Problem 64 - Chemistry and Chemical Reactivity

Question

Draw a Lewis structure for diimide, \(\mathbf{H}-\mathbf{N}=\mathbf{N}-\mathbf{H}\) Then, using valence bond theory, describe the bonding in this compound. What orbitals overlap to form the bond between nitrogen atoms in this compound?

Step-by-Step Solution

Answer

The N=N bond in diimide consists of one sigma and one pi bond; the sigma bond is formed by sp3-sp3 overlap, and the pi bond by p-p overlap.

Step 1: Draw the Skeletal Structure

Start by arranging the atoms in the sequence given: H-N=N-H. This sequence suggests that the nitrogen atoms are double-bonded to each other, with each nitrogen atom bonded to one hydrogen atom.

Step 2: Count the Total Valence Electrons

Each hydrogen atom contributes 1 valence electron, and each nitrogen atom contributes 5 valence electrons. Thus, the total number of valence electrons is \(2(1) + 2(5) = 12\) valence electrons.

Step 3: Place Single Bonds

Place single bonds between each nitrogen and hydrogen, and a double bond between the two nitrogen atoms. Each single bond consists of 2 electrons: H-N N-H. The double bond also consists of 2 pairs of electrons: N=N.

Step 4: Distribute Remaining Electrons

After placing 4 pairs of electrons for the bonds, you have used 8 electrons. Distribute the remaining 4 electrons to complete the octet for each nitrogen atom. As both nitrogen atoms are part of a double bond, they already have a complete octet.

Step 5: Verify the Octet Rule

Ensure that each hydrogen has 2 electrons (full valence shell for hydrogen), and each nitrogen atom has 8 electrons (4 pairs). This is consistent with a stable configuration.

Step 6: Discuss Valence Bond Theory

In valence bond theory, the nitrogen-nitrogen double bond is formed by the overlap of one sp3 orbital from each nitrogen atom. Additionally, one p-orbital from each nitrogen forms a pi (\(π\) bond), creating the double bond.

Step 7: Identify Overlapping Orbitals

The sigma bond between the nitrogen atoms is formed by the overlap of sp3 hybrid orbitals. The pi bond, which is the second part of the double bond, is formed by the side-to-side overlap of unhybridized p orbitals from each nitrogen atom.

Key Concepts

Valence Bond Theory

Valence Bond Theory (VBT) is a fundamental theory used to explain how atoms in a molecule are held together through covalent bonds. The concept revolves around the idea that a covalent bond forms when the orbitals of two atoms overlap, allowing their valence electrons to be shared. This theory emphasizes the role of atomic orbitals and the electron-pair bond in the formation of molecules.

In the context of diimide ( H-N=N-H ), VBT explains the formation of the nitrogen-nitrogen double bond. Here, the bonding between nitrogen atoms involves the overlapping of atomic orbitals to form sigma ( σ ) and pi ( π ) bonds. The sigma bond arises from the end-to-end overlap of orbitals, while the pi bond results from the side-to-side overlap of orbitals. Thus, VBT effectively illustrates how these atomic interactions result in the stable structure of diimide.

Hybrid Orbitals

Hybrid orbitals are a concept that arises when atomic orbitals of an atom mix to form new orbitals. These new orbitals, called hybrid orbitals, have different shapes and energies compared to the original atomic orbitals. Hybridization is a crucial concept that explains the geometry and bond angles in molecules.

In diimide, the nitrogen atoms undergo sp^3 hybridization to form the sigma bond with each other and with the hydrogen atoms. This hybridization results in the formation of four hybrid orbitals for each nitrogen, one of which participates in forming the sigma bond with the adjacent nitrogen atom. The other three hybrid orbitals accommodate the nitrogen's other bonds, i.e., one orbital overlaps with a hydrogen to create an H-N sigma bond.

• sp^3 hybridization involves the mixing of one s and three p orbitals.
• These orbitals orient in such a way to minimize repulsion and form bonds.

By understanding hybrid orbitals, we can predict molecular shapes and bond properties, providing a clearer picture of chemical bonding in diimide.

Nitrogen Double Bond

A nitrogen double bond consists of a combination of one sigma bond and one pi bond. This type of bond is significant because it influences a molecule's stability, reactivity, and structure. In a double bond, the sigma bond forms due to the head-on overlap of hybrid orbitals, whereas the pi bond arises from the lateral overlap of parallel atomic p orbitals.

For diimide, the nitrogen-nitrogen double bond is essential for its linear structure. The double bond results from the overlap of sp3 hybrid orbitals (forming the sigma bond) and unhybridized p orbitals (forming the pi bond) from each nitrogen. Here’s a closer look:

• The sigma bond, being the primary bond, holds the atoms in place along the internuclear axis.
• The pi bond offers additional stability and fixes the positions of the atoms relative to each other.

This double bond not only contributes to the molecule's geometry but also impacts its electronic properties and chemical behavior, making it an integral feature of the diimide structure.