Molecular Models 1
Chem 102, Fall 2005
The purpose of this exercise is to help you visualize the three-dimensional
structure of organic molecules using molecular models. You will study structures and
conformations of alkanes, structures of cycloalkanes, alkenes and alkynes, and cis-trans
isomers of cycloalkanes and alkenes.
The molecular model kit contains plastic balls and sticks to represent atoms and
bonds. The black balls with 4 holes represent carbons, the white balls represent
hydrogen, and the colored balls represent other atoms, such as nitrogen, oxygen or the
halogens. Joining the balls together with gray sticks depicts a bond between two atoms.
(The longer gray sticks are used in the formation of double and triple bonds.)
Procedures:
Take one model kit per pair of students. Partners should take turns building the
models so that each student gets hands-on practice. Although you will work together,
and may discuss any aspect of the assignment, each student must make their own
drawings and write their own answers to the questions.
Exercise 1: Conformational Analysis of Alkanes
1. Construct a model of ethane (CH3-CH3) by joining two carbon atoms with a single
bond (shorter grey sticks) and then attaching three hydrogens to each of the carbon
atoms. Sight down the carbon-carbon bond and rotate one carbon until all the
hydrogens of one carbon atom are directly behind the hydrogens of the other carbon.
This is referred to as the eclipsed conformation.
a) Draw the structure (using wedges and dashes) for the eclipsed conformation.
b) Rotate one of the carbon atoms about the C-C single bond by 60° with respect to the
other carbon. This is referred to as the staggered conformation. Draw the structure
(using wedges and dashes) for the staggered conformation.
2. Construct a model of butane (CH3-CH2-CH2-CH3) and arrange it in a fully staggered
conformation (such that the two methyl groups are 180° from each other). This is called
the anti conformation.
a) Draw the structure (using wedges and dashes) for the anti conformation. (Don’t
draw out the C-H bonds on the methyl groups, just write them as CH3)
b) Keeping the position of C3 constant, rotate C2 clockwise by 60°. This conformation is
eclipsed. Draw the structure (using wedges and dashes).
c)
Rotate C2 by another 60°. This is called the gauche conformation (a type of
staggered). Draw the structure (using wedges and dashes).
d) Rotate C2 by another 60° to get the fully eclipsed conformation. Draw the structure
(using wedges and dashes).
Exercise 2: Structures and cis-trans isomers of cycloalkanes
3. Construct a model of cyclopropane.
a) Draw the structure, either from the top view (use wedges and dashes) or from the
side view (show all H’s).
b) Replace two of the H’s on the cyclopropane with chlorines to make cis-1,2dichlorocyclopropane. Draw the structure, either from the top view (use wedges and
dashes) or from the side view (show all H’s).
c) Now make trans-1,2-dichlorocyclopropane and draw the structure, either from the
top view (use wedges and dashes) or from the side view (show all H’s).
4. Construct a model of cyclohexane. Cyclohexane is not a flat molecule, but rather
takes on various conformations, the most common being the “chair” conformation.
Arrange the cyclohexane so that all of the C-H bonds are staggered. This is the chair
conformation. See me if you need help. Draw the structure, both from the top view
(use wedges and dashes) and from the side view (show all H’s).
Exercise 3: Structures of alkenes and alkynes and cis-trans isomers of alkenes
5. Make a model of ethene. Use two of the longer sticks to form the double bond.
a) Draw the structure.
b) Convert your model to cis-2-butene. Draw the structure (show stereochemistry).
c) Convert your model to trans-2-butene. Draw the structure (show stereochemistry).
d) Convert your model to 2-methylpropene. Draw the structure.
7. Make a model of 2-butyne, using three of the longer sticks to form the triple bond.
Draw the structure.
Additional Questions:
1. Arrange the conformations of butane from parts a-d of question 2 (anti, eclipsed,
gauche and fully eclipsed) from lowest to highest energy.
2. Cyclohexane is not flat. Explain why in one sentence.
3. Do the model kits allow you to accurately portray double and triple bonds? Briefly
explain why or why not.
4. Why is there no cis or trans for 2-methylpropene (exercise 6d above)?