Question

Cyanamide \(\left(\mathrm{H}_{2} \mathrm{NCN}\right),\) an important industrial chemical, is produced by the following steps: $$ \begin{array}{c}{\mathrm{CaC}_{2}+\mathrm{N}_{2} \longrightarrow \mathrm{CaNCN}+\mathrm{C}} \\ {\mathrm{CaNCN} \stackrel{\mathrm{Acid}}{\longrightarrow} \mathrm{H}_{2} \mathrm{NCN}} \\\ {\mathrm{Cyanamide}}\end{array} $$ Calcium cyanamide (CaNCN) is used as a direct-application fertilizer, weed killer, and cotton defoliant. It is also used to make cyanamide, dicyandiamide, and melamine plastics: a. Write Lewis structures for \(\mathrm{NCN}^{2-}, \mathrm{H}_{2} \mathrm{NCN}\) , dicyandiamide, and melamine, including resonance structures where appropriate. b. Give the hybridization of the C and N atoms in each species. c. How many \(\sigma\) bonds and how many \(\pi\) bonds are in each species? d. Is the ring in melamine planar? e. There are three different \(C-N\) bond distances in dicyandiamide, \(\mathrm{NCNC}\left(\mathrm{NH}_{2}\right)_{2}\) , and the molecule is nonlinear. Of all the resonance structures you drew for this molecule, predict which should be the most important.

Short answer

The Lewis structures for the given species are as follows: \(NCN^{2-}\) has two resonance structures with a triple bond between N and C, and a single bond between C and N with a negative charge on the terminal nitrogen atoms. Cyanamide (H₂NCN) has a double bond between N and C and another double bond between C and N. Dicyandiamide has a more important resonance structure with a double bond between N and C, and a triple bond between C and N. Melamine has a planar ring structure with alternating single and double bonds. The hybridization of C and N atoms in each species are: cyanamide - H₂N has sp², C has sp, and N has sp; dicyandiamide - terminal N has sp³, central N has sp, and C has sp; melamine - central N has sp², terminal N has sp³, and C has sp². Cyanamide has 4 σ bonds and 2 π bonds, the more important resonance structure of dicyandiamide has 7 σ bonds and 2 π bonds, and melamine has 12 σ bonds and 3 π bonds. The ring in melamine is planar, as the central nitrogen atoms are sp² hybridized and have a trigonal planar geometry. The most important resonance structure of dicyandiamide is H-N-C=N-C=N-NH₂, as it has more resonance stabilization due to the delocalization of π electrons.

Step-by-step solution

Draw the Lewis structures

We need to draw Lewis structures for NCN^2-, H₂NCN, dicyandiamide, and melamine. I will include resonance structures where appropriate. (a) For NCN^2-, the total number of valence electrons is 2(5) + 4 - 2(charge) = 12 electrons. The Lewis structure (with one resonance structure) is: N≡C-N^(-) ⇌ N^(-)-C≡N For H₂NCN (cyanamide), we have 2(1) + 2(5) + 4 = 16 electrons. The Lewis structure is: H-N=C=N-H For dicyandiamide (NCNC(NH₂)₂), there are 2(1) + 4(5) + 4 = 24 electrons. The two resonance structure (with one minor and one dominant resonance structure) are as follows: H-N-C≡N-C(-)-NH₂ ⇌ H-N-C=N-C=N-NH₂ (More important) For melamine, we have 6(1) + 6(5) + 6 = 42 electrons. The Lewis structure is: NH₂ | NH₂-N-C=N | \ NH₂ C=N \ NH₂

Determine the hybridization

We need to determine the hybridization of the C and N atoms in each species. (a) Cyanamide (H₂NCN): - H₂N: sp² hybridized (trigonal planar) - C: sp hybridized (linear) - N: sp hybridized (linear) b) Dicyandiamide (NCNC(NH₂)₂): - Terminal N (NH₂): sp³ hybridized (tetrahedral) - Central N (NCNC): sp hybridized (linear) - C: sp hybridized (linear) c) Melamine: - Central N (N-C=N): sp² hybridized (trigonal planar) - Terminal N (NH₂): sp³ hybridized (tetrahedral) - C=N: sp² hybridized (trigonal planar)

Count the number of σ and π bonds

We need to determine the number of σ and π bonds in each species. (a) Cyanamide (H₂NCN): 4 σ bonds, 2 π bonds (b) Dicyandiamide (NCNC(NH₂)₂): - More important resonance structure: 7 σ bonds, 2 π bonds - Less important resonance structure: 6 σ bonds, 3 π bonds (c) Melamine: 12 σ bonds, 3 π bonds

Determine if melamine's ring is planar

The central nitrogen atoms in melamine are sp² hybridized, and their geometry is trigonal planar. Therefore, the ring in melamine is planar.

