Text reference:
Section 24.1
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. donsequently, an understanding of molecular
shape is very important to an nnderstanding of chemistry or biology. Like
many people, you may find it easier to work in three dimensions if you
use molecular models. By working with models, you can learn to visualize aod understand molecular shapes.
In this experiment, you will use ball-and-stick models to study the
shapes of hydrocarbon molecules.
Make models of hydrocarbon molecules.
Compare the threeodimensional models with the structural formulas
of chemical conlpounds.
Make models of structural and geometric isomers.
safety goggles
1 ball- and~stick molecular
model ldt/4 students
¯ Note the Safety Symbol used here. Review safety information on
pages 7-9.
. Always wear safety goggles when working in the lab.
Refer to Table 49.1 for the color code of the atoms. As you construct each
model, complete Table 49.2a and Table 49.2b,
Table 49.! Color Code for Models
Color
Atom Represented
black
yellow
red
blue
green
purple
orange
carbon
hydrogen
oxygen
nitrogen
chlorine
iodine
bromine
1. Make a model of methane, CHv 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 carbou and its hydrogens, without breaking the C--C bond?
3. Make a model of propane, Calls. Can this model be rearraoged to
form a different molecule?
4. Make a model of butane, C4HI0. Can this model be rearrar~ged 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, C5H~2. Construct as many structural isomers of pentane as you can. For each of these structt~ral isomers, give the
name of the structural formula here and draw a sketch in Table 49.2a.
298 Experiment 49
6. Construct a model of hexane, CaHt4. 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 molecule 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 conformatiot~. In Table
49.2a, draw these two conformations.
Is there free rotation about the C--C bond in cyclohexane?
7. Make a model of ethene, C~H4. Can you rotate the carbons about tbe
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-dicbloroethene, C~H~C12. There are t~vo structures possib!e for this cmupmmd.
They are called geometric isomers, and are disfinguisbed by the prefix cis
or the pre~L’¢ tratzs added to the name. Construct models of both geometric isomers.
9. Make a model of botene, C4Hs. This compound has two structnra[
isomers. Name these isomers and, in Table 49.2b, give their molecular
and structural formulas. Are there also geometric ("cis" and "trans").
isomers for butene?
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?
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 aud chair conformations?
Hydrocarbons: A Structural Study 299
Name
Class
Date
Table 49.2a In formation on Molecular Models
Methane
Stroctural Formula
molecular formula
Sketch
H--C--H
angle?
Propane
Structura! Formula
Ethmm
molecular fornmla
Structural Fornmla
Rotation around C
molecular formula
Sketch
C bolnd?
molecular formula
Sketch
Sketch
Other isomers?
Pentaue
Structural Formulas
molecular forlllOla
Sketch
Cyclohexane
molecular fornlola
Structural Fommla
Sketch
Conformations
Boat
Chair
Rotation arom ~ C Cbond?
Hydrocarbons: A Structural Study 301
Table 49.2b Information on Molecular Models
~}~’~ ~(o
molecular formula
Structural Formula
Sketch
1,2-Dichloroetheoe
molecular formula
Structural Formula
Sketcii
cis-
Rotation around C:C bond?
molecnlar formula _
Structt~ra[ Formu[as
Structural isomers
Ethyne
molecolar formula
Structural FormuIa
Sketch
Sketch
Shape?
Rotation around C~,C bond?
Benzene
Structural Formula:
is the molecnle planar?
Geometric Isomers
302 Experirnent 49
Sketch
Chair and boat conforlnations?
Sketch