24 Jun Chemistry 212, Problem Set #3: Geometr
Chemistry 212, Problem Set #3: Geometric Structure and Localized Bonding Models Due in class, Wednesday, 5 February 2014 1. For each of the following chemical species: (i) write acceptable Lewis structures (including major resonance contributors and indicating formal charges); (ii) designate oxidation states for all atoms; and (iii) draw the stereochemical structures predicted by VSEPR arguments, noting deviations from ideal bond angles and predicting the direction of molecular dipoles. a. b. [OTeF5]– c. XeCl4 d. H2C=SF4 e. 2. ClOF3 BF3 f. N2O5 (indicate CH2 orientation) Polynitrogen species, [Nn]z (n > 2, z = charge), “are of significant interest as high energy density materials (HEDM) for propulsion or explosive applications.” The [N5]+[AsF6]– salt, prepared in 1999, contains the first new polynitrogen species reported in over a century. The compound is described as a “highly energetic, strongly oxidizing material that can detonate violently. It should be handled only on a very small scale while using appropriate safety precautions (face shields, leather gloves, and protective clothing).” a. Write acceptable Lewis structures (including major resonance contributors and formal charges) for the cation. What stereochemistry is predicted by the VSEPR model? Are all N–N bonds the same length? Explain. b. Only one other polynitrogen molecule is known. What is its trivial name? Give its Lewis structure and geometry. Are all N–N bonds the same length? Where might you find this molecule in daily life and in what application? c. Theoretical chemists have explored the possible existence of other polynitrogen species, including neutral molecules of composition N4. Write Lewis structures for two different molecules with this composition. Why are they predicted to be unstable? d. With the exception of conditions of extreme pressure, there is only one stable polyoxygen species. What is its name, Lewis structure, and geometry? What is the significance of this species? PS # 3, p. 1 of 2 3. Consider the trigonal planar NO2Cl molecule oriented against the coordinate system shown at right: y z-axis is perpendicular to the plane of the page O ∠O–N–O = 130° ∠O–N–Cl = 115° N Cl O z x a. b. Demonstrate that the hybrid orbitals derived in part (a) are orthogonal to one another. c. Plot one of the oxygen-directed hybrids as a cross-section on the x-y plane. d. 4. Using the bond angles provided in the figure and the general properties of hybrid orbitals, construct normalized hybrid orbitals for nitrogen that describe the σframework in this system. Using two different theoretical models, predict how the structure of NO2F will differ relative to the structure of NO2Cl. Consider the cyclobutane molecule (C4H8), idealized as a planar (flat) square of carbon atoms and oriented against the coordinate system shown below: H2C H2C CH2 x CH2 z y z-axis extends out of the plane of the page a. Consider a single carbon atom in cyclobutane. Using the atomic valence orbitals for carbon (s, px, py, pz), and applying valence bond considerations, construct appropriate normalized hybrid orbitals on this carbon to maximize overlap for carbon-carbon (C–C) bonding. b. Given the orbitals chosen in part (a) to describe the C–C bonding, construct appropriate normalized hybrid orbitals on carbon to form the C–H bonds. c. What stereochemistry is predicted at carbon based on the orbitals chosen in parts (a) and (b)? What is the H–C–H bond angle? d. The carbon atoms in cyclohexane are nearly tetrahedral geometry. Is cyclobutane, as described via (a)-(c), more or less acidic than cyclohexane? Explain. e. Consider the cyclopropane molecule. It is not possible to construct valid hybrid orbitals that point directly along the C-C vectors in this system. Why not? PS # 3, p. 2 of 2
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