Predict the most important resonance structure of dicyandiamide

As mentioned in Step 1, the most important resonance structure of dicyandiamide has more resonance stabilization due to the delocalization of π electrons in the molecule, which means that this structure is more stable. The most important resonance structure is: H-N-C=N-C=N-NH₂

Key concepts

Hybridization

Hybridization is a key concept in understanding the geometry and bonding of molecules. It describes the mixing of atomic orbitals to form new hybrid orbitals. These orbitals can help explain the shape of molecules.

• In cyanamide (\(\mathrm{H}_2\mathrm{NCN}\)), the nitrogen attached to hydrogen (\(\mathrm{H}_2N\)) is \(\text{sp}^2\) hybridized, resulting in a trigonal planar geometry. The carbon and nitrogen with a triple bond (\(\text{C} \equiv \text{N}\)) are \(\text{sp}\) hybridized, giving them a linear shape.


• In dicyandiamide, both central nitrogen and carbon atoms are \(\text{sp}\) hybridized, indicating linear sections, while the terminal nitrogen connected to hydrogen is \(\text{sp}^3\) hybridized, resulting in a tetrahedral geometry.


• In melamine, the central nitrogen atoms have \(\text{sp}^2\) hybridization, which supports a planar arrangement. Meanwhile, terminal nitrogen connected to hydrogens is \(\text{sp}^3\) hybridized, adding tetrahedral geometry to the molecule.

Understanding hybridization helps to predict the molecule's overall structure and the angles between bonds, an important aspect when analyzing any chemical structure.

Sigma and Pi Bonds

Sigma (\(\sigma\)) and pi (\(\pi\)) bonds are the two main types of covalent bonds formed between atoms.

• \(\sigma\) bonds are the first bonds formed between two atoms; they result from the direct overlap of orbitals and are stronger due to this direct overlap.


• \(\pi\) bonds are additional bonds to \(\sigma\) bonds and occur from the sideways overlap of p orbitals. They provide additional strength but allow less rotation around the bond axis.

In cyanamide (\(\mathrm{H}_2\mathrm{NCN}\)), there are four \(\sigma\) bonds and two \(\pi\) bonds resulting from the carbon-nitrogen triple bond. Dicyandiamide has more complexity, with seven \(\sigma\) bonds and two \(\pi\) bonds in its more stable form. Melamine contains a network of twelve \(\sigma\) bonds and three \(\pi\) bonds, forming a rigid, planar structure. Knowing the type and number of bonds can help predict the molecule's reactivity and other chemical properties.

Resonance Structures

Resonance structures are multiple ways of drawing a molecule that illustrate the different possible arrangements of electrons. They are essential for understanding the real, delocalized nature of electron distribution in molecules.

• In the case of \(\text{NCN}^{2-}\), resonance is evident as electrons shuffle between different bonding configurations, stabilizing the molecule.


• For dicyandiamide, having resonance structures implies delocalization of electrons. The most stable form will be the one where electrons are spread out to minimize energy, suggesting the structure with balanced bonds is most significant.

Resonance does not imply the molecule flips between structures. Rather, the true one is a hybrid of all possible versions— a useful tool in predicting stability and reactivity.

Planar Molecular Geometry

Planar molecular geometry occurs when all atoms in a molecule lie on the same plane, often due to \(\text{sp}^2\) hybridization. This geometry significantly affects molecular interactions.
In the case of melamine, the planar arrangement of nitrogen and carbon within the ring, due to \(\text{sp}^2\) hybridization, ensures a flat structure. The planarity allows the molecule to stack and interact with others effectively, which can be fundamental for certain materials like plastics and resins.
Understanding whether a molecule is planar helps predict both its physical properties and potential chemical behaviors, like interactions with other molecules or surfaces. This concept is crucial especially in materials science and nanotechnology where surface interactions impact functionality. Planarity can be a profound influence on a molecule's properties, both in isolations and as part of larger structures.