Lab Exper. S 5
Models - Stereoisomers
Check out and use the plastic model kits to build models. You can save some time by not putting in every
single hydrogen atom, but be sure that you are aware of what you are leaving out. This exercise was written
assuming students would be working together.
A. REVIEW (You should get through this part quickly)
1. Chain isomers differ in their carbon skeletons.
Two students should each make a model of butane. If the models the same convert one to the other isomeric
form of C4H10.
Draw the structural formulas of the isomers on the Report Sheet and name them.
2. Position isomers have the same molecular formula, but a substituent (group attached) occurs in a different
position in each isomer, giving the two compounds different physical and chemical properties. Make two
isomeric structures for butanol (C4H10O) in which there is a straight carbon chain.
Draw the structures and name them.
3. Functional group isomers again have the same molecular formula, but because arrangements of the
elements differ, the isomers belong to different classes (families) of compounds.
Write the structural formulas of ethanol and of dimethyl ether with their names. Are there any other
functional group isomers of ethanol?
4. An important example of a pair of cis-trans (or geometric) isomers is fumaric acid, which occurs in the
citric acid cycle that you will study in carbohydrate metabolism. The cis-isomer is named maleic acid and is
not usable by the body in the citric acid cycle. Chemists do not usually give cis/trans isomers different
names, but this one is an exception.
Construct models of both isomers, and draw their structures.
H
COOH
C
HOOC
C
H
Fumaric Acid
B. Enantiomers and other stereoisomers
1.
Construct models of methane, chloromethane, chlorobromomethane, and chlorobromoiodomethane. A
chiral carbon or tetrahedral stereocenter is an atom attached to 4 different groups. Among these four
models identify the one with a chiral carbon. Construct a second, different, model of this compound. Is one
the mirror image of the other? If not, make one that is the mirror image of the other. Can you superimpose
these two models?
Compare this action to superimposing your hands, putting your right palm on the back of your left hand.
Now put your palms together. Imagine a mirror between your palms. Your left hand is the mirror image of
your right hand. A pair of mirror image isomers are called enantiomers. Think of them as twins, one lefthanded and the other right-handed.
Look at the first two models on the S5 Chime page. Draw sketches of your enantiomer models in a way that
clearly shows their mirror image relationship.
Next construct a mirror image model of one of the four compounds above that does not have a chiral
carbon. Try to superimpose this model on its mirror image.
2. Look at the structures of the 4 isomers of butanol on the Chime page. Find the one with a chiral carbon. A
chiral carbon is attached to 4 different groups, not necessarily 4 different elements. Build two models of
this butanol isomer that are not the same. These are a pair of enantiomers. Are they superimposable? Are
they mirror images of each other? On the report sheet draw both of the models in a way that clearly shows
their relationship. Mark the chiral carbons with asterisks (*). List the four different groups that are attached
to this carbon. Identify the groups with numbers on the structures you drew.
3. Glyceraldehyde, the simplest sugar occurs as a pair of enantiomers. Make one model of glyceraldehydes
(you can find the structure in your textbook). Make a model of its mirror image. Try to superimpose them.
Compare your models to those in Chime. Using sticky tape, label them “D” and “L”. Draw both models in
perspective and mark the chiral carbons with asterisks (*). Draw both models using Fisher projections.
What are the four different groups that are attached to the chiral carbon? Identify them with numbers on the
structures you drew.
4. Many compounds contain more than one chiral carbon. When a compound has two chiral carbons it usually
exists as 4 isomers. The compound below (2,3-dihydroxybutyraldehyde) has two chiral carbons. Make
models of its 4 isomers. Compare your models to the Chime models. Sort the 4 models into two sets of
enantiomers. Using Fisher projection formulas, draw the enantiomers side by side, Orient the carbon chains
vertically with the aldehyde at top.
OH
* CH
H 3C
O
*
CH
OH
CH
2,3-dihydroxybutyraldehyde
5. Some compounds with two chiral carbons have no enantiomers (mirror image isomers). They are called
meso compounds. A meso compound has no enantiomer because it is identical to its mirror image. Build
two identical models for the compound below (2,3-dihydroxybutane). You’ll notice that these identical
models are also mirror images even though they contain chiral carbons. Compare with the Chime versions.
Draw a Fisher projection of this structure.
H 3C
CH
CH
OH
OH
CH3
2,3-dihydroxybutane
A compound can be meso only when it has a plane of symmetry. Imagine a plane that cuts the molecule
into two halves, one half being the mirror image of the other. On your Fisher projection draw the plane of
symmetry as a dashed line.
C. Prebuilt models – plastic and Chime
Five pairs of models are distributed about the lab. They may be structural isomers, functional group
isomers, cis/trans-isomers, enantiomers, or stereoisomers which are not enantiomers. The same model sets
are on the Chime pages. Draw their structural formulas, then state which type of isomerism each pair
represents. If they are cis/trans -isomers, tell which formula is which. If they are chiral compounds, mark
each chiral carbon with an asterisk.
Name
Date
Please print
S5 STEREOISOMERS VIA MODELS
Report Sheet
A. REVIEW 1. Chain isomers
2. Position isomers
3. Functional group isomers
4. Cis-trans isomers
B. Enantiomers and other stereoisomers
1. Enantiomers The model with the chiral carbon is
.
2. Butanol enantiomers
The four different groups that are attached to this chiral carbon are 1.
2.
3.
4.
3. Glyceraldehyde enantiomers
The four different groups that are attached to this chiral carbon are 1.
2.
3.
4.
D
L
D
L
4. 2,3-dihydroxybutyraldehyde - 2 pairs of enantiomers
5. Meso 2,3-dihydroxybutyraldehyde
C. Prebuilt model pairs – plastic and chime
Compounds A1 and A2: type of isomerism:
Compounds B1 and B2: type of isomerism:
Compounds C1 and C2: type of isomerism:
Compounds D1 and D2: type of isomerism:
Compounds E1 and E2: type of isomerism: