ChemActivity 7
ChemActivity
Chirality
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Part A: Chirality
(How can you tell if an object is chiral?)
Model 1: Criterion for Identical-ness
Two molecules are identical if models of the two molecules can be superimposed
without breaking any bonds.
and
identical if
super-imposable
Critical Thinking Questions
1. Make two identical models of the following molecule and confirm that they can be
superimposed on one another. Use different colored balls for each of the four
substituents. If your set has green, orange and purple, use the following color code.
H
Cl
C
I
green ball = chlorine atom
orange ball = bromine atom
purple ball = iodine atom
Br
2. Switch any two balls on one of your models (leave the other model unchanged). Is
this new model identical to your original model?
No.
3. Which of the following words describe the relationship between these two models?
Circle more than one choice, if appropriate.
A.
B.
C.
D.
E.
B and E
Identical
Stereoisomers (same atom connectivity, but not identical)
Conformers (can be made identical via single bond rotation)
Constitutional isomers (same formula, different atom connectivity)
Mirror images (look like reflections of one another in the mirror)
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4. Consider the following set of molecules. Note: assume each molecule is in its
simplest conformation, even if it is not the lowest energy conformation. In this case,
assume each cyclohexane ring is in a planar conformation (rather than a chair).
a)
b)
c)
d)
A
B
C
trans
trans
cis
Label each of A-C with the word cis or trans, as appropriate.
Are any two of the set an identical pair? If so which ones.
No.
Which are more similar A&B or B&C [circle one]? A&B
What do A&B have in common that B&C do not? A&B are both trans.
Definitions:
chiral
= not identical to its mirror image.
(Chiral is Latin for “handed.”)
enantiomers = a pair of objects that are mirror images but not identical.
(a relationship between two objects)
(Enantiomers is Latin for “opposites.”)
diastereomers= a pair of objects that are stereoisomers but not enantiomers
(a property of an object)
(a relationship between two objects)
racemic mixture = a 1:1 mixture of a pair of enantiomers.
(a special sample of objects)
Critical Thinking Questions
5. Give an example of an everyday object that is chiral and one that is not chiral.
Chiral: glove, screw, car, fan, your hand
Not Chiral: soccer ball, your face (disregarding moles, scars and other
imperfections), pencil, any 2D object such as a letter or written word
Note: students often say that writing is chiral because, if you hold a page up to a
mirror it is impossible to read. But if the writing itself could be lifted off the page and
flipped over like a pancake, it would be correct again.
6. Give an example of a pair of everyday objects that are enantiomers.
a pair of gloves, or your hands, R and L bicycle pedals, righty and lefty scissors,
shoes
7. Circle each structure in CTQ 4 that is chiral (it may help to draw the mirror image
of each). A and B are chiral.
a) What is the relationship between A&B (be as specific as possible).
A& B are enantiomers (they are also stereoisomers but this is not as
specific.)
b) What is the relationship between B&C (be as specific as possible).
B&C are diastereomers (they are also stereoisomers but this is not as
specific.)
c) What is the relationship between A&C (be as specific as possible).
A&C are diastereomers (they are also stereoisomers but this is not as
specific.)
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8. T or F: All chiral objects have exactly one enantiomer. If false, give an example
from CTQ 4.
True! Your left hand has exactly one mirror image. An object can not have two
different mirror images!!
9. T or F: All diastereomers are chiral. If false, give an example from CTQ 4.
False. Structure C in CTQ 4 is a diastereomer of A and B, but it is identical to its
mirror image, so it is not chiral.
Model 2: Internal Plane of Symmetry (Mirror Plane)
It is always true that an object (or molecule) with an internal plane of symmetry is
not chiral (not chiral = achiral = identical to its mirror image).
internal plane
of symmetry
Object A
Object B
Critical Thinking Questions
10. Object C has a very obvious plane of symmetry (marked with a dotted line), and a
not so obvious one. Where is this second plane of symmetry? (Assume the balls are
uniform spheres.)
The second plane of symmetry is in the plane of the paper.
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Object C
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Chirality
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Chirality
12. Objects D and E are NOT chiral. Indicate the internal mirror plane in each.
Object D
Object E
In Object D there is a mirror plane that cuts the white and gray spheres in half and
crosses halfway between the two black spheres.
