Organic Chemistry Fifth Edition

1
Isomers
constitutional isomers
stereoisomers
2
Chirality

• A molecule is chiral if its two mirror image
  forms are not superposable upon one another.
• A molecule is achiral if its two mirror image
  forms are superposable.

3
The Chirality Center

• a carbon atom with fourdifferent groups attached
  to it
• also called
• chiral centerasymmetric centerstereocenter
• stereogenic center

4
Examples of molecules with 1 chirality center
a chiral alkane
5
Examples of molecules with 1 chirality center
1,2-Epoxypropane a chirality center can be part
of a ring

• attached to the chirality center are
• H
• CH3
• OCH2
• CH2O

6
Examples of molecules with 1 chirality center
Chiral as a result of isotopic substitution
7
Optical Activity

• A substance is optically active if it rotates
  the plane of polarized light.
• In order for a substance to exhibit
  opticalactivity, it must be chiral and one
  enantiomer must be present in excess of the
  other.

8
Polarization of light
9
Rotation of plane-polarized light
10
Specific rotation

• observed rotation (?) depends on the number of
  molecules encountered and is proportional
  to path length (l), and concentration (c)
• therefore, define specific rotation ? as

11
Racemic mixture

• a mixture containing equal quantities of
  enantiomers is called a racemic mixture
• a racemic mixture is optically inactive (? 0)
• a sample that is optically inactive can
  beeither an achiral substance or a
  racemicmixture

12
Optical purity

• an optically pure substance consists exclusively
  of a single enantiomer
• enantiomeric excess one enantiomer
  other enantiomer
• optical purity enantiomeric excess

13
Configuration

• Relative configuration compares the arrangement
  of atoms in space of one compound with those of
  another.
• Absolute configuration is the precise
  arrangement of atoms in space.

14
The Cahn-Ingold-Prelog Rules(Table 7.1)

• 1. Rank the substituents at the chirality
  center according to same rules used in E-Z
  notation.
• 2. Orient the molecule so that lowest-ranked
  substituent points away from you.
• 3. If the order of decreasing precedence traces
  a clockwise path, the absolute configuration is
  R. If the path is counterclockwise, the
  configuration is S.

15
Very important! Two different compounds with
the same sign of rotation need not have the same
configuration.

• Verify this statement by doing Problem 7.7 on
  page 291. All four compounds have positive
  rotations. What are their configurations
  according to the Cahn-Ingold-Prelog rules?

16
Chirality center in a ring
CH2CC gt CH2CH2 gt CH3 gt H
17
7.7 Fischer Projections
Purpose of Fischer projections is to show
configuration at chirality center without
necessity of drawing wedges and dashes or using
models.
18
Physical properties of enantiomers

• Same melting point, boiling point, density,
  etc
• Different properties that depend on shape of
  molecule (biological-physiological properties)
  can be different

19
Odor
CH3
CH3
O
O
H3C
H3C
CH2
CH2
()-Carvonespearmint oil
()-Carvonecaraway seed oil
20
Chiral drugs

• Ibuprofen is chiral, but normally sold asa
  racemic mixture. The S enantiomer is the one
  responsible for its analgesic and
  antiinflammatory properties.

21
Many reactions convert achiral reactants to
chiral products.

• It is important to recognize, however, that if
  all of the components of the starting state
  (reactants, catalysts, solvents, etc.) are
  achiral, any chiral product will be formed as a
  racemic mixture.
• This generalization can be more simply stated
  as "Optically inactive starting materials can't
  give optically active products." (Remember In
  order for a substance to be optically active, it
  must be chiral and one enantiomer must be present
  in greater amounts than the other.

22
Example

• Chiral, but racemic

Achiral
23
epoxidation from this direction gives R epoxide
R
24
Isomers
constitutional isomers
stereoisomers
enantiomers
diastereomers
25
Erythro and Threo

• stereochemical prefixes used to specify relative
  configuration in molecules with two chirality
  centers
• easiest to apply using Fischer projections
• orientation vertical carbon chain

26
Erythro

• when carbon chain is vertical, same (or
  analogous) substituents on same side of Fischer
  projection

CO2H
H
HO
HO
H
CH3
9.5
9.5
27
Threo

• when carbon chain is vertical, same (or
  analogous) substituents on opposite sides of
  Fischer projection

17.8
17.8
28
Many reactions convert achiral reactants to
chiral products.

• It is important to recognize, however, that if
  all of the components of the starting state
  (reactants, catalysts, solvents, etc.) are
  achiral, any chiral product will be formed as a
  racemic mixture.
• This generalization can be more simply stated
  as "Optically inactive starting materials can't
  give optically active products." (Remember In
  order for a substance to be optically active, it
  must be chiral and one enantiomer must be present
  in greater amounts than the other.

29
Example

• Chiral, but racemic

Achiral
30
epoxidation from this direction gives R epoxide
R
31
Isomers
constitutional isomers
stereoisomers
enantiomers
diastereomers
32
Erythro and Threo

• stereochemical prefixes used to specify relative
  configuration in molecules with two chirality
  centers
• easiest to apply using Fischer projections
• orientation vertical carbon chain

33
Erythro

• when carbon chain is vertical, same (or
  analogous) substituents on same side of Fischer
  projection

CO2H
H
HO
HO
H
CH3
9.5
9.5
34
Threo

• when carbon chain is vertical, same (or
  analogous) substituents on opposite sides of
  Fischer projection

17.8
17.8