Chapter 7 Chirality"
CHAPTER 7
"Chirality"
In this chapter, the concept of stereochemistry is advanced by examining chiral
molecules."
This chapter begins by defining basic terms and demonstrating a new form of
stereoisomerism generated by having four different groups attached to a
tetrahedral carbon."
The remainder of the chapter explores the physical and chemical implication of
this new type of stereoisomerism."
Chapter 7 Problems: 32, 33, 38, 39, 49, 50, 53, 56 "
CHM2211 "
Chapter 7 - 2"
DEFINITIONS
STEREOISOMER
isomers that have the same connectivity but differ by the
arrangement of atoms in space.
CHIRAL
an adjective used to describe an object that has an nonsuperimposable mirror image (i.e. the mirror image is
different from the original).
ENANTIOMER
a stereoisomer that is a non-superimposable mirror image of
the original molecule.
DIASTERIOMER
a term used to describe stereoisomers that are not
enantiomers (e.g. E/Z stereoisomers of alkenes, cis/trans
disubstituted cycloakanes).
CHIRALITY CENTER a tetrahedral carbon with four different groups bonded to it
(other terms with same meaning are asymmetric-,
stereogenic-, chiral-, stereo-, -center, or –carbon).
CHM2211 "
Chapter 7 - 3"
PROPERTIES OF CHIRAL OBJECTS
Chirality is a general property that applies to all objects, small and big.
On the molecular level, chiral molecules have non-superimposable mirror images,
the mirror image molecule is called an enantiomer.
A tetrahedral carbon with four different coordinating groups is a chirality
center and, by itself, will make the molecule chiral.
CHM2211 "
Chapter 7 - 4"
Br
H
H
PROPERTIES OF ACHIRAL
OBJECTS
F
F
A tetrahedral carbon with at least two identical groups will be achiral (mirror images
A
B!
are the same).
Achiral objects have an internal symmetry element, mirror plane or center of
symmetry (inversion center).
7.2
age forms of chlorodifluoroare superimposable on each
orodifluoromethane is achiral.
Cl
F
F
H
05.indd 264
Cl
F
F
H
10/2/12 11:28 AM
CHM2211 "
Chapter 7 - 5"
OPTICAL ACTIVITY
Nearly all physical properties of a chiral molecule are identical to its enantiomer
(energy, boiling point, melting point, etc), except optical activity.
Optical activity is the ability of a chiral molecule to rotate the polarization angle of
plane-polarized light.
Degree and direction of optical activity for a chiral molecule is equal and opposite for
the enatiomer.
CHM2211 "
Chapter 7 - 6"
ABSOLUTE AND RELATIVE CONFIGURATIONS
Relative configurations are determined experimentally and designated by the
direction of the rotation of polarized light (+) or (-).
Absolute configuration determined by the structure of the stereoisomer.
Relative and absolute configurations are independent of each other.
CHM2211 "
Chapter 7 - 7"
R/S NOTATION SYSTEM
Absolute configurations denoted as R (rectus) or S (sinister).
To determine R or S, follow this procedure, use the Cahn-Ingold-Prelog rules and the
following procedure.
1. assign priority to each group on chirality center.
2. orient the molecule so that the lowest priority group points away from your
viewpoint.
3. for the remaining groups, they will orient in either a clockwise (R) or
counterclockwise (S) going from highest to lowest priority.
CHM2211 "
Chapter 7 - 8"
METHOD FOR REORIENTING MOLECULES
Often the molecule will not be drawn with the lowest priority group pointing away.
To redraw the chirality center in the proper orientation,
1. Assign priorities and redraw the chirality center.
2. Swap the position of the group pointing away from you with the lowest
priority group.
3. Keeping the lowest priority group pointing away, swap two other groups.
4. Determine the absolute configuration.
CHM2211 "
Chapter 7 - 9"
PRACTICE PROBLEMS
How many chirality centers in strychnine? What are their absolute configurations?
CHM2211 "
Chapter 7 - 10"
FISCHER PROJECTIONS
A crossed line is sometime used to denote the absolute configuration of a chirality
center.
The horizontal lines denote bonds the project in front of the paper.
The vertical lines denote bonds that project behind the plane of the paper.
CHM2211 "
Chapter 7 - 11"
PROPERTIES OF ENANTIOMERS
Physical properites of enantiomers are identical except optical activity (see slide #6)
Chemical properties can be different, notably when chiral molecules interact with
other chiral molecules.
