Cyanamide (H2 NCN), an important industrial chemical, is...

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.

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.

Key Concepts

Resonance Contribution and Bond Distance Variations

Resonance contribution involves evaluating which resonance structure most accurately represents the true electron distribution within a molecule. Factors such as bond order, formal charge distribution, and observed bond lengths (which may deviate from ideal single or double bonds) are considered to determine the major contributing structure that most effectively stabilizes the molecule. This concept is crucial in understanding variations in bond distances in molecules with delocalized bonds.

Molecular Geometry and Planarity

Molecular geometry and planarity refer to the three-dimensional arrangement of atoms within a molecule, which greatly influences its chemical properties and reactivity. Planarity, in particular, is important in systems where conjugation is present, as it allows for effective overlap of p orbitals, leading to electron delocalization, while deviations from planarity can affect resonance and bond characteristics.

Sigma and Pi Bonds

Sigma (?) and pi (?) bonds are types of covalent bonds formed by the overlap of atomic orbitals. A sigma bond is the first bond formed between two atoms, characterized by head-on orbital overlap and is typically stronger, while a pi bond is formed by the side-to-side overlap of p orbitals and adds additional bonding in cases of double or triple bonds, affecting the molecule's reactivity and electron distribution.

Hybridization

Hybridization is the concept of mixing atomic orbitals to form new hybrid orbitals that can explain the geometry of molecular bonding. By accounting for the observed bond angles and structures, hybridization provides a model for the distribution of electron density in bonds and lone pairs, which is essential for understanding the shape and reactivity of organic and inorganic molecules.

Lewis Structures

Lewis structures are diagrams that represent the bonding between atoms of a molecule and the lone pairs of electrons that may exist. They serve as a fundamental graphical tool in chemistry to depict valence electrons and bond formation, providing insight into how atoms achieve a stable electron configuration that often follows the octet rule, albeit with known exceptions.

Resonance Structures

Resonance structures are alternative Lewis structures for a single molecule which depict delocalized electrons where the actual electronic structure is a hybrid of several contributing forms. They are used to explain bonding that cannot be adequately described by a single classical Lewis formula, showing electron distribution and stabilization in molecules with conjugated systems or multiple bonding possibilities.

Transcript

00:01 Okay, let's begin by drawing the lewis structures for the four species that they have identified.

00:07 The first one is n -c -n -2 -minus with a total of 16 valence electrons.

00:16 We draw the lewis structure.

00:18 We end up getting two double bonds on each side as one of the possible resonant structures.

00:26 For the next one, we would have a triple bond between.

00:34 One of the carbons and the nitrogen's, and a triple bond for the third resonant structure between the other carbon and nitrogen.

00:44 Then they didn't really tell us, they just said that we had h2ncn.

00:55 And so if we are putting the hydrogens on each end, then these would be the possible resonant structures, three different resonant structures.

01:05 We could then begin to assign the hybridization on the carbons and the nitrogen's.

01:12 We've got sp2 with the three electron groups on the nitrogen's, and there's two electron groups on the carbon, so just sp, and that's the way it's going to be with all of these resonant structures.

01:25 Then for this next one, we've got the next molecule, h2n -cn.

01:31 We've got three electron groups, so sp2 hybridized on each nitrogen, and as p hybridized on the carbon, similar to the previous molecule.

01:43 We see that we have just two sigma bonds and two pi bonds in the first molecule, or the first anion.

01:49 Second molecule, we have four sigma bonds and two pi bonds.

01:55 Or another way to draw this molecule would be to put both hydrogens on one nitrogen, which is maybe what is being suggested with the way they wrote the chemical formula.

02:05 We would then have these two resonant structures with sp3 hybridization on the nitrogen in this case, sp hybridization on the carbon, and then we would have sp hybridization here.

02:23 Then we go to the second structure, and we have sp2 hybridized with three electron groups, sp and then sp2 hybridized...

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