Chapter 5 Stereochemistry

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Chapter 5Stereochemistry
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Chirality

• Handedness right glove doesnt fit the left
  hand.
• Mirror-image object is different from the
  original object.
  gt

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Chiral Carbons

• Tetrahedral carbons with 4 different attached
  groups are chiral.
• Its mirror image will be a different compound
  (enantiomer). gt

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Mirror Planes of Symmetry

• If two groups are the same, carbon is achiral.
  (animation)
• A molecule with an internal mirror plane cannot
  be chiral.

Caution! If there is no plane of symmetry,
molecule may be chiral or achiral. See if mirror
image can be superimposed. gt
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Are these two structures identical?
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Stereoisomers
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Stereoisomers
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Stereoisomers
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Stereoisomers
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Stereoisomers
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Stereoisomers
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Stereoisomers
enantiomers
Stereoisomers that are nonidentical mirror images
are called enantiomers.
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Stereoisomers

• Geometric isomers cis-trans isomers.
• Enantiomers nonsuperimposable mirror images,
  different molecules.
  gt

trans -1,2-dichlorocyclopentane
cis-1,2-dichlorocyclopentane
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Stereoisomers
diasteriomers
cis
Stereoisomers that are not mirror images are
called diastereomers.
trans
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Diastereomers

• Stereoisomers that are not mirror images.
• Geometric isomers (cis-trans)
• Molecules with 2 or more chiral carbons.
  
  gt

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Alkenes

• Cis-trans isomers are not mirror images, so these
  are diastereomers.

gt
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Properties of Diastereomers

• Diastereomers have different physical properties
  m.p., b.p.
• They can be separated easily.
• Enantiomers differ only in reaction with other
  chiral molecules and the direction in which
  polarized light is rotated.
• Enantiomers are difficult to separate.
  
  gt

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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.

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Bromochlorofluoromethane is chiral
Cl

• It cannot be superposed point for point on its
  mirror image.

Br
H
F
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Bromochlorofluoromethane is chiral
Cl
Cl
Br
Br
H
H
F
F

• To show nonsuperposability, rotate this model
  180 around a vertical axis.

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Bromochlorofluoromethane is chiral
Cl
Br
Cl
Br
H
H
F
F
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Another look
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Enantiomers
nonsuperposable mirror images are called
enantiomers
and

• are enantiomers with respect to each other

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Isomers
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Chlorodifluoromethaneis achiral
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Chlorodifluoromethaneis achiral

• The two structures are mirror images, but are
  not enantiomers, because they can be superposed
  on each other.

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The Stereogenic Center
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The Stereogenic Center

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

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Chirality and stereogenic centers

• A molecule with a single stereogenic center is
  chiral.
• Bromochlorofluoromethane is an example.

H
F
Cl
Br
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Chirality and stereogenic centers

• A molecule with a single stereogenic center is
  chiral.
• 2-Butanol is another example.

H
CH3
CH2CH3
OH
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Examples of molecules with 1 stereogenic center
a chiral alkane
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Examples of molecules with 1 stereogenic center
Linalool, a naturally occurring chiral alcohol
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Examples of molecules with 1 stereogenic center
1,2-Epoxypropane a stereogenic center can be
part of a ring

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

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Examples of molecules with 1 stereogenic center
Limonene a stereogenic center can be part of a
ring

• attached to thestereogenic center are
• H
• CH2CH2
• CH2CH
• C

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A molecule with a single stereogenic centermust
be chiral.

• But, a molecule with two or more stereogenic
  centers may be chiral or it may not

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Symmetry in Achiral Structures
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Symmetry tests for achiral structures

• Any molecule with a plane of symmetryor a center
  of symmetry must be achiral.

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Plane of symmetry

• A plane of symmetry bisects a molecule into two
  mirror image halves. Chlorodifluoromethane has
  a plane of symmetry.

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Plane of symmetry

• A plane of symmetry bisects a molecule into two
  mirror image halves. Chlorodifluoromethane has
  a plane of symmetry.

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• A molecule with a plane of symmetry cannot be
  chiral
• Example
• 2-Chloropropane (a) has a plane of symmetry but
  2-chlorobutane (b) does not

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Plane of symmetry

• A plane of symmetry bisects a molecule into two
  mirror image halves.1-Bromo-1-chloro-2-fluoroeth
  ene has a planeof symmetry.

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Plane of symmetry

• A plane of symmetry bisects a molecule into two
  mirror image halves.1-Bromo-1-chloro-2-fluoroeth
  ene has a planeof symmetry.

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Center of symmetry

• A point in the center of themolecule is a center
  of symmetry if a line drawn from it to some
  element, when extended an equal distance in
  the opposite direction, encounters an
  identical element.

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Locating a Stereocenter (Chiral Carbon)

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Locating a Stereocenter

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Locating a Stereocenter

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Locating a Stereocenter
no stereocenter
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Carvone mirror-image isomers have different
flavors and odors!

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Enantiomers. Many of the properties of
enantiomers are identical.
boiling point
melting point
solubility in water
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Enantiomers. However, if enantiomers are
different compounds they must have a different
names and different properties.
Taste and smell
biological activity
(R)-2-chlorobutane
Rotation of the plane of polarized light (optical
rotation).
(S)-2-chlorobutane
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Enantiomers have different properties
Enantiomers have different biological activity.
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(-)-carvoneoil of spearmint
()-carvoneoil of caraway
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• The Biological Importance of Chirality
• The binding specificity of a chiral receptor site
  for a chiral molecule is usually only favorable
  in one way

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"How would you like to live in Looking-Glass
House, Kitty? I wonder if they'd give you milk in
there? Perhaps Looking-glass milk isn't good to
drink...".
Through the Looking Glass by Lewis Carroll
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Is looking glass milk safe to drink?
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Different enantiomers of a drug can have
different effects.
stimulant
adverse cardiovascular effects
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• Problem Are A and B identical or enantiomers?
• Manipulate B to see if it will become
  superposable with A
• Exchange 2 groups to try to convert B into A
• One exchange of groups leads to the enantiomer of
  B
• Two exchanges of groups leads back to B

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The Cahn Ingold PrelogR-S Notational System
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Assign (R) or (S)

• Assign a priority number to each group attached
  to the chiral carbon.
• Atom with highest atomic number assigned the
  highest priority 1
• Working in 3D, rotate molecule so that lowest
  priority group is in back.
• Draw an arrow from highest to lowest priority
  group.
• Clockwise (R), Counterclockwise (S)

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If the order of decreasing precedence traces a
clockwise path, the absolute configuration is R.
If the path is anticlockwise, the configuration
is S.

• Order of decreasing ranka gt b gt c gt d

clockwise
anticlockwise
R
S
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Enantiomers of 2-butanol
(S)-2-Butanol
(R)-2-Butanol
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(No Transcript)
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The Cahn-Ingold-Prelog Rules
Orient the molecule so that lowest-ranked
(lightest) substituent points away from you.
Trace the path from highest to lowest. Clockwise
(R), Counter Clockwise (S)
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Enantiomers of 2-butanol
(S)-2-Butanol
(R)-2-Butanol
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Stereogenic center in a ring R or S ?
CH2CC gt CH2CH2 gt CH3 gt H
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• Groups with double or triple bonds are assigned
  priorities as if their atoms were duplicated or
  triplicated

Vinyl vs isopropyl
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• Problem Are A and B identical or enantiomers?
• Name them using R- S- designation

(R)-1-bromo-1chloroethane
http//www.cem.msu.edu/reusch/VirtualText/sterism
3.htm
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Practice

• 5-6