Problem 7 - A Complete Resource Book in Chemistry for JEE Main

Question

Among the following compounds of boron, the spe cies which also forms \(\pi\)-bond in addition to \(\sigma\)-bond: is (a) \(\mathrm{BF}_{3}\) (b) \(\mathrm{BF}_{4}^{-}\) (c) \(\mathrm{B}_{2} \mathrm{H}_{6}\) (d) \(\mathrm{BH}_{3}\)

Step-by-Step Solution

Answer

The compound with a c-bond is (a) .

Step 1: Understanding the bonds

In molecular chemistry, a c-bond (pi bond) involves the side-to-side overlap of orbitals, often found in double and triple bonds. A c-bond (sigma bond) is the first bond formed between two atoms and involves head-on overlap. We need to identify which boron compound features both c and c bonds.

Step 2: Looking at each compound

Let's analyze the boron compounds one by one. (a)  is known to form a double bond with each fluorine through a sigma bond and a pi bond due to p overlap. (b)  only forms sigma bonds in tetrahedral configuration, as it is sp
1 regulation. (c)  features three-center two-electron bonds (3c-2e), known for sigma bonding but not pi bonds. (d)  forms only sigma bonds due to its instability as it dimerizes to form .

Step 3: Conclusion about the compound

The only compound that forms both c and c bonds is , due to the pc overlap with fluorine atoms.

Key Concepts

Sigma Bond

Sigma bonds play a foundational role in molecular chemistry. When two atoms form a sigma bond, their atomic orbitals overlap head-on, creating a strong bond. This overlap generally involves s and p orbitals, combining to form a new molecular orbital. Sigma bonds are the strongest type of covalent bond due to this direct overlap.

Here's why sigma bonds are unique:

• They allow for free rotation around the bond axis, offering flexibility in molecular structures.
• Sigma bonds are usually the first bonds formed when two atoms come together, establishing the primary connection.
• These bonds are often present in single bonds but are also found in double and triple bonds, where they coexist with pi bonds.

Most importantly, sigma bonds are crucial for stabilizing molecules, providing the framework upon which other interactions, such as pi bonds, are added.

Pi Bond

Pi bonds are a fascinating aspect of molecular chemistry, often paired with sigma bonds in double and triple bonds. These bonds occur due to the side-to-side overlap of atomic orbitals, typically p orbitals, perpendicular to the axis of the atoms involved.

Key points about pi bonds include:

• Unlike sigma bonds, pi bonds do not allow for free rotation because the electron density is concentrated above and below the plane of the atomic nuclei.
• Pi bonds are generally weaker than sigma bonds due to the nature of their overlap.
• They add strength and rigidity to molecular structures, making them crucial in compounds like ethene and benzene.

The presence of a pi bond alongside a sigma bond forms the double bond, which is much more rigid and less flexible than a single bond. In the case of boron compounds like \ (\( \text{BF}_3 \)), pi bonds are formed due to p-p orbital interactions, which enhance the molecular architecture.

Molecular Chemistry

Molecular chemistry is a branch of science focused on understanding the formation, composition, and properties of molecules. At its core, this field examines how atoms bond and interact to form the vast array of compounds found in nature.

Fundamentals of molecular chemistry include:

• Understanding atomic structure, which is central to predicting and explaining molecular shapes and reactions.
• Exploring bonding types, like covalent, ionic, and metallic bonds, each contributing differently to molecular properties.
• Studying molecular geometry, which involves the spatial arrangement of atoms within a molecule, significantly influencing physical and chemical properties.

In the world of boron compounds, molecular chemistry helps us see how different configurations and bonds (sigma and pi) lead to diverse chemical behaviors. For instance, in \ (\( \text{BF}_3 \)), the molecular structure is partly defined by how sigma and pi bonds work together, showcasing the intricate dance of atomic interactions in determining a compound's characteristics.