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

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

Step 1: Identify the Diborane Structure

Diborane is a chemical compound with the formula \( \text{B}_2\text{H}_6 \). Its structural peculiarity comes from its bonding. The structure consists of two boron (B) atoms and six hydrogen (H) atoms. The boron atoms are bonded through two hydrogen atoms acting as bridges.

Step 2: Analyze the Hydrogen Bonding

In diborane, there are two types of hydrogens: bridging hydrogens and terminal hydrogens. The two hydrogen atoms that form bridges between the boron atoms create bonds known as 3-center-2-electron bonds, or banana bonds. The remaining four hydrogens are bonded directly to the boron atoms and are known as terminal hydrogens.

Step 3: Verify the Given Options

From the structure analysis, we know diborane has 2-bridged hydrogens and four terminal hydrogens. Look through the given options: Option (a) states 2-bridged hydrogens and four terminal hydrogens, which matches our analysis. Options (b), (c), and (d) do not match this description.

Key Concepts

Bridged Hydrogens

In the fascinating structure of diborane (\(\text{B}_2\text{H}_6\) ), the concept of bridged hydrogens plays a pivotal role. These are two specific hydrogen atoms that bind the two boron (\(B\)) atoms together in a unique manner.
They form bonds known as "bridge" bonds, which are quite distinct from typical covalent bonds.
Unlike regular bonds where two atoms share a pair of electrons, in bridged hydrogens, the electron pair is shared between three atoms - two boron atoms and the bridging hydrogen in between. This kind of bonding configuration, referred to as a 3-center-2-electron bond or "banana" bond due to its curved shape, facilitates the connection between these atoms in a non-standard yet stable manner.

• Two bridging hydrogens in diborane
• Each bridges both borons, forming "banana" bonds
• Central to diborane’s unique bonding nature

Terminal Hydrogens

Terminal hydrogens in diborane structure are those hydrogen atoms that are directly bonded to each boron atom without bridging or sharing electrons with other neighboring atoms.
In diborane, there are four terminal hydrogen atoms, each one forming a typical covalent bond with a boron atom.
These bonds are straightforward and resemble the single bonds we often envision when thinking of hydrogen's attachment to other elements. The presence of these terminal hydrogens is what differentiates the diborane structure from other potential boron-hydride configurations.

• Four terminal hydrogens in diborane
• Form single covalent bonds directly with boron atoms
• Provide stability alongside bridged hydrogens

3-center-2-electron Bonds

The 3-center-2-electron bond (also known as a banana bond) is a peculiar type of bonding that is fundamental to the structure of compounds like diborane.
This bond is characterized by the sharing of two electrons among three atoms – unlike typical covalent bonds where two atoms share two electrons.
In the case of diborane, these bonds form between two boron atoms and one bridging hydrogen atom, effectively making each bond a participant in this stable, electron-efficient structure.

• Shares electrons across three atoms
• Stabilizes without needing multiple electron pairs
• Unique to certain boron compounds, adding to diborane's interest

Understanding the role of these bonds helps clarify the complex geometry and unusual stability of boron-hydride molecules.