Molecular Models Exercise

1

Molecular Models Exercise

Name: ______

In this lab period, you will be working with a "ball-and-stick" molecular models kit. The wooden balls represent atoms, and the sticks represent bonds. The short sticks are mostly used for bonds to H, while the long sticks are used for single bonds between other atoms (C-C or C-O, for example}. The springs are used to represent double or triple bonds (C=O, C≡C, for example}, two springs for a double bond, and three springs for a triple bond.

The two objectives of this lab are, first, to give you a better idea of the 3-dimensional (3-D) shape of organic compounds, and second, to help you understand the concept of isomers. Isomers are compounds with the same molecular formula but different structures. As you work through these exercises, you should gain a better understanding of these concepts.

1a. Make a model of methane, CH4, the simplest organic compound. Note that all the H's are as far apart as possible (if the C was drilled properly}.

b. Replace one of the H's with a Cl. You now have chloromethane. Notice that no matter which H you replace with a Cl, you still have chloromethane. Another way to say this is that all the H's in methane are equivalent.

c. Replace a second H with a Cl. You now have dichloromethane.

d. Build another molecule of dichloromethane. Is it identical with the first? Why or why not? How many different isomers of dichloromethane, CH2Cl2, are there?

e. Hold the dichloromethane model by one of the H's so that this H is pointing directly up as shown below.

Slowly rotate the molecule by turning the top H. Note the relationship of each atom as you rotate the molecule. The drawing shown above is difficult to draw. To represent dichloromethane or another 3-dimensional molecule on paper, we use an easier representation consisting of dashes (׀׀׀׀), wedges (◄), and normal lines (—). A dashed line indicates that an atom is going back behind the plane of the paper, a wedge indicates that an atom is projecting in front of the plane of the paper, and a normal line indicates that the atoms connected by it are in the plane of the paper. Using this notation, the molecule of dichloromethane could now be represented as:

or by other representations, depending on how the molecule is rotated.

2. Make models of CH3-CH3 (ethane), H2C=CH2 (ethylene or ethene), and HC≡CH (acetylene or ethyne), using 2 springs for a C=C, and 3 springs for a C≡C. Complete the table for each model.

CH3-CH3 / CH2=CH2 / HC≡CH
Describe the shape
of each
Approximate C-C-H bond angle (120°, 180° or 109.5°)
Complete each using the "wedge-dash" notation, if needed. / C-C /

C=C

/

C≡C

3. Using the ethane model you built above, replace one of the H's with a Cl atom to form chloroethane.

a. Draw a structural formula for chloroethane below. Structural formulas use a line for a single bond. CH4 is drawn as follows.

Structural formulas may not show the spatial relationship between atoms. They only show which atoms are connected and the number of bonds between atoms.

b. Draw a 3-dimenstional representation of your chloroethane molecule, filling in the missing atoms on the drawing below.

c. Replace the chlorine with a hydrogen to reform ethane. Remove a different hydrogen atom and replace it with a C1 atom. Draw the structural formula and a 3-dimensional representation for this molecule.

d. Are the two chloroethane molecules you made isomers? Explain.

e. Now replace a second hydrogen with a Cl to make a dichloroethane. Draw the structural formula and name this molecule.

f. There is one isomer of the molecule you made above. Draw its structural formula and name it.

4a. Make a model of propane (C3H8). How many different ways can you replace an H in propane with a Cl? Draw them and name them.

b. Draw the structural formulas of all possible dichloropropane isomers and name each one.

5. Make a model of C2H6O.

a. Draw a structural formula for your model.

b. Make a model of an isomer of the molecule above, and draw it below.

6. Make a model of C2H4O.

a. Draw a structural formula for your model.

b. Make a model of an isomer of the molecule above, and draw it below.

7. Make a model of cyclohexane. Is it flat, like the drawing below? What shape is it?

8. Stereoisomers are isomers with the same structural formula, but they differ in the three-dimensional arrangement of atoms in space.

a. Enantiomers are one type of stereoisomers. Enantiomers are mirror images that cannot be superimposed.

Make two identical models of CH3CHBrCl. Superimpose the two models on each other. That is, slide the models on top of each other so that Cl is on top of Cl, Br on Br, CH3 on CH3, and H on H.

b. Now, take one of the two models and interchange the positions of the Cl and the Br. Can you now superimpose every atom in the two models?

Are the models mirror images?

Mirror images can easily be drawn by first drawing a three-dimensional structure and then reflecting each atom or groups of atoms across a mirror plane.

Mirror