Chem& 162
General Chemistry w/Lab II

Experiment: VSEPR & 3–D Molecular Structures

Purpose
In this laboratory activity you will gain additional practice predicting the three–dimensional structure of molecules and ions without the use of models. You will also use your knowledge of geometry and bond angles to predict the relative stability of several cyclic compounds, and will compare the structures of some larger drug molecules.

Introduction
For small molecules the shape or geometry can be accurately predicted using Valence–Shell Electron–Pair Repulsion (VSEPR) theory. This model is discussed at some length in your text, pages 373–382. Be sure to read this section carefully prior to attempting this activity.

In this exercise you will begin by predicting the geometry of several molecules and ions. You will then be able to check your work by viewing a computer generated 3–D model of the molecule or ion. Be sure that you complete all work in your lab notebook.

Procedure
You will work with a partner for this exercise.

Part I — VSEPR
Inorganic Molecules:For each molecule or ion draw the Lewis structure. Then make a 3–D sketch of the molecule or ion that shows the arrangement of all electron pairs about the central atom, name the arrangement, label each using the AXmEn notation, and predict its shape/geometry (including angles). Each substance has one central atom.

    1. BrF5 2. ClF3 3. PBr4+
    4. I3-5. SO36. CCl2O
    7. SF48. PCl39. XeF410. XeF2

Organic Molecules:These molecules all have more than one central atom. After drawing each Lewis structure, again make a 3–D sketch of the molecule, name the arrangement about each central atom, label each central atom using the AXmEn notation, and predict its shape/geometry (including angles.) Note that the formulas given below have been written so as to indicate how the atoms are bonded.

    11. CH3CHOHCOOH12. COOHCH2COOH
    13. CH2CHCONH214. CH3CCCH2Cl

Cyclic Organic Compounds:The following compounds are all cyclic, with the carbon atoms arranged in a ring:

    15. C3H616. C4H817. C6H12

Some of these are more stable than others. Given what you know about the possible geometries around carbon, and considering how well (or not) your models fit the ideal geometries, which of these compounds do you think is the most stable? The least stable? To answer these questions first draw the Lewis structures, look at the bond angles, and then explain your choice.

Once you have finished predicting the geometry for each molecule or ion above, show your work to your instructor before continuing on to the next step.

Now it’s time to check your work. Using MS Internet Explorer, go to the Molecular Models web page, at Okanagan University College, to find the correct structures. Once at the web page, select the link to the “Formula Index”.

Use this index to look up each of the substances. (Note that the index does not show the charge of an ion.) Each will first appear in a “ball and stick” representation. To change the way the molecule is displayed, choose the desired representation from the options to the right of the display. (Note that the models displayed on this web site do not show non–bonding pairs.) You can rotate a molecule by dragging on any part of the model.

Part II — Structural Similarities
The three–dimensional structure of a molecule is an essential part of its chemical function and reactivity. Sometimes molecules with similar structures have similar properties. Other times they are vastly different. Consider the following drugs. Two of them have very similar 3–D structures. Which two are they?

    Caffeine Nicotine Morphine Cocaine LSD Heroin
    C8H10N4O2 C10H14N2C17H19NO3C17H21NO4C20H25N3OC21H23NO5

To find out, use MS Internet Explorer to return to the Molecular Models web page, and select “Drugs”. The drugs are listed by the number of carbon atoms. Compare the structures (Again, you can rotate a molecule by dragging on any part of the model. It is easiest to compare structures by opening two different “windows” and viewing the structures side by side.), and try to find the two which are most alike. In your lab analysis explain your choice in a few brief sentences.

Analysis and Report
In your report be sure that you answer the questions posed above, and include a paragraph summarizing the concepts you have learned in this exercise.

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Last Revised: 4/8/12