Question

Draw the Lewis structure for 1,1 -dimethylhydrazine \([\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NNH}_{2}, \mathrm{a}\) compound used as a rocket fuel]. What is the hybridization for the two nitrogen atoms in this molecule? What orbitals overlap to form the bond between the nitrogen atoms?

Short answer

Both nitrogen atoms are sp³ hybridized with sp³-sp³ overlap forming the N-N bond.

Step-by-step solution

Identify the Atoms

1,1-Dimethylhydrazine has the chemical formula \(( ext{CH}_3)_2 ext{NNH}_2\). It consists of two CH₃ (methyl) groups, two nitrogen (N) atoms, and two hydrogen (H) atoms bonded to one of the nitrogen atoms.

Determine the Number of Valence Electrons

Each carbon atom has 4 valence electrons, each nitrogen atom has 5 valence electrons, and each hydrogen atom has 1 valence electron. Thus, the total number of valence electrons is \(2(4) + 2(5) + 2(1) = 20\) electrons.

Connect the Atoms

Connect the atoms in the sequence: H₂N-N, with each nitrogen atom attached to a methyl group (CH₃). This is a skeleton structure showing the connectivity of the atoms.

Distribute the Remaining Valence Electrons

Start by placing electrons to satisfy the octet rule for each atom. Fill in the electrons as lone pairs around the nitrogen atoms. Ensure that all atoms (except hydrogen) achieve an octet configuration.

Verify the Structure

Check the structure to see if all atoms except hydrogen have 8 electrons in their valence shell. Adjust any misplaced electrons if needed.

Determine Hybridization of Nitrogen Atoms

The nitrogen atoms are each bonded to three groups, so they are \( ext{sp}^3\) hybridized. This means they form tetrahedral geometry.

Determine the Orbitals Overlapping

The nitrogen-nitrogen bond is formed by the overlap of two \( ext{sp}^3\) hybrid orbitals, each from one nitrogen atom.

Key concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom and play a crucial role in chemical bonding. They are responsible for forming bonds with other atoms. To determine the Lewis structure of a molecule, counting the valence electrons is often the first step.
In 1,1-dimethylhydrazine, each carbon atom contributes 4 valence electrons, nitrogen contributes 5, and hydrogen contributes 1. When calculating the total number of valence electrons, we need to account for every atom in the molecule. For this compound, we have:

• 2 carbon atoms: \(2 \times 4 = 8 \text{ electrons}\)
• 2 nitrogen atoms: \(2 \times 5 = 10 \text{ electrons}\)
• 6 hydrogen atoms: \(6 \times 1 = 6 \text{ electrons}\)

Adding them together gives us a total of 20 valence electrons that will be arranged around the atoms to satisfy each element's desire for a stable electron configuration, typically an octet for most atoms.

Hybridization

Hybridization is the process of mixing atomic orbitals to form new hybrid orbitals that are suitable for pairing electrons to form chemical bonds in a molecule. This concept helps to explain the geometry of molecular structures.
In 1,1-dimethylhydrazine, each nitrogen atom undergoes sp^3 hybridization. This happens because each nitrogen is bonded to three other atoms: one is the other nitrogen atom, and the remaining bonds connect to either hydrogen or a methyl group.
The result of sp^3 hybridization is the formation of four equivalent hybrid orbitals that are arranged in a tetrahedral geometry around each nitrogen atom, although pairs of electrons occupy two of these positions, contributing to the geometry observed.

Chemical Bonding

Chemical bonding is the attraction between atoms that allows the formation of chemical substances containing two or more atoms. In the case of 1,1-dimethylhydrazine, multiple bonds are formed, including covalent bonds between nitrogen atoms and carbon or hydrogen.
The nitrogen-nitrogen single bond is particularly interesting because it involves the overlap of sp^3 hybrid orbitals from each nitrogen. This type of covalent bond shares a pair of electrons, one from each nitrogen atom. Meanwhile, the nitrogen-carbon and nitrogen-hydrogen bonds are also formed by the overlap of these sp^3 hybrid orbitals with the corresponding orbitals of carbon and hydrogen.
Such bonding creates a stable arrangement for the molecule, ensuring that each nitrogen atom is involved in the maximum possible bonding interactions, satisfying its valence shell.

Molecular Geometry

Molecular geometry describes the three-dimensional shape of a molecule determined by the spatial arrangement of atoms. Understanding geometry is vital as it influences the physical and chemical properties of a substance.
For 1,1-dimethylhydrazine, each nitrogen atom's sp^3 hybridization leads to a tetrahedral geometry. However, because nitrogen atoms have lone pairs, the actual geometry around each nitrogen won't be a perfect tetrahedron.

• The nitrogen atom has two bonding pairs and two lone pairs, which results in a trigonal pyramidal shape.
• This shape is because lone pairs take up more space than bonding pairs, pushing bonded atoms closer and creating this unique geometry.

Understanding these geometries helps predict interactions with other substances and is crucial when using such compounds in rocket fuel applications.