Question

Reagent ' \(\mathrm{X}^{\prime}\) in the following reaction may be: (A) \(\mathrm{Br}_{2} / \mathrm{h} v\) (B) N-bromosuccinimide (C) \(\mathrm{HBr}\) (D) \(\mathrm{NaBr}\)

Short answer

Reagent X' in the following reaction may be N-bromosuccinimide (B), as it is suitable for allylic and benzylic bromination reactions.

Step-by-step solution

Analyze properties and reactions of the choices

Let's analyze the properties and main reactions of each choice to deduce which one could be the correct reagent: (A) \(\mathrm{Br}_{2} / \mathrm{h} v\): Bromine (Br2) can react with alkenes and alkynes in the presence of light (hv) to produce vicinal dibromides through halogenation reaction. (B) N-bromosuccinimide (NBS): NBS is a brominating and oxidizing agent that is particularly used in allylic and benzylic bromination reactions. (C) \(\mathrm{HBr}\): Hydrogen bromide (HBr) is a strong acid and a reagent in many reactions, such as hydration of alkenes, where it protonates the double bond, leading to the formation of a carbocation intermediate and eventually the addition of a bromide ion to form an alkyl halide. (D) \(\mathrm{NaBr}\): Sodium bromide (NaBr) is a simple bromide salt, which can act as a weak nucleophile in certain reactions but is generally not considered a reactive reagent in organic chemistry.

Determine which choice is the most suitable reagent

By analyzing the properties and reactions of the given choices, we can deduce the suitable reagent for the reaction: (A) \(\mathrm{Br}_{2} / \mathrm{h} v\): Suitable for halogenation reactions with alkenes and alkynes. (B) N-bromosuccinimide: Suitable for allylic and benzylic bromination reactions. (C) \(\mathrm{HBr}\): Suitable for hydration of alkenes and other reactions where an acidic or halide reagent is needed. (D) \(\mathrm{NaBr}\): Generally not suitable as a reactive reagent in organic chemistry. Considering the reactivity and reactions of the given choices, we can conclude that reagent X' in the following reaction may be: (B) N-bromosuccinimide

Key concepts

Allylic Bromination

Allylic bromination is a specific type of reaction in organic chemistry that involves the substitution of a hydrogen atom located on an allylic position with a bromine atom. This process is vital as it modifies the molecular structure, making it possible to derive useful products from simple organic compounds. The versatility of allylic bromination stems from its ability to introduce a bromine atom without disrupting the core carbon-carbon double bond.

Allylic positions are characterized by the presence of a hydrogen atom on the carbon adjacent to a carbon-carbon double bond. Because these positions are more reactive, due to the resonance stability of the allylic radical formed during the reaction, the substitution can occur cleanly and under controlled conditions.

N-bromosuccinimide (NBS) is the preferred reagent for allylic bromination. This is due to its effective way of providing a low concentration of bromine through equilibrium, allowing for cleaner reactions and minimizing side-products. The controlled release of bromine ensures that the double bonds remain intact while brominating the allylic position specifically.

Organic Reagents

Organic reagents are substances used in chemical reactions to bring about a desired transformation in organic compounds. Understanding their roles and reactivity is crucial for efficiently carrying out chemical reactions.

They can act as catalysts, reactants, or modifiers of reaction conditions. Their function relies heavily on the specific reaction they're involved in, which includes forming or breaking chemical bonds, and rearranging molecular structures.

Reagents like N-bromosuccinimide (NBS) in the context of allylic bromination, work as a source of bromine for selective bromination without needless overreaction. This selectivity is vital for retaining functional group integrity during transformations.

When selecting reagents, it is essential to consider their compatibility with the substrate, reaction conditions, and the type of transformation desired. Reactivity, selectivity, and stability are all important parameters to evaluate when choosing the right organic reagent for a reaction.

Halogenation Reactions

Halogenation is the process of adding halogen atoms—like chlorine or bromine—to organic compounds. It's one of the key reactions used widely in organic chemistry to synthesize a wide array of products. The inclusion of halogen atoms can significantly change the properties of the original molecule, often increasing its reactivity and altering its physical characteristics.

There are two types of halogenation reactions: electrophilic and radical. In the context of allylic bromination, radical halogenation is more relevant. For instance, bromination with elements like N-bromosuccinimide (NBS) under light or heat initiates the process by creating bromine radicals. These radicals facilitate the abstraction of allylic hydrogen atoms, paving the way for bromine substitution.

Halogenations are valuable as they afford straightforward paths to synthesis, modify molecular structures, and can introduce haloalkanes capable of further functionalization. Understanding the type of halogenation suitable for specific substrates is crucial for achieving the desired chemical transformations without unwanted side reactions.