Problem 2 - A Complete Resource Book in Chemistry for JEE Main

Question

In diborane (a) 2 -bridged hydrogens and four terminal hydrogens are present (b) 3-bridged hydrogens and three terminal hydrogens are present (c) 4-bridged hydrogens and two terminal hydrogens are present (d) none of the above

Step-by-Step Solution

Answer

(a) 2-bridged hydrogens and four terminal hydrogens are present.

Step 1: Identifying the Structure of Diborane

Diborane (B2H6) consists of two boron atoms and six hydrogen atoms. Its structure is unusual among boranes, as it includes both terminal and bridging hydrogen atoms.

Step 2: Understanding Bridged and Terminal Hydrogen Atoms

In diborane, four hydrogen atoms are attached directly to the boron atoms. These are called terminal hydrogen atoms. The remaining two hydrogen atoms act as bridges between the two boron atoms, known as bridged hydrogens.

Step 3: Analyzing the Options

Option (a) mentions 2-bridged hydrogens and four-terminal hydrogens, which matches the structure of diborane as it has exactly 2 bridged hydrogens coordinating the two borons and 4 terminal hydrogens. Options (b) and (c) do not match as they describe different numbers of bridged and terminal hydrogens.

Step 4: Conclusion

Therefore, option (a) correctly describes the hydrogens in diborane with 2 bridged and 4 terminal hydrogens.

Key Concepts

Bridging Hydrogen

In diborane (B\(_2\)H\(_6\)), the concept of bridging hydrogen is truly fascinating.
These hydrogens do not just connect with one boron atom each. Instead, they serve as a bridge, connecting the two boron atoms in a shared bonding arrangement.
This creates a unique three-center, two-electron bond, often symbolized as a B-H-B bond, where the hydrogen atom partly bonds with each boron atom.
The presence of bridging hydrogens is one of the reasons why diborane has its distinctive structural geometry and reduced association when compared to other hydrides and elements.

• Bridging hydrogens form a chemical bridge between two boron atoms.
• The unusual bonding is described as a three-center, two-electron bond.
• This type of bond is less common and does not occur in simpler hydrocarbon structures.

The function of bridging hydrogen atoms is crucial in understanding how diborane is held together, ultimately affecting its reactivity and properties.

Terminal Hydrogen

Terminal hydrogens in diborane are, in a sense, much more conventional than bridging hydrogens.
Each terminal hydrogen atom is bonded directly to a boron atom, much like a simple hydrogen atom in hydrocarbon compounds.
This linear bonding offers a clearer picture of what we usually expect from hydrogen in terms of bonding. In terms of position:

• Terminal hydrogens are located at the "ends" of the borane structure.
• They allow the diborane molecule to maintain a degree of structural stability.
• Typically, there are four terminal hydrogen atoms in diborane.

Understanding the role of terminal hydrogens enriches our view of diborane's structure and illustrates how varied hydrogen's role in bonding can be.
When thinking of diborane, bear in mind how these hydrogens complement the bridging counterparts.

Boron Atoms

In the diborane molecule, the boron atoms play essential roles as molecular anchors that define the unique architecture of the compound.
There are two boron atoms within diborane, each contributing to the formation of the compound's structure.
Boron atoms in diborane are:

• Central to connecting the terminal and bridging hydrogen atoms.
• Linked together by the bridging hydrogens, exhibiting an unusual electron-deficient bond.
• Responsible for the compound's overall stability and chemical ity.

This configuration leads to an electron-deficient species, different from the traditional covalent bonding seen in typical hydrocarbons.
The boron atoms and their bonding configurations highlight how different bonding types can merge to create stable, yet unusual, molecular structures.