Cyanamide \(\left(\mathrm{H}_{2} \mathrm{NCN}\right)\), an important industrial chemical, is produced by the following steps: $$ \begin{aligned} \mathrm{CaC}_{2}+\mathrm{N}_{2} & \longrightarrow \mathrm{CaNCN}+\mathrm{C} \\\ \mathrm{CaNCN} & \stackrel{\text { Acid }}{\longrightarrow} \mathrm{H}_{2} \mathrm{NCN} \end{aligned} $$ Cyanamid 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 \(\mathrm{C}\) and \(\mathrm{N}\) 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 \(\mathrm{C}-\mathrm{N}\) 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.

: For each compound, we will first count the valence electrons, then arrange the atoms, and finally distribute the electrons in pairs to complete the octet rule. Consider resonance structures where appropriate. 1. NCN²⁻: Total valence electrons = 5 (from N) + 4 (from C) + 5 (from N) + 2 (for 2- charge) = 16. The Lewis structure is: \[ \mathrm{N} \equiv \mathrm{C} \equiv \mathrm{N}^{2-} \] 2. H₂NCN (Cyanamide): Total valence electrons = 2 (from 2H) + 5 (from N) + 4 (from C) + 5 (from N) = 16. The Lewis structure is: \[ \mathrm{H}_{2} \mathrm{N} \equiv \mathrm{C} = \mathrm{N} \] 3. Dicyandiamide: The molecular formula is H₄C₂N₄. Total valence electrons = 4 (from 4H) + 8 (from 2C) + 20 (from 4N) = 32. The Lewis structure is: \[ \mathrm{H}_{2} \mathrm{N} - \mathrm{C} = \mathrm{N} - \mathrm{C} - \mathrm{N} \mathrm{H}_{2} \] 4. Melamine: The molecular formula is C₃H₆N₆. Total valence electrons = 6 (from 6H) + 12 (from 3C) + 30 (from 6N) = 48. The Lewis structure is: \[ \begin{array}{cccccc} & & {\mathrm{NH}_{2}}\\ &-&{\mathrm{CN}_{2}-}\\ \boldsymbol{-} & {\circlearrowright} & {\circlearrowright} & {\circlearrowright} \\ &{\mathrm{CN}_{2}-}\\ & & {\mathrm{NH}_{2}}\\ \end{array} \] b.

: 1. In NCN²⁻: N (sp), C (sp), N (sp) 2. In H₂NCN: N (sp³), C (sp), N (sp²) 3. In dicyandiamide: N (sp³), C (sp²), N (sp²), C (sp²), N (sp³) 4. In melamine: N (sp³), C (sp²) c.

: 1. In NCN²⁻: 2 σ bonds and 4 π bonds 2. In H₂NCN: 4 σ bonds and 2 π bonds 3. In dicyandiamide: 8 σ bonds and 2 π bonds 4. In melamine: 12 σ bonds and 3 π bonds d.

: The ring in melamine is composed of alternating Carbon (sp²) and Nitrogen (sp³) atoms. The sp² hybridization of Carbon atoms and the presence of a continuous π electron cloud around the Carbon and Nitrogen atoms in the ring make it planar. e.

: Since the molecule is nonlinear and has three different C-N bond distances, the most important resonance structure should have higher electron density around the central Carbon atom with single bonds to both Nitrogen atoms. This effectively increases its bond length as compared to double-bonded C-N. Thus, the most important resonance structure is: \[ \mathrm{H}_{2} \mathrm{N} - \mathrm{C} - \mathrm{N} - \mathrm{C} - \mathrm{N} \mathrm{H}_{2} \]