Problem 8 - Chemistry in 30 days

Question

In diborane: (a) four bridged hydrogens and two terminal hydrogens are present (b) two bridged hydrogens and four terminal hydrogens are present (c) three bridged hydrogens and three terminal hydrogens are present (d) none of these

Step-by-Step Solution

Answer

(b) two bridged hydrogens and four terminal hydrogens are present.

Step 1: Identify the Diborane Structure

Diborane (\( B_2H_6 \)) is a compound containing boron and hydrogen. To solve this problem, it's essential first to understand the molecular structure of diborane. This molecule is known for having unique bonding, including both bridge and terminal hydrogens.

Step 2: Review the Molecular Structure

Diborane consists of two boron atoms and six hydrogen atoms. It forms \( B_2H_6 \), with each boron atom bonded to two terminal hydrogens (directly attached) and two borons connected via bridge hydrogens.

Step 3: Count the Bridged Hydrogens

In diborane, two of the hydrogen atoms form bridges, sharing a single electron between two boron atoms. This arrangement accounts for the presence of two bridged hydrogen atoms.

Step 4: Count the Terminal Hydrogens

The remaining four hydrogen atoms are terminal hydrogens. These are directly connected to the two boron atoms, with each boron atom bearing two terminal hydrogens.

Step 5: Analyze the Options

Given the structure of diborane, two bridged hydrogens and four terminal hydrogens are correct. Thus, the correct choice from the options provided would be: (b) two bridged hydrogens and four terminal hydrogens are present.

Key Concepts

Bridged Hydrogens

Diborane is a fascinating molecule known for its unique arrangement of hydrogen atoms. Among them are the bridged hydrogens. These hydrogens are noteworthy because they do not connect to one boron atom as typical hydrogen bonds might suggest. Instead, they form a bridge between two boron atoms.

Imagine the diborane structure as two boron atoms connected by a pair of 'holding hands.' The bridged hydrogens are these 'hands,' shared between the borons. This type of bond is called a three-center two-electron bond.

Here's how it works:

• Each of the two hydrogen atoms involved contributes one electron each.
• These two electrons are shared among the two hydrogen and two boron atoms.

This arrangement is unusual and not typical in most molecules, making bridged hydrogens a point of interest in chemistry studies.

Terminal Hydrogens

On the other hand, diborane also contains terminal hydrogens. These hydrogens are more straightforward in their bonding compared to bridged hydrogens.

Each boron atom in diborane is bonded directly with two hydrogen atoms. These are the terminal hydrogens. If you visualize diborane, picture the boron atoms having 'arms' that directly reach out and connect to these hydrogens.

Key characteristics include:

• These are two-center two-electron bonds, which are standard covalent bonds.
• They involve two electrons shared between one boron atom and one hydrogen atom.
• In total, diborane has four terminal hydrogens.

Understanding how terminal hydrogens bond helps distinguish them from the more complex bridging hydrogens, and this clarity benefits a deeper grasp of diborane's overall structure.

Diborane Bonding

The bonding in diborane is quite unusual and intriguing. It merges two different bonding styles within a single compound, making it a unique study in molecular chemistry.

Diborane bonding can be summarized with these insights:

• The compound consists of two types of hydrogen bonds: bridging and terminal.
• In bridging bonds, we see the rarely occurring three-center two-electron bond.
• Terminal bonds display standard two-center two-electron covalent bonding.

These two forms come together to meet the electron needs of boron. Normally, boron doesn't satisfy the typical octet rule because it has only three valence electrons.

However, by adopting both bridging and terminal hydrogens, diborane accomplishes stable interactions despite this electron shortage, creating a remarkable and stable structure. Exploring these bonds reveals a rich tapestry of chemical creativity and adaptability in nature's bonding toolkit.