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Welcome to Week 5 of PEP, and the beginning of Chapter 2!

Updated September 19th, 2019
Recollect that this is your landing page for announcements and current news for your organic chemistry class. Please check this page often.

Summary of Week 4

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Congratulations on finishing Chapter 1, the first of many chapters we will be covering in this class. You probably noticed that much of the chapter was a review of what you already learned in general chemistry. Recollect what you have learned or have been reminded of from Chapter 1:
  • why carbon (C) is a unique element, and perfectly suited for life
  • atomic structure and orbitals
  • how to represent molecules by Lewis diagrams, condensed structures and line structures
  • how electronegativity differences between bonding atoms gives rise to a polar covalent bond and bond dipole
  • how to determine the formal charge and oxidation state of every atom within a molecule
  • rules for drawing resonance structures and how to derive the resonance hybrid from these
  • how to identify and draw the seventeen functional groups we will work with
  • biological molecules
These skills all provide for you an excellent background for moving on to the next chapter!

What to expect in Week 5

This week we will start Chapter 2 - Three-Dimensional Geometry, Intermolecular Interactions, and Physical Properties, advancing on the idea of molecular geometry and seeing how both it and polar bonds/ net dipole moments impact the physical properties of organic molecules, such as boiling point, melting point, solubility, etc. Below is a bird's eye-view of what you will cover in the chapter:
  1. Valence Shell Electron Pair Repulsion (VSEPR) Theory/ Angle Strain (pp. 77 - 81)
  2. Dash-Wedge Notation (pp. 81 - 86)
  3. Strategies for Success: The Molecular Modeling Kit
  4. Net Molecular Dipoles and Dipole Moments (pp. 84 - 86)
  5. Physical Properties, Functional Groups, and Intermolecular Interactions (pp. 87 - 97)
  6. Melting Points, Boiling Points, and Intermolecular Interactions
  7. Solubility (pp. 98 - 103)
  8. Strategies for Success: Ranking Boiling Points and Solubilities of Structurally Similar Compounds (pp. 103 - 106)
  9. Protic and Aprotic Solvents
  10. Soaps and Detergents
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Introduction to Structure
If we were to compare two molecular species: carbon dioxide (CO2) and formic acid (HCO2H) we would find they are quite similar in their chemical makeup, the only difference being that formic acid possesses two hydrogens; note their structures at right.

Although similar in chemical makeup, their physical properties are wildly different: the boiling point of carbon dioxide is -78 degrees C, and that of formic acid is 101 degrees C. In addition, carbon dioxide is only slightly soluble in water (as evidenced by carbonated beverages that lose carbonation very readily), while formic acid is infinitely soluble. What accounts for these huge differences?

You will learn in this Chapter that the difference in these physical properties is due to the fact these molecules experience intermolecular interactions that are very different from one another; interactions that are governed by their three-dimensional geometry, dipoles, and the presence of what are called functional groups. These very factors impact not only their physical properties (boiling/ melting points, solubility), but chemical properties, such as reactivity. For this chapter, we will be principally concerned with their physical properties.

There are two models that describe molecular geometry: (1) valence shell electron pair repulsion (VSEPR) theory, and (2) molecular orbital (MO) theory; while molecular orbital theory is more powerful, we will focus on VSEPR for this week, as it is still pretty useful because of its simplicity. In future chapters we will learn about and examine MO theory in more detail.
Basic Principles of VSEPR Theory
According to Joel Karty (2012) the basic ideas of VSEPR theory are as follows:
  • Electrons in Lewis structures are viewed as groups; with a lone pair of electrons, a single bond, a double bond, and a triple bond each constituting a single group of electrons.
  • The negatively charged electron groups exert repulsion forces on one another, so they tend to orient themselves as far away from each other as possible: two electron groups around a central atom form a linear configuration with a 180 degree bond angle; three groups surrounding a central atom form a triangular, planar configuration, forming a 120 degree bond angle; four groups form a tetrahedral configuration with a 109.5 degree bond angle.
  • Electron geometry describes the orientation of the electron groups around a central atom (often carbon, nitrogen, sulfur or oxygen).
  • Molecular geometry describes more specifically the arrangement of atoms around a central atom. Since atoms must be attached by bonding pairs of electrons, an atom's molecular geometry is governed by it's electron geometry.
While not perfect and complete, VSEPR theory is still quite useful. You will see the application of this theory throughout this course.
Image Credits:
Nanocar ball-and-stick model in banner retrieved from: http://news.rice.edu/2015/12/14/rice-to-enter-first-international-nanocar-race/
Nanocar with fullerene wheels in banner by Materialscientist at en.wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=16454593
Glucose ball-and-stick model retrieved from: https://www.thoughtco.com/pathway-most-atp-per-glucose-molecule-608200