Problem 109 - Chemistry: The Central Science

Question

Azo dyes are organic dyes that are used for many applications, such as the coloring of fabrics. Many azo dyes are derivatives of the organic substance azobenzene, \(\mathrm{C}_{12} \mathrm{H}_{10} \mathrm{N}_{2}\) . A closely related substance is hydrazobenzene, $\mathrm{C}_{12} \mathrm{H}_{12} \mathrm{N}_{2}$ . The Lewis structures of these two substances are (Recall the shorthand notation used for benzene.) (a) What is the hybridization at the N atom in each of the substances? (b) How many unhybridized atomic orbitals are there on the N and the C atoms in each of the substances? (c) Predict the \(N-N-C\) angles in each of the substances. (d) Azobenzene is said to have greater delocalization of its \(\pi\) electrons than hydrazobenzene. Discuss this statement in light of your answers to (a) and (b). (e) All the atoms of azobenzene lie in one plane, whereas those of hydrazobenzene do not. Is this observation consistent with the statement in part (d)? (f) Azobenzene is an intense red-orange color, whereas hydrazobenzene is nearly colorless. Which molecule would be a better one to use in a solar energy conversion device? (See the "Chemistry Put to Work" box for more information about solar cells.)

Step-by-Step Solution

Answer

The N atoms in both azobenzene and hydrazobenzene are sp² hybridized with no unhybridized orbitals. The N-N-C angles in both substances are 120°. Azobenzene has greater π electron delocalization, a planar structure, and is an intense red-orange color. In contrast, hydrazobenzene has less delocalized π electrons, a nonplanar structure, and is nearly colorless. Azobenzene is a better choice for solar energy conversion devices due to its effective light absorption in the visible spectrum.

Step 1: (a) Hybridization at N atom

In azobenzene, the nitrogen atom forms two single bonds and one double bond with the carbon atoms. The hybridization of N atoms in azobenzene is sp². In hydrazobenzene, each nitrogen atom forms two single bonds and one lone pair. Therefore, the hybridization at N atoms in hydrazobenzene is also sp².

Step 2: (b) Number of unhybridized atomic orbitals

In azobenzene, the nitrogen atoms are sp² hybridized, resulting in three hybridized orbitals. There are no unhybridized orbitals on the N atoms. None of the carbon atoms in the benzene rings has any unhybridized orbital. In hydrazobenzene, the nitrogen atoms are also sp² hybridized with three hybridized orbitals. The carbon atoms in the benzene rings again do not have unhybridized orbitals on them.

Step 3: (c) Predict N-N-C angles

In both azobenzene and hydrazobenzene, the nitrogen atoms are sp² hybridized. This leads to a trigonal planar arrangement around those nitrogen atoms. The angle between sp² hybridized orbitals is 120°. Therefore, the N-N-C angles in both substances would be 120°.

Step 4: (d) Delocalization of π electrons

Azobenzene exhibits greater delocalization of π electrons within its structure compared to hydrazobenzene. In azobenzene, the π electrons participate in resonance across the entire molecule, contributing to its stability. In hydrazobenzene, there is no resonance between the lone pair on nitrogen and the benzenoid system; thus, the π electrons are less delocalized.

Step 5: (e) Plane of the atoms

The observation that all the atoms of azobenzene lie in one plane is consistent with the statement that it has greater delocalization of π electrons. In azobenzene, the π electrons are spread out across the entire molecule within a plane, resulting in a planar structure. In contrast, for hydrazobenzene, the lone pair on nitrogen does not resonate with the benzenoid-system, leading to a nonplanar structure.

Step 6: (f) Solar energy conversion device

Azobenzene is an intense red-orange color, which means it absorbs light energy in the visible spectrum effectively. Hydrazobenzene, on the other hand, is almost colorless, indicating that it doesn't absorb light energy efficiently in the visible region. For a solar energy conversion device, the molecule that effectively absorbs light energy is preferable. Therefore, azobenzene would be a better choice for a solar energy conversion device.