In Object E there is a mirror plane in the plane of the paper.
Part B: Absolute Configuration
(How can you tell if a chiral center is right handed “R” or left handed “S”?)
Model 3: A Trick for Determining If a Molecule Is Chiral
CH3
*C
HO
H3C
CH3
H
HO
D
chiral
H3C
H
*
C
H
H
H
not chiral
O
O
chiral
not chiral
chiral center = carbon (or other atom) with four different groups attached to it.
• By convention, chiral centers are marked with an *.
• A molecule with one chiral center is always chiral.
• If a molecule has more than one chiral center it is usually chiral.
Critical Thinking Questions
13. Construct an explanation for why each carbon indicated with a “1” is considered to
have two identical groups, while the carbons with an * are chiral, with four
different groups. (Hint: It has to do with weather the ring is symmetrical or not.)
Not Chiral
Br
H
Chiral
H3C
1
H
1
Br
H
H3C
*
H
*
CH3
considered two
identical groups
H3C
CH3
CH3
considered two different groups
For each of the chiral centers, a mental journey around the ring encounters the substituents in a
different order depending on which way you go. For the two C’s labeled “1”, a journey around the
ring in either direction is indistinguishable. This means the two parts of the ring attached to this
carbon are considered identical groups.
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14. Consider the following structures, some of which are chiral.
OH
*N
*
OH
Cl
HO
CH3
NH2
H
*
Br
Cl
*
Cl
Cl
*
Cl
*
*
Cl
*
N
O
* O
NH
*
MESO COMPOUND
a)
For some of the chiral centers above (for example, the first one), the H is not
shown. How can you tell if an H is going into the page or coming out of the page?
The "implied" H is going the opposite direction of the group shown. For example, in the top-left
structure the H is going into the page since the OH group is coming out of the page.
b)
c)
d)
Mark each chiral center on the structures above with an *.
Circle the three structures at the top of the page that are NOT chiral, and for
each circled structure, indicate the plane of symmetry (mirror plane).
One of the structures you circled is an example of the rare case where a
structure contains chiral centers but is not chiral. These molecules are
called meso compounds. Label the meso compound above.
Information
Like gloves each chiral center is either right or left handed. The convention for
determining handedness requires we rank four groups attached to a chiral center.
Rules for Ranking the Four Groups from highest (3) to lowest (0):
• The higher the atomic number of an element, the higher its rank. For example: N
(atomic # = 7) beats C (atomic # = 6), and everything beats H (atomic # = 1)
• In the first round, look at the 1st “shell.” This is comprised of the four atoms
directly attached to the chiral center of interest. (see “1st shell”, below)
• If there is a tie in the first round, a second round is necessary in which we look to
break the tie by examining atoms attached to the tied groups that are in the 2nd
“shell”. (see example).
• If there is a tie in the 2nd round, atoms in the third “shell” are compared, etc.
3rd shell
2nd shell
H
H
1st shell
H
H
H
H
C
O
C
*
C
H
O
C
H
C
H
H
H
H
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Critical Thinking Questions
15. For the example on the previous page,
a) Based on examination of the first shell, an overall winner (3) and an overall
loser (0) can be determined. Mark these atoms with 3 and 0, respectively.
b) In the first round there is a tie for ranks 2 and 1. Identify the two atoms that
are tied based only on examination of the 1st shell.
c) In this case, examination of the 2nd shell does NOT break the tie and help
determine ranks 2 and 1. Explain.
d) Based on the 3rd shell. Mark the two carbons in the 1st shell as “2” and “1”
to complete your ranking of the four atoms attached to the chiral center.
Model 4: Nomenclature for Chiral Centers
• Each chiral center is designated as either right handed (R) or left handed (S).
Right handed chiral centers are called R from the Latin word for right: rectus.
Left handed chiral centers are called S from the Latin word for left: sinister.
• The R or S assignment is called the absolute configuration of that chiral center.
• To determine the absolute configuration of a chiral center such as the one below:
H
OH
H
H
C
*
C
C
H
H
H
H
I.
II.
