Problem 18 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Read the following Statement-1 (Assertion) and Statement -2 (Reason) and answer as per the options given below: Statement-1 : Bromobenzene upon reaction with \(\mathrm{Br}_{2} / \mathrm{Fe}\) gives \(1,4-\) dibromobenzene as the major product. [2008S] Statement-2 : In bromobenzene, the inductive effect of the bromo group is more dominant than the mesomeric effect in directing the incoming electrophile. (a) Statement-1 is True, Statement-2 is True; Statement-2 is a correct explanation for Statement-1. (b) Statement- 1 is True, Statement- 2 is True; Statement- 2 is NOT a correct explanation for Statement- 1 (c) Statement-1 is True, Statement- 2 is False (d) Statement-1 is False, Statement-2 is True

Step-by-Step Solution

Answer

(c) Statement-1 is True, Statement-2 is False.

Step 1: Analyze Statement-1

Statement-1 claims that bromobenzene reacts with \( \mathrm{Br}_{2} / \mathrm{Fe} \) to produce 1,4-dibromobenzene as the major product. Bromobenzene has a bromine atom that is an ortho-para directing group due to its resonance (mesomeric effect), making it more likely that incoming electrophiles will substitute at the ortho or para positions relative to the existing bromine. Therefore, 1,4-dibromobenzene, which is para to the original bromine, is indeed the major product. Statement-1 is true.

Step 2: Analyze Statement-2

Statement-2 suggests that in bromobenzene, the inductive effect of the bromo group is more dominant than the mesomeric effect in directing the incoming electrophile. However, the mesomeric (or resonance) effect of the bromo group is stronger than the inductive effect when it comes to directing the site of electrophilic aromatic substitution. This means the mesomeric effect leads to the para substitution, not the inductive effect. Therefore, Statement-2 is false.

Step 3: Determine the Relationship Between The Statements

Given the truth values of the statements from the previous steps, Statement-1 is true and Statement-2 is false. Therefore, Statement-2 cannot explain the reaction described in Statement-1.

Key Concepts

Bromobenzene

When we talk about bromobenzene, we're referring to a chemical compound where a bromine atom is attached to a benzene ring. This compound is particularly interesting in organic chemistry, especially in reactions known as Electrophilic Aromatic Substitution (EAS). In EAS reactions, bromobenzene can act as a starting material for substitutions where the benzene ring reacts with electrophiles.

The presence of the bromine atom on benzene affects the substitution pattern of the reaction. Unlike benzene, which typically undergoes reaction without preference for any position, bromobenzene shows a preference for substitution at specific positions on the ring. This is due to the electron-withdrawing properties of bromine, which influence where new bonds are formed in the benzene ring.

Understanding the behavior of bromobenzene is crucial in predicting the outcome of electrophilic substitution reactions because it sets a pattern for where new substituents will most likely appear.

Ortho-Para Directing Groups

Ortho-para directing groups, such as the bromine in bromobenzene, are groups that direct incoming electrophiles to the ortho (positions closest to the substituent) and para (position directly opposite to the substituent) positions on an aromatic ring. In the case of bromobenzene, the bromine atom acts as an ortho-para director due to its electron donation ability through resonance.

Here's how it works: the resonance effect from the bromine atom introduces additional stability to the intermediates formed during the reaction. This resonance effect is also known as a mesomeric effect, where the delocalization of pi electrons stabilizes certain positions on the ring, making them more reactive to electrophilic attacks.

In essence, because these positions are more stable, incoming electrophiles are more likely to attach at these ortho and para positions, leading to specific substitution patterns, such as the formation of 1,4-dibromobenzene from bromobenzene.

Inductive and Mesomeric Effects

The inductive and mesomeric effects are two critical concepts in understanding how substituents influence the reactivity of the benzene ring in electrophilic aromatic substitution. The inductive effect is a result of the electronegativity of the substituent, which can pull electron density away from or push electron density towards the aromatic ring through sigma bonds.

However, when it comes to bromobenzene and similar compounds, the mesomeric effect plays a more significant role. The mesomeric effect involves the p-orbitals of the substituent (like bromine) overlapping with the pi-system of the benzene ring, enhancing the electron density at specific positions (ortho and para).

This is why, in the case of bromobenzene, the mesomeric effect dominates over the inductive effect. As a result, the directing influence in EAS reactions is primarily determined by the mesomeric effect, which promotes substitution at these more stabilized ortho and para positions, leading to predictable outcomes in chemical syntheses.