoms much easier and quicker, enhancing the overall efficiency of the reaction. Making use of catalysts like iodine is crucial in industrial processes to save time and energy.

Electrophilic Aromatic Substitution

Electrophilic Aromatic Substitution (EAS) is a key reaction mechanism when it comes to aromatic compounds like benzene. It's how we replace hydrogen atoms on a benzene ring with something else, like a bromine atom. The reaction revolves around an electrophile attacking the electron-rich benzene ring.
In bromination, the bromine atom acts as the electrophile, thanks to the iodine catalyst, which helps create the reactive bromonium ion needed for substitution. When benzene undergoes electrophilic aromatic substitution, the active bromine ion attaches to one of the carbon atoms in the benzene ring, replacing a hydrogen atom.
The result is a brominated benzene derivative. Understanding EAS is crucial for designing new aromatic compounds and plays an essential role in pharmaceuticals and dye manufacturing.

Reaction Mechanism

The reaction mechanism is like a step-by-step guide showing how reactants turn into products during a chemical reaction. It involves detailing each intermediate and transition state the reactants go through.
For the bromination of naphthalene, for example, the proposed mechanism signifies two key steps in glacial acetic acid: an initial complex forms when one bromine molecule interacts with naphthalene, followed by this complex's reaction with a second bromine molecule.
The presence of 50% aqueous acetic acid introduces an alternate route, featuring bromine hydrate. Details like these illustrate why the rate expression simplifies under different conditions, showcasing the pathway's changes. In essence, diving into the reaction mechanism unravels the complexity of chemical transformations, helping chemists understand and manipulate them effectively.