H
C
H
Rank the four groups attached to the chiral center 3, 2, 1 and 0, respectively.
With your RIGHT hand, grab the bond to the lowest rank group (use a model
at first). Be sure that the 0 group (usually H) is at the pinky end, and the three
larger groups are sticking out at the thumb end like a bouquet of flowers.
If the natural curl of your fingers matches the progression from
rank = 1, to rank = 2, to rank = 3 (highest) than the center is right handed or R.
If the curl of the fingers on your right hand does not match this progression
(1-2-3) then the chiral center is left handed (S). Confirm the S assignment
by grabbing this same bond with your left hand and checking that the
progression from 1-2-3 (highest rank) matches the curl of your fingers.
III.
IV.
Try your technique on the following examples. Rankings are shown using the
numbers 0 = lowest, 3 = highest.
3
3
0 H OH
*R
1
3
OH
2
NH2
*R
1
2
H (#4) going into paper is implied
*S
1
2
H (#4) coming out of paper is implied
ChemActivity 7
H3C
2
C*
0 H
H3C
2
F
*
0H C
CH3
1
Cl
2
R
Cl
CH3
C*
3 Cl
1
0
3
3
8
CH2
CH2
R
Chirality
S
CH3
H
1
Critical Thinking Questions
16. Determine the absolute configuration of each chiral center in the following
structures.
H3C
H3C
F
Cl
CH2
S *
C
Cl
CH2
R
S *
C
H
CH3
C*
CH3
H
CH3
H
Cl
17. Note that the structures in the previous question are the enantiomers of the first row
of structures at the top of the page. What happens to the absolute configuration of a
molecule if you switch exactly two groups (e.g. Cl and H) attached to a chiral
center.
If you switch exactly two groups attached to a chiral center, it changes the absolute configuration
from S to R or vice versa.
18. Determine the absolute configuration (R or S) of each chiral center in CTQ 14.
OH
S
R
*
*N
OH
Cl
HO
NH2
H
R
S
*
Br
Cl
R
*
Cl
Cl
S
S
*
*
CH3
Cl
Cl
S
R
*
*
S
N
O
* O
NH
*
MESO
COMPOUND
Exercises for Part A:
1. Draw an example of a pair of stereoisomers that are NOT enantiomers. What is the
name for the relationship between such structures?
2. Shade or mark the balls on the object below to generate a chiral object.
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Chirality
3. Consider the structures below.
H 3C
CH3
C*
Cl
*C
H
Cl
H 3C
C*
*C
Cl
H
H
CH3
CH3
Cl
*
H
*
H
H
CH3
mirror image
H 3C
CH3
CH3
C*
Cl
*C
*
Br
H
*
H
H
H
F
a)
b)
Draw the mirror image of each molecule (the first one is done for you.)
Circle the structures that are not chiral and use a dotted line to show any
internal symmetry planes. Label these structures meso compound.
4. Read the assigned sections in your text and do the assigned problems.
Exercises for Part B
5. Sometimes a molecule with one or more chiral centers is drawn without giving
enough information to determine if the chiral center/s are R or S. Mark each chiral
center below with an *.
Cl
OH
By convention, if there is not enough information given to assign R or S, we are to
assume that the structure represents an even mixture of all stereochemical possibilities.
For each structure above, draw all stereoisomers implied by the drawing. Circle those
structures that imply a racemic mixture (racemic mixture = 1:1 mixture of exactly two
enantiomers).
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6. Determine the absolute configuration of each chiral center in Exercise 3. Explain
why a meso compound must always have a 1:1 ratio of R chiral centers to S chiral
centers. (Hint: consider what happens to an R chiral center when it is reflected in an
internal mirror plane.)
7. Label every chiral center on the following molecules with an * and an R or S; and
state whether the molecule as a whole is chiral.
H
S
H
N
OH
N
OH
O
O
O
O
Penicillin G
O
Alleve (Naproxen)
8. How many different stereoisomers are there of each molecule in previous question?
9. Most books teach a different and equally good way of figuring out R and S. Find
this section in your textbook by looking in the index under chiral center or absolute
configuration.
10. Read the assigned sections in your text and do the assigned problems.
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