Organic Structure Part 2

Objectives: The primary objective of this lab is to build simple organic molecules using model kits in order to visualize the 3-D structure, then to become familiar with depicting the 3-D structures as 2-D using the MarvinSketch applet.

Note: There are some changes in this lab report form and the lab that arrived in your kit. Please complete this form and submit.

Alkenes
A. Cis-Trans (Geometric) Isomerism.
Alkenes are hydrocarbons containing one or more double bonds. They are unsaturated. They are named according to the longest continuous chain of carbon atoms, adding -ene to the end, and using a number to indicate the position of the double bonds, if necessary. The simplest alkene is ethene, CH2=CH2, also known by the common name, ethylene.

1. Make a model of butane. The structure is shown below using the line drawing and the structural formula.
or CH3 - CH2 - CH2 - CH3

Rotate the single carbon-carbon bonds such that the end methyl (CH3) groups are on opposite sides of the center carbons.

Rotate the single carbon-carbon bonds such that the end methyl groups are on the same side of the center carbons.

2. Would the structures you have produced in parts a and b be considered isomers of one another? Explain your answer.

3. What are the bond angles and molecular geometry around the carbon atoms?

4. Make a model of cis-2-butene (both methyl groups are on the same side of the double bond). The structural formula for cis-2-butene is shown below:

CH3 CH3
| |
H - C = C - H

Or use the drawing program to get:
cis-2-butene

5. Try to rotate or "spin" the middle carbon atoms. Is this possible with the double bond? Describe what you see.

6. What is the bond angle and molecular geometry around the 2 middle carbon atoms?

Now, make a model of trans-2-butene (the end methyl groups are on the opposite sides of the carbon-carbon double bond).

The structural formula for trans-2-butene is shown in two ways below:

Note: this depicts 4 carbons, and it is understood that each C has 4 bonds. Therefore the end Carbons have 3 H's each and the middle Carbons have 1 H each.

H - C = C - CH3
| |
CH3 H

7. What are the bond angles and molecular geometry around the double bonded carbons?

8. Can your cis-2-butene model be converted into the trans-2-butene model without breaking bonds?

9. The cis- and trans-2-butenes are called geometric isomers. Why are they given this name?

10. Define the term structural isomer (from Organic Structure I) and explain how it differs from a geometric isomer.