Problem 140 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following is the correct statement? [2008] (a) Boric acid is a protonic acid (b) Beryllium exhibits coordination number of six.(c) Chlorides of both beryllium and aluminium have bridged chloride structures in solid phase. (d) \(\mathrm{B}_{2} \mathrm{H}_{6} .2 \mathrm{NH}_{3}\) is known as 'inorganic benzene'

Step-by-Step Solution

Answer

The correct statement is (c) Chlorides of both beryllium and aluminium have bridged chloride structures in solid phase.

Step 1: Identifying Boric Acid Nature

Boric acid, with the chemical formula \( \text{H}_3\text{BO}_3 \), is known for not donating protons in its reactions. Instead, it acts as a Lewis acid by accepting hydroxide ions \( \text{OH}^- \). Therefore, boric acid is not a protonic acid.

Step 2: Evaluating Beryllium's Coordination Number

Beryllium typically forms covalent compounds and does not have a coordination number of six. Generally, its coordination number is usually four, given its small atomic size and tendency to form complexes with short coordination numbers.

Step 3: Analyzing Chloride Structures

Both beryllium chloride (\( \text{BeCl}_2 \)) and aluminum chloride (\( \text{AlCl}_3 \)) are known to have bridged chloride structures in the solid phase. In these structures, chloride ions bridge between metal atoms, leading to polymeric chain or layered structures.

Step 4: Understanding \( \mathrm{B}_{2} \mathrm{H}_{6} .2 \mathrm{NH}_{3} \)

The compound \( \mathrm{B}_{2} \mathrm{H}_{6} .2 \mathrm{NH}_{3} \) is not referred to as 'inorganic benzene'. Instead, \( \text{B}_3\text{N}_3\text{H}_6 \) is known as 'inorganic benzene' because it mimics the aromatic structure of benzene with alternating boron and nitrogen atoms.

Key Concepts

Boric Acid

Boric acid is an interesting compound often misunderstood in terms of its acidic properties. Unlike typical acids, boric acid, with the formula \( \text{H}_3\text{BO}_3 \), does not donate protons easily in reactions. Instead, it uniquely acts as a Lewis acid, which means it has the ability to accept hydroxide ions \( \text{OH}^- \). This behavior clearly distinguishes it from protonic acids, which readily donate protons. Boric acid's molecular structure allows it to form a tetrahedral complex by bonding with hydroxide ions, which plays a pivotal role in many industrial applications involving this compound.

Lewis Acids

Lewis acids are fascinating because they broaden the traditional definition of acids and bases. Unlike the classical Arrhenius or Bronsted-Lowry definitions, which involve proton donation or acceptance, Lewis acids focus on accepting electron pairs. This acceptance is vital to their role in forming coordinate covalent bonds. Boric acid is an excellent example of a Lewis acid, as it can accept an electron pair from a base. This concept isn't limited to boron compounds; various other substances, including several metal ions, can also function as Lewis acids, which is critical in many chemical reactions, especially in organic and inorganic synthesis.

Bridged Chloride Structures

Certain compounds, like beryllium chloride \( \text{BeCl}_2 \) and aluminum chloride \( \text{AlCl}_3 \), form bridged chloride structures. These structures are unusual but fascinating because they involve chlorine atoms linking multiple metal atoms. In the solid state, both \( \text{BeCl}_2 \) and \( \text{AlCl}_3 \) can polymerize through chloride bridges. These bridges create extended networks or layers that are crucial in determining the compound's properties such as electrical conduction and melting points. Understanding these structures is important, as they exemplify the diversity of bonding and its implications in solid-state chemistry.

Inorganic Benzene

'Inorganic benzene' is a term used to describe a compound structurally similar to benzene but made of alternating boron and nitrogen atoms. The correct formula for inorganic benzene is \( \text{B}_3\text{N}_3\text{H}_6 \). Its analogy to benzene comes from its similar planar structure and aromatic characteristics, such as electron delocalization, giving it stability akin to organic benzene. However, unlike carbon-based benzene, the presence of boron and nitrogen brings different chemical reactivity and properties to this compound. This distinction is crucial for fields like synthetic chemistry and materials science, where such compounds can provide unique functionalities.