Name
Lab Day
Optical Isomers
Introduction:
Stereoisomers are compounds that have the same structural
formulas, but differ in their spatial arrangements. Two major types of
stereoisomers are geometric isomers (cis-trans) and optical isomers (the
ability to rotate plane-polarized light). In this lab, you will construct and
study models of optical isomers to observe the spatial structures and the
criteria for compounds which have this property.
Definitions:
Chiral (asymmetric) carbon atom: A carbon atom that is bonded to
four different atoms or groups of atoms.
Superimposable molecules: When one molecule is laid on top of
another molecule and all the atoms of both molecules coincide
exactly, the molecules are identical.
Optical activity: Molecules that have the ability to rotate planpolarized light.
Enantiomers: Non-superimposable molecules that appear as
mirror images of one another (right hand vs. left hand).
Diastereomers: stereoisomers of the same compound, but are not
enantiomers (opposites).
Chiral molecule: A molecule that is not superimposable on its
mirror image.
Meso compounds: Stereoisomers that contain chiral carbon atoms
and are superimposable on their own mirror images.
Projection formula: The three-dimensional and projection formulas
for 1-bromo-1-chloroethane, CH3CHBrCl, are shown below:
#1
Br
Br
#1
C
H
C
Cl
Cl
H
CH3
Three-dimensional
CH3
Projection formula
2
Carbon #1 is chiral with four different groups attached to it: CH3, H, Cl, and
Br. In the three-dimensional formula the bonds from carbon to H and Cl are
coming out of the plane of the paper (like a hug) toward you. Likewise, in
the projection formula the horizontal bonds to the H and Cl are coming out
of the plane towards you. In the three-dimensional formula the bonds from
carbon to Br and CH3 are projecting behind the plane of the paper away
from you. Likewise, in the projection formula the vertical bonds to the Br
and CH3 are going behind the plane away from you.
Experimental:
Obtain a Model Kit containing a collection of plastic sticks and balls.
The black balls with four holes drilled at angles of approximately 109.5º
represent chiral carbons. The white balls represent hydrogen. The green
balls represent chlorine. The red balls represent bromine or the OH group.
The blue balls will be used to represent the CH3 group.
1-bromo-1-chloroethane, CH3CHBrCl can be represented then as
shown below:
red
Br
H
C
Cl
white
black
green
CH3
Projection formula
blue
Model Kit representation
Activities:
A.
Construct two models of 1-bromo-1-chloroethane. Check to see if the
two models are superimposable on one another. If they are superimposable,
swap the position of the Br and Cl on one of the models. You should now
have two models that are not superimposable. Draw these molecules so that
they appear as mirror images. Put the Br on the top position and the CH3 on
the bottom, and the H and Cl on the horizontal positions. Draw the
projection formulas below (assume the chiral carbon is located where the
lines cross).
3
Since these two molecules are not superimposable and are mirror images,
they must be different compounds. These molecules are opposites or
.
B.
Construct two models of lactic acid. Carbon #2 is chiral.
#3
CH3
#2
CH
OH
#1
COOH
Use a yellow ball
to represent the
COOH group
Check to see if the two models are superimposable on one another. If they
are superimposable, swap the position of the H and OH on carbon #2 on one
of the models. You should now have two models that are not
superimposable. Draw these molecules so that they appear as mirror
images. Put the COOH on the top position and the CH3 on the bottom, and
the H and OH on the horizontal positions. Draw the projection formulas
below (assume the chiral carbon is located where the lines cross).
4
C.
#1
#2
#3
#4
Construct two models of the compound CH3CHClCHBrCH3
Carbon #2 and #3 are both chiral. Position the two CH3 groups on the top
and bottom vertical positions. Now arrange the Cl on carbon #2 and the Br
on carbon #3 so that both groups are on your right on one model and both
are on your left on the other model. Draw the projection formulas below
(assume the chiral carbon is located where the lines cross).
#1
#2
#3
#4
Construct two more models of the compound CH3CHClCHBrCH3
Position the two CH3 groups on the top and bottom vertical positions. Now
arrange the Cl on carbon #2 on the right and the Br on carbon #3 on the left
on one model, and the Cl on carbon #2 on the left and the Br on carbon #3
on the right on the other model. Draw the projection formulas below
(assume the chiral carbon is located where the lines cross).
5
You now have two pairs of mirror images in these four structures. All four of
these isomers are optically active. If you compare either of the first two pair
with either of the second pair, you will notice although they are
stereoisomers of the same compound, they are not mirror images of each
other. Instead they are
.
D.
#1
#2
#3
#4
Construct four models of the compound CH3CHClCHClCH3
Take two of these models and position the two CH3 groups on the top and
bottom vertical positions. Now arrange the Cl on carbon #2 on the right and
the Cl on carbon #3 on the left on one model, and the Cl on carbon #2 on the
left and the Cl on carbon #3 on the right on the other model. Draw the
projection formulas below (assume the chiral carbon is located where the
lines cross).
Take the other two models and position the two CH3 groups on the top and
bottom vertical positions. Now arrange the Cl on carbon #2 on the right and
the Cl on carbon #3 on the right on one model, and the Cl on carbon #2 on
the left and the Cl on carbon #3 on the left on the other model. Draw the
projection formula below (assume the chiral carbon is located where the
lines cross).
6
Did you notice anything about these last pair? The last pair of models
constructed are superimposable. The molecule is symmetrical about a
horizontal plane passing through its center, between carbon #2 and carbon
#3. The molecule shows no optical activity. Stereoisomers that contain chiral
carbon atoms and are superimposable on their own mirror images are
compounds .
E.
#1
#2
#3
Draw all projection formulas for CH3CHBrCHBrCl
Put the CH3 on the top and the Cl of carbon #3 on the bottom vertical
positions.
7
F.
Draw all projection formulas for CH2BrCHBrCHBrCH2Br
Use a blue or yellow ball to represent the two CH2Br groups and put
the groups on the top and bottom vertical positions.
Circle the two identical molecules.
8
Question 1. For an organic compound to show optical activity, it must have
at least on chiral carbon atom. Place a check mark next to the following
compounds that would show optical activity.
CHClBrI
CH3CHCl2
CH3CHBrCl
Draw expanded structural
formulas for these condensed
formulas before checking for
chiral carbon atoms.
CH3CH2CH(CH3)CH2CH3
CH3CH2CH(CH3)CH2CH2CH3
CH3CBrClCH2CH(CH3)CH3
CH2ClCHClCH2Cl
CH2ClCH2Br
Question 2. The maximum number of stereoisomers for a chiral compound
is given by the formula 2n, where n is the number of chiral carbon atoms in
the compound. If n = 0, the compound is not chiral. What is the maximum
number of stereoisomers for the following compounds:
CH2ClCHBrCH2Cl
Draw expanded structural
formulas for these condensed
formulas before checking for
chiral carbon atoms.
CH3CHClCHClCH3
CH2=CHCHClCH3
CH2ClCHClCOOH
Question 3. Construct the following compounds using a blue ball for CH3,
a white ball for H, a red ball for Br, and a yellow ball for OH. Are the
compounds mirror images or are they the same molecule?
CH3
H
C
Br
Circle the correct answer:
OH
OH
CH3
C
Br
Mirror images
H
Same molecule