Ch.9 - Molecular Geometry and Bonding Theories

Problem 109c

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Chapter 9, Problem 109c

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, C12H10N2. A closely related substance is hydrazobenzene, C12H12N2. The Lewis structures of these two substances are

(Recall the shorthand notation used for benzene.) (c) Predict the N¬N¬C angles in each of the substances.

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Identify the Lewis structures of azobenzene (C12H10N2) and hydrazobenzene (C12H12N2) from the given image.

Note that azobenzene has a double bond between the nitrogen atoms (N=N) and hydrazobenzene has a single bond between the nitrogen atoms (N-N).

Recall that the bond angles around a nitrogen atom in a double bond (sp2 hybridization) are approximately 120 degrees.

Recall that the bond angles around a nitrogen atom in a single bond (sp3 hybridization) are approximately 109.5 degrees.

Predict that the N-N-C angles in azobenzene will be around 120 degrees and in hydrazobenzene will be around 109.5 degrees.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Lewis Structures

Lewis structures are diagrams that represent the bonding between atoms in a molecule and the lone pairs of electrons that may exist. They help visualize the arrangement of electrons and the connectivity of atoms, which is crucial for predicting molecular geometry and reactivity. Understanding how to draw and interpret Lewis structures is essential for analyzing the properties of organic compounds like azo dyes and their derivatives.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. It is determined by the number of bonding pairs and lone pairs of electrons around the central atom, which influences bond angles and overall shape. For azo dyes and related compounds, knowing the molecular geometry helps predict the N-N-C angles and the physical properties of the substances.

Bond Angles

Bond angles are the angles formed between two adjacent bonds at an atom. They are influenced by the hybridization of the atom and the presence of lone pairs, which can repel bonding pairs and alter angles. In the context of azo dyes and hydrazobenzene, predicting the N-N-C bond angles requires an understanding of the molecular geometry and the steric effects of the surrounding atoms.

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Bond Angles

Related Practice

One of the molecular orbitals of the H2- ion is sketched below: (d) Compared to the H¬H bond in H2, the H¬H bond in H2- is expected to be which of the following: (i) Shorter and stronger, (ii) longer and stronger, (iii) shorter and weaker, (iv) longer and weaker, or (v) the same length and strength?

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Textbook Question

Place the following molecules and ions in order from smallest to largest bond order: N22+, He2+, Cl2 H2-, O22-.

Molecules that are brightly colored have a small energy gap between filled and empty electronic states (the HOMOLUMO gap; see Exercise 9.104). Suppose you have two samples, one is lycopene which is responsible for the red color in tomato, and the other is curcumin which is responsible for the yellow color in turmeric. Which one has the larger HOMO-LUMO gap?

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Textbook Question

a) Using only the valence atomic orbitals of a hydrogen atom and a fluorine atom, and following the model of Figure 9.46, how many MOs would you expect for the HF molecule?

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Textbook Question

Carbon monoxide, CO, is isoelectronic to N2. (d) Would you expect the p2p MOs of CO to have equal atomic orbital contributions from the C and O atoms? If not, which atom would have the greater contribution?

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Textbook Question

The energy-level diagram in Figure 9.36 shows that the sideways overlap of a pair of p orbitals produces two molecular orbitals, one bonding and one antibonding. In ethylene there is a pair of electrons in the bonding π orbital between the two carbons. Absorption of a photon of the appropriate wavelength can result in promotion of one of the bonding electrons from the p2p to the p*2p molecular orbital. (a) Assuming this electronic transition corresponds to the HOMO-LUMO transition, what is the HOMO in ethylene?

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