Before Coming to Lab . . .

Look over the following to make sure you have a basic understanding of the topics presented.

Introduction

The shape of a molecule is extremely important in determining its physical properties and reactivity. A multitude of shapes are possible, and in today’s lab, you will be looking at several.

In Part 1, you will be exploring the various symmetry elements that can be present in molecules. The symmetry elements you will be looking for are mirror planes, rotation axes, and inversion centers. Being able to determine which symmetry elements are present in a molecule help in understanding its chemistry. If there is a plane present in the molecule that has the exact same arrangement of atoms on either side of the plane, then the molecule has a mirror plane (σ). It is important to note that a molecule can have more than one mirror plane. Rotation axes are represented as Cn (n = 1, 2, 3 . . .). The subscript indicates how many degrees of rotation (360^o/n) are needed in order to return to the same orientation of atoms with which you started. So if there is a C₂ axis, the rotation would be 180^o. An example of a molecule having a C₂ axis is H₂O.

Figure 1

The third symmetry element is an inversion center (i). In such molecules, starting at any position and drawing a line through the center and an equal distance to the opposite side of the molecule, you will end up at a position with an identical environment to the one you started from.

Figure 2

Part 2 of the lab introduces the concept of enantiomers. Enantiomers are molecules sharing the same molecular configuration, but they are non-superimposable images of each other. This concept should become clearer as you build the models for this part of the lab. Enantiomers share many of the same physical properties. The property which distinguishes them is the direction in which they rotate plane-polarized light. They will rotate the light in equal amounts but in different directions (plane-polarized light is just light in which all wave vibrations have been filtered out except for those in one plane). If both enantiomers are present in a 1:1 ratio, the effects of the rotation of light cancel and no net rotation is observed. Such a mixture of isomers is known as a racemic mixture or as a racemate. Because these isomers rotate plane-polarized light, they are also known as optical isomers. Compounds that form optical isomers are said to be chiral.

The chemistry of enantiomers is of great importance in the field of medicine. It has been discovered that with many drugs, one enantiomer will be biologically active while the other will be inactive or even produce undesired side effects. For this reason, it has become advantageous for pharmaceutical companies to try to synthesize the active enantiomer exclusively.

The next part of the lab deals with isomers. Isomers are molecules having the same molecular formula, but the atoms are arranged in a different manner, while still obeying the rules of bonding. There are different classifications for isomers. For example, structural isomers differ from one another in the order in which the atoms are bound to each other (connectivity is different). On the other hand, geometrical isomers have the same order of atoms, but the spatial arrangement of atoms is different (connectivity is the same). A common example of geometrical isomers is the cis and trans forms of double bonds:

Figure 3

** NOTE: Remember that molecules having single carbon-carbon bonds cannot have cis/trans isomers because there is free rotation about single bonds.

By building the models of various molecules during this lab, you will be able to better understand molecular symmetry and isomers. Building models is not difficult; however, the chemical principles involved are very important and you may find some surprises in how atoms can be fit together.

Finally, in Part 4, you will be applying your knowledge of VSEPR (Valence Shell Electron Pair Repulsion) Theory in order to determine the geometry of several different molecules. VSEPR theory is useful in helping to determine how atoms will orient themselves in molecules. Basically, the idea is that the arrangement adopted by a molecule will be the one in which the repulsions among the various electron domains are minimized. The two kinds of electron domains are bonding (electron pair shared by two atoms) and non-bonding (electron density centralized on one atom) pairs of electrons.

Experimental Procedure

For Parts 1 & 2: You and your lab partner are to work with one other lab group in preparing these models (no more than 3 - 4 students). Your TA will assign each group a certain set of molecules to make and answer questions pertaining to those molecules. Each group will then present their answers to the class. These models will need to be completed and answers determined within 30 minutes so that we can continue to Parts 3 & 4 as soon as possible.

For Parts 1-4, the work should be divided among the group members. Be sure to discuss the questions and answers among yourselves, but put your own conclusions on the Report Form.

1. Symmetry Elements

Using the Molecular Framework models, make models of the following compounds:

Choose a color to represent each atom. For example, make all C atoms black, all H atoms white, etc.

Once the models are created, look for symmetry elements that may be present. Ask yourselves the following questions:

Determine which of these symmetry elements are present in your assigned molecules. All of the columns of the table on the report form should be filled out. If you have any difficulty determining whether such symmetry elements are present in the molecules you are assigned, your TA can provide examples of each symmetry element.

Extra credit points can be earned by indicating in the table how many of each symmetry element are present for each molecule (i.e. How many mirror planes are present?).

 2.Mirror Images

Using the model kits, build models which are the mirror images of the models you were assigned to build (b, c, d, e, f, g, h, i and j) in Part 1. With the two mirror images in hand, try to place the models on top of one another, atom for atom.

If you can do this, the model and its mirror image are superimposable mirror images of one another. If not, the molecule and its mirror image form nonsuperimposable mirror images. Nonsuperimposable mirror images are also known as enantiomers.

For each compound, decide whether the mirror image is superimposable or nonsuperimposable. Can you make a generalization about when to expect molecules to have nonsuperimposable mirror images?

 3.Isomers

In this exercise you will build models of compounds which are structural and/or geometrical isomers of one another.

Make the following models:

A. Structural Isomers

 B. Geometrical Isomers

C. Aromatic Ring Compounds

4. Hypervalent Structures

Hypervalent compounds are those that have more than an octet of electrons around them. Such compounds are formed commonly with the heavier main group elements such as Si, Ge, Sn, Pb, P, As, Sb, Bi, S, Se, Te, etc. but rarely with C, N or O. Refer to the rules for Electron Domain theory in order to assign Lewis structures to the following molecules. Describe possible isomeric forms and the bond angles between the atoms. How many lone pairs of electrons are present on the central atom of each molecule, if any? (** Your book will be very useful in aiding you with these structures **)