CHM2211 "
Chapter 7 - 12"
AXIS OF CHIRALTIY
Restricted rotation about a single bond can lead to chiral molecules without a
chirality center.
CHM2211 "
Chapter 7 - 13"
REACTIONS THAT CREATE A CHIRALITY CENTER
Many previous reactions can create a chirality center.
If the starting materials and reagents are achiral (no optical activity) and a chirality
center is created, the reaction must produce equal amounts of the R and S.
A 50:50 mixture of enantiomers is called a racemic (“ra-see-mik”) mixture.
Example: Addition of HBr to 1-butene
CHM2211 "
Chapter 7 - 14"
ACHIRAL INTERMEDIATES
Reactions that go through a an achiral intermediate and create a chirality center, will
produce racemic mixtures.
Example: (S) 2-butanol to 2-bromobutane via SN1
CHM2211 "
Chapter 7 - 15"
CHIRAL MOLECULES WITH TWO CHIRALITY CENTERS
A molecule with one chirality center will have two stereoisomers.
A molecule with two chirality center will have four stereoisomers.
A molecule with n chirality centers will have 2n stereoisomers.
CHM2211 "
Chapter 7 - 16"
PHYSICAL/CHEMICAL PROPERTIES OF DIASTEREOMERS
Unlike enantiomers, the physical/chemical properties of diastereomers are different.
CHM2211 "
Chapter 7 - 17"
FISCHER PROJECTIONS WITH TWO CHIRALITY CENTERS
Each cross treated independently.
Example: tartaric acid
CHM2211 "
Chapter 7 - 18"
ACHIRAL MOLECULES WITH TWO CHIRALITY CENTERS
For molecules with two chirality centers and an internal mirror plane or inversion
center, two stereoisomers will be identical and achiral.
In these achiral molecules, one chirality center is S and the other is R.
These molecules are called meso (mee-zo).
CHM2211 "
Chapter 7 - 19"
REACTIONS THAT CREATE DIASTEREOMERS
All addition reactions have the possibility to create zero, one, or two chirality centers.
Stereospecific addition reactions (syn or anti) may create only two of the four
possible stereoisomers.
Non-stereospecific reactions will generate all four stereoisomers.
Example #1: hydrogenation of E-3,4-dimethyl-3-hexene
Example #2: Bromination of cis-2-butene
CHM2211 "
Chapter 7 - 20"
REACTIONS THAT CREATE DIASTEREOMERS
Example #1: Acid-catalyzed hydration of 3-methyl-3-hexene
Example #2: Hydrogenation of 3-methyl-3-hexene
CHM2211 "
Chapter 7 - 21"
RESOLUTION OF ENANTIOMERS
A major synthetic challenge is separating enantiomers, aka resolving enatiomers.
Resolving techniques take advantage of the physical/chemical differences of
diastereomers.
CHM2211 "
Chapter 7 - 22"
Before the development of the Ziegler–Natta catalyst systems (Section 6.14), polymerization of propene was not a reaction of much value. The reason for this has a stereochemical
basis. Consider a section of polypropylene:
STEREOCENTERS IN POLYMERS
Many common monomers
polymerize
to form chirality centers.
CH3 willCH
3 CH3 CH3 CH3 CH3
Depending on the catalyst and reaction conditions, some control over the regularity
of these chirality centers can be achieved.
Three distinct structural possibilities that differ with respect to the relative configurations of the carbons that bear the methyl groups are apparent. In one, called isotactic, all
the methyl groups are oriented in the same direction with respect to the polymer chain.
CH3
CH3
CH3
CH3
CH3
CH3
A second, called syndiotactic, has its methyl groups alternating front and back along the
chain.
CH3
CHM2211 "
CH3
CH3
CH3
CH3
CH3
Chapter 7 - 23"
Both isotactic and syndiotactic polypropylene are stereoregular polymers; each is
characterized by a precise stereochemistry at the carbon atom that bears the methyl group.
CHIRALITY CENTERS AT OTHER ATOMS
Chirality centers may exist at other tetrahedral atoms.
Examples include amines and sulfoxides.
In typical amines, the lone pair will rapidly invert through the nucleus to form an
racemic mixture.
Sulfoxides have a lone pair and are tetrahedral. The lone pair on a sulfoxide does not
invert through the nucleus and generates stable chiral molecules.
CHM2211 "
Chapter 7 - 24"