Chapter 2 - Three-Dimensional Geometry, Intermolecular Interactions and Physical Properties
Updated September 18th, 2021 Recollect that this is your landing page for announcements and current news for your organic chemistry class. Please check this page often.
Learning Outcomes for Chapter Two: in case you are interested... Upon successful completion of this chapter, you will be able to:
Using valence shell electron pair repulsion (VSEPR) theory,indicatethe bond angles along with both electron and molecular geometry of organic species.
Examineorganic species to determine whether angle strain is present in any of the bonds.
Applythe rules for dash-wedge notation by re-drawing organic species, provided only the Lewis structure or chemical formula.
Identifythe various intermolecular interactions between solvent molecules (ion-ion, dipole-dipole, hydrogen bonding, London dispersion forces, etc.) when provided a Lewis structure of the solvent particle.
Examineorganic species to determine how many potential H-bond donors and H-bond acceptors are present.
Comparea pair or trio of organic molecules and determine which would have the highest boiling point.
Examinea potential solvent and solute pair, and determine solubility by identifying their intermolecular interactions.
Arrangean array of different organic molecules, ranking them in order of boiling points.
Week 4 Summary
This week we started 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, solubility, etc. Below is a bird's eye-view of what you will cover in the chapter:
Melting Points, Boiling Points, and Intermolecular Interactions
Solubility (pp. 98 - 103)
Strategies for Success: Ranking Boiling Points and Solubilities of Structurally Similar Compounds (pp. 103 - 106)
Protic and Aprotic Solvents
Soaps and Detergents
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 as well. 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. This past Tuesday we did a lab on VSEPR, although MO Theory is more powerful. In future chapters we will learn about and examine MO theory in more detail.
As a reminder, the Basic Principles of VSEPR Theory
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.
Chapter 1 Test - Open Book I already emailed the pdf file for the Chapter 1 Test this week. When taking, keep this in mind:
This is to be an open book exam - please use only the resources provided by your textbook, and not from the internet
You might want to allot about two hours to take this exam
Since this particular one is open book, it does not need to be proctored by a parent, but future exams will need to be
Due Tuesday, September 21st, along with the Chapter 1 Homework
What to expect in Week 4
This week we will start in Section 2.4 of your text. Please take the time to watch the video linked right so you are prepared to engage in a lively class discussion. Intermolecular Interactions As already discussed, the physical properties of a compound are impacted by both the compound's molecular geometry and the functional groups present. These functional groups exert an impact because they may give rise to differences in the distribution of charge within the compound. This charge distribution may be partial or full; with a greater concentration of charge producing a stronger intermolecular interaction. For this class, these are the intermolecular interactions of interest:Interestingly, there are five kinds of interactions defined based upon the mechanism by which the molecules are attracted to one another. These interactions come into play when considering melting points and boiling points of compounds, and can be characterized accordingly:
ion-ion interactions - the strongest because ions have very high concentrations of positive and negative charges.
dipole-dipole interactions - occur when the positive end of one molecule's net dipole is attracted to the negative end of another's, as shown in Figure 2-12 (right).
hydrogen bonding - are a form of dipole-dipole interaction occurring between a hydrogen-bond donor (hydrogen atom covalently bonded to F, O, or N, and hydrogen-bond acceptor (any atom with a large concentration of negative charge and lone pair of electrons, which is really only F, O, and N).
Induced dipole-induced dipole (London Dispersion Forces) - are the dominant intermolecular interaction between nonpolar molecules. These occur because at a given instant of time, there will be more electrons on one side of a molecule than on the other; the extra electrons give rise to an instantaneous dipole, which can in turn, impact the electron distribution on an adjacent molecule (Figure 2 - 17, right).
Ion-dipole interactions - interactions between dissolved ions and polar solvents, such as what we would see when NaCl dissolves in water.
When you are asked to rank boiling points or solubilities of compounds, you need to keep these intermolecular interactions in mind - try to imagine how the molecules will interact with one another - will they provide surfaces that allow interaction (take a look at Figure 2 - 18). Do the molecules permit hydrogen bonding? If so, to what degree? Learning to analyze organic species in this manner will be powerful when you later have to look at reactants and reaction conditions, and then make predictions with respect to products.
Check-off List of Things to Do:
In order to be adequately prepared for classes next week, please make sure you do the following by: Tuesday, September 21st
Come to class prepared to take notes as we continue through Chapter 2.
Watch videos covering Sections 2.1 through 2.4, if you need the review
Due at the beginning of class: Test #1, and Chapter 1 Homework
Thursday, September 23rd
Prepare for Quiz #5, writing the basic functional groups
Figure 2-12 Dipole-dipole interaction. Note the positive end of one ether molecule (top) attracts the negative end of the other (bottom).
Figure 2 - 17 Induced dipole-induced dipole interaction. While the molecules of propane are nonpolar, since the electrons are mobile, electron density can still build up on one side of the molecule, resulting in a temporary dipole.
Figure 2 - 18 Note that the contact surface area of pentane is greater than dimethylpropane, which gives pentane a higher boiling point.