49 Hydrocarbons: A Structural Study
Background
The physical, chemical, and biological properties of molecules are determined, to a large extent, by their three-dimensional shapes. Molecular substances made up of molecules that pack tightly together often form large, beautiful crystals. Other substances made up of molecules that do not pack together remain liquids even at low temperatures. Many medicinal drugs are effective because their shapes resemble those of molecules in the body. Consequently, an understanding of molecular shapes is very important to an understanding of chemistry or biology. Like many people, you may find it easier to working in three dimensions if you use molecular models. By working with models, you can learn to visualize and understand molecular shapes.

In this experiment, you will use ball-and-stick models to study the shapes of hydrocarbon molecules.
Goals

Equipment
safety goggles
1 ball-and-stick molecular mode
l kit/4 students

Safety

Procedure
For each molecular model, indicate the molecular and structural formula in your laboratory notebook. Also provide sketches of any structural or geometric isomers, and any different conformations. Refer to Table 49.1 for the color code of the atoms.
 
   
Part A. Continuous-Chain and Branch Alkanes
1. Make a models of methane, CH4. Are all the angles formed by any two C-H bonds the same?
2. Make a model of ethane, C2H6. Can you hold one carbon and its hydrogens in a fixed position and rotate the other carbon and its hydrogen, without breaking the C-C bond?
3. Make a model of propane, C3H8. Can this model be rearranged to form a different molecule?
4. Make a model of butane, C4H10. Can this model be rearranged to form a new molecule that has the same molecular formula but a different structural formula? If so, name the structures. Molecules that have the same molecular formula, but different structural formulas, are called structural isomers.
5. Make a model of pentane, C5H12. Construct as many structural isomers of pentane as you can. For each of these structural isomers, give the name of the structural formula here and draw a sketch in Table 49.2a.
Part B. Cycloalkanes
6. Construct a model of hexane, C6H14. Manipulate the structure to form a ring. (You will have to remove two hydrogens to join the ring.) This ring structure is cyclohexane.

Manipulate your cyclohexane molecules so that two carbons directly across the ring from each other are above the plane of the other four carbons. This is called the boat conformation. Now manipulate the molecule so that one of these carbons is above, and the other below, the plane of the remaining four carbons. This is the chair conformation. In Table 49.2a, draw these two conformations.

Is there free rotation about the C-C bond in cyclohexane?.

Part C. Alkenes and Geometric Isomers
7. Make a model of ethene, C2H4. Can you rotate the carbons about the double bond?
8. Remove one hydrogen from each carbon in ethene and replace it with a chlorine. The name of the resulting compound is 1,2-dichloroethene, C2H2Cl2. There are two structures possible for this compound. They are called geometric isomers, and are distinguished by the prefix cis or the prefix trans added to the name. Construct models of both geometric isomers.
9. Make a model of butene, C4H8. This compound has two structural isomers. Name these isomers and in Table 49.2, give their molecular and structural formulas.

Are there also geometric ("cis" and "trans") isomers for butene?

Part D. Alkynes
10. Make a model of ethyne, C₂H₂. In the space provided in Table 49.2b, describe the shape of the molecule. Can you rotate the molecule about the triple bond?

Part E. Arenes
11. Make a model of benzene, C6H6, using alternating single and double bonds to approximate the aromatic bonds. Do all the atoms in this molecule lie in the same plane?

Can benzene exist in the boat and chair conformations?

Table 49.1 Color Code for Models

Color Atom Represented

black carbon

yellow hydrogen

red oxygen

blue nitrogen

green chlorine

purple iodine

orange bromine