Problem 1 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The reaction of \(\mathrm{H}_{3} \mathrm{~N}_{3} \mathrm{~B}_{3} \mathrm{Cl}_{3}(\mathrm{~A})\) with \(\mathrm{LiBH}_{4}\) in tetrahydrofuran gives inorganic benzene (B). Further, the reaction of (A) with (C) leads to \(\mathrm{H}_{3} \mathrm{~N}_{3} \mathrm{~B}_{3}(\mathrm{Me})_{3}\). Compounds (B) and (C) respectively, are: [Main Jan. 09, 2020 (II)] (a) Borazine and \(\mathrm{MeBr}\) (b) Diborane and \(\mathrm{MeMgBr}\) (c) Boron nitride and MeBr (d) Borazine and \(\mathrm{MeMgBr}\)

Step-by-Step Solution

Answer

Compounds (B) and (C) are borazine and MeMgBr, respectively (option d).

Step 1: Analyze the Reaction with LiBH4

The compound \( \mathrm{H}_{3} \mathrm{N}_{3} \mathrm{B}_{3} \mathrm{Cl}_{3} \) (A) is treated with \( \mathrm{LiBH}_{4} \) in tetrahydrofuran. This reaction typically replaces chlorine atoms with hydrogen in compounds like borazine, resulting in the formation of inorganic benzene, which is known as borazine, \( \mathrm{B}_{3} \mathrm{N}_{3} \mathrm{H}_{6} \) (B). Thus, compound (B) formed from (A) is borazine.

Step 2: Analyze the Reaction with (C)

Next, \( \mathrm{H}_{3} \mathrm{N}_{3} \mathrm{B}_{3} \mathrm{Cl}_{3} \) (A) reacts with another compound to form \( \mathrm{H}_{3} \mathrm{N}_{3} \mathrm{B}_{3}(\mathrm{Me})_{3} \). This suggests a methylation reaction, where chlorine atoms in \( \mathrm{H}_{3} \mathrm{N}_{3} \mathrm{B}_{3} \mathrm{Cl}_{3} \) are replaced by \( \mathrm{Me} \) groups. A common reagent for methylation is \( \mathrm{MeMgBr} \), a Grignard reagent. Thus, compound (C) is \( \mathrm{MeMgBr} \).

Key Concepts

Inorganic Chemistry

Inorganic chemistry is a branch of chemistry that primarily deals with compounds that do not contain carbon-hydrogen (C-H) bonds. This field encompasses a wide range of substances, such as metals, minerals, and elements in their purest form. Inorganic chemistry is pivotal for understanding various chemical reactions and processes that do not fall under the umbrella of organic chemistry.

Many topics in inorganic chemistry involve the study of chemical bonding, structure, and the reactivity of inorganic compounds. This is often seen in the reactions involving coordination compounds, which are central to many processes in biology and industry.

• Catalysis: Many inorganic compounds serve as catalysts, speeding up reactions without being consumed.
• Material Science: Inorganic substances are crucial in the development of ceramics, semiconductors, and superconductors.
• Bioinorganic Chemistry: This subfield explores the role of metals in biological systems.

In the exercise we analyzed, the chemical transformations of \(\mathrm{H}_{3} \mathrm{N}_{3} \mathrm{B}_{3} \mathrm{Cl}_{3}\), which is a chlorine-containing compound, illustrate how inorganic chemistry principles are applied to transform these compounds using lithium borohydride.

Borazine

Borazine, often referred to as inorganic benzene, is a chemical compound with the formula \( \mathrm{B}_{3} \mathrm{N}_{3} \mathrm{H}_{6} \). Its structure is similar to benzene, comprising a six-membered ring alternating between boron and nitrogen atoms. This fascinating compound exhibits properties similar to those of aromatic compounds due to its electronic structure and layout.

Some characteristics of borazine include:

• Stable Ring Structure: While borazine maintains a stable ring, it is more polar than benzene.
• Aromaticity: Borazine displays some aromatic character due to delocalized electrons across the ring.
• Chemical Reactivity: Despite its stability, borazine can engage in various reactions, such as hydrogenation.

The concept of borazine being analogous to benzene is important as it helps understand aromatic stability within inorganic molecules. It’s also remarkable due to its potential application in materials science and understanding the chemistry of boron and nitrogen. In our exercise, the interaction with \( \mathrm{LiBH}_{4} \) in tetrahydrofuran helps convert the original compound into borazine, shedding chlorine atoms and gaining hydrogen.

Methylation

Methylation is a chemical process that involves the transfer of a methyl group (\( \mathrm{-CH_3} \)) to a substrate, introducing new properties to the molecule. This modification significantly alters the chemical behavior, physical properties, and sometimes even the biological functions of the original compound.

Key aspects of methylation include:

• Common in Organic Chemistry: Methylation is widely used in modifying organic molecules to impact solubility, reactivity, or stability.
• Methylating Agents: Chemicals like methyl iodide (\( \mathrm{CH_3I} \)) and methyl magnesium bromide (\( \mathrm{MeMgBr} \)) are frequently used in these reactions.
• Biological Relevance: Methylation plays a crucial role in regulating DNA expression and other biological processes.

In the chemical reaction explored in our exercise, \( \mathrm{MeMgBr} \) acts as a methylating agent. This reagent replaces chlorine atoms in the starting compound, \( \mathrm{H}_{3} \mathrm{N}_{3} \mathrm{B}_{3} \mathrm{Cl}_{3} \), with methyl groups to produce the compound \( \mathrm{H}_{3} \mathrm{N}_{3} \mathrm{B}_{3} (\mathrm{Me})_{3} \). This change illustrates how methylation can transform the structure and properties of inorganic compounds dramatically.