chapter9 stereokimia.ppt.........................

9. Stereochemistry
Based on McMurry’s Organic Chemistry, 7th
edition

2
Stereochemistry
 Some objects are not the
same as their mirror
images (technically, they
have no plane of
symmetry)
 A right-hand glove is
different than a left-
hand glove. The
property is commonly
called “handedness”
 Organic molecules
(including many drugs)
have handedness that
results from substitution
patterns on sp3
hybridized
carbon

3
Why this Chapter?
 Handedness is important in organic and
biochemistry
 Other types of stereoisomers besides
cis/trans
 Molecular handedness makes possible
specific interactions between enzymes and
substrates—effecting metabolism and
pharmaceuticals

4
Examples of Enantiomers
 Molecules that have one carbon with 4 different substituents
have a non-superimposable mirror image
 Enantiomers = non-superimposable mirror image
stereoisomers
 Enantiomers have identical physical properties (except for one)
 Build molecular models to see this

5
Chirality
 If an object has a plane of symmetry it’s the same as its mirror image
 A plane of symmetry divides an entire molecule into two pieces that are exact mirror
images
 Achiral means that the object has a plane of symmetry
 Molecules that are not superimposable with their mirror images are chiral (have
handedness)
 Hands, gloves are prime examples of chiral object
 They have a “left” and a “right” version
 Organic molecules can be Chiral or Achiral

6
Chiral Centers
 A point in a molecule where four different groups (or
atoms) are attached to carbon is called a chiral center
 There are two nonsuperimposable ways that 4 different
different groups (or atoms) can be attached to one carbon
atom
 If two groups are the same, then there is only one way
 A chiral molecule usually has at least one chiral center

7
Chiral Centers in Chiral Molecules
 Groups are considered “different” if there is any
structural variation (if the groups could not be
superimposed if detached, they are different)
 In cyclic molecules, we compare by following in each
direction in a ring

8
9.3 Optical Activity
 Light restricted to pass through a plane is plane-polarized
 A polarimeter measures the rotation of plane-polarized light that
has passed through a solution
 Rotation, in degrees, is []
 Clockwise (+) = dextrorotatory; Anti-clockwise (-) = levorotatory
 Plane-polarized light that passes through solutions of achiral
compounds remains in that plane ([] = 0, optically inactive)
 Solutions of chiral compounds rotate plane-polarized light and the
molecules are said to be optically active

9
Measurement of Optical Rotation
 The more molecules of a chiral sample are present the greater the rotation of the
light = concentration dependent
 To have a basis for comparison, define specific rotation, []D for an optically
active compound
 Specific rotation is that observed for 1 g/mL in solution in a cell with a 10 cm path
using light from sodium metal vapor (589 nm)
 The specific rotation of the enantiomer is equal in magnitude but opposite in sign
(+)-lactic acid = +3.82; (-)-lactic acid = -3.82
C
l
D





g/ml)
in
tration
dm)(concen
in
h
(pathlengt
degrees)
in
rotation
(observed
]
[

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9.4 Pasteur’s Discovery of Enantiomers
 Louis Pasteur discovered that sodium ammonium salts of
tartaric acid crystallize into right handed and left handed forms
 The solutions contain mirror image isomers, called
enantiomers and they crystallized in distinctly different
shapes – such an event is rare
 A (50:50) racemic mixture of both crystal types dissolved
together was not optically active
 The optical rotations of equal concentrations of these forms
have opposite optical rotations

11
Sequence Rules for Specification of Configuration
 A general method applies to determining the configuration at each
chiral center (instead of to the whole molecule)
 The configuration is specified by the relative positions of all the
groups with respect to each other at the chiral center
 The groups are ranked in an established priority sequence and
compared—use the same priority ranking as we did for E/Z names
 The relationship of the groups in priority order in space determines
the label applied to the configuration, according to a rule
Method
• Assign each group priority 1-4 according to Cahn-Ingold-Prelog
• Rotate the assigned molecule until the lowest priority group (4) is in
the back, look at remaining 3 groups in a plane
• Clockwise 1-2-3 movement is designated R (from Latin for “right”)
• Counterclockwise is designated S (from Latin word for “left”)

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Priority (Cahn-Ingold-Prelog) Rules [REVIEW CH. 6]
Rule 1:
 Look at the atoms directly attached to the chiral carbon and
assign priority based on highest atomic number (O > N > C >
H)
Rule 2:
 If decision can’t be reached by ranking the first atoms in the
substituents, look at the second, third, or fourth atoms until
difference is found
Rule 3:
 Multiple-bonded atoms are equivalent to the same number of
single-bonded atoms

14
9.6 Diastereomers
 Molecules with more than one chiral center have mirror image
stereoisomers that are enantiomers
 In addition they can have stereoisomeric forms that are not mirror
images, called diastereomers

15
 Diastereomers are similar, but they aren’t mirror images
 Enantiomers have opposite configurations at all chiral centers;
Diastereomers are opposite at some, but not all chiral centers
 Diastereomers have different physical properties
 Epimers are diastereomers different at only 1 chiral center
Diastereomers

16
9.7 Meso Compounds
 Tartaric acid has two chiral centers and two diastereomeric forms
 One form is chiral and one is achiral, but both have two chiral centers
 An achiral compound with chiral centers is called a meso compound
– it has a plane of symmetry
 The two structures on the right in the figure are identical so the
compound (2R, 3S) is achiral
 Identical substitution on both chiral centers

17
9.8 Racemic Mixtures and The
Resolution of Enantiomers
 A 50:50 mixture of two chiral compounds that are
mirror images does not rotate light – called a
racemic mixture (named for “racemic acid” that was
the double salt of (+) and (-) tartaric acid
 The pure compounds need to be separated or
resolved from the mixture (called a racemate)
 To separate components of a racemate (reversibly)
we make a derivative of each with a chiral substance
that is free of its enantiomer (resolving agent)
 This gives diastereomers that are separated by their
differing solubility
 The resolving agent is then removed

18
Using an Achiral amine doesn’t change the relationship of the products
Still can’t separate the Enantiomeric Salts

19
Using a Chiral amine changes the relationship of the products
Now we can separate the Diastereomeric Salts

20
9.9 A Review of Isomerism
 The flowchart summarizes the types of isomers we
have seen

21
Constitutional Isomers
 Different order of connections gives different carbon
backbone and/or different functional groups

22
Stereoisomers
 Same connections, different spatial arrangement of atoms
 Enantiomers (nonsuperimposable mirror images)
 Diastereomers (all other stereoisomers)
 Includes cis, trans and configurational

23
9.10 Stereochemistry of Reactions:
Addition of H2O to Alkenes
 Many reactions can produce new chiral centers from
compounds without them
 What is the stereochemistry of the chiral product?
 What relative amounts of stereoisomers form?
 Example addition of H2O to 1-butene

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Achiral Intermediate Gives Racemic Product
 Addition via carbocation
 Top and bottom are equally accessible
 Achiral reactant + Achiral reactant = Optically Inactive Product
 Optical Activity doesn’t come from nowhere

25
Addition of H2O to a Chiral Alkene
 What is the sterochemical result of the addition of H2O to a chiral
alkene R-4-methyl-1-hexene
 Product has 2 chiral centers
-Chiral + Achiral = Optically Active
-Chiral Intermediate has different top
and bottom sides
-Amounts of the two products will be
different
-Product will have optical activity

26
9.12 Chirality at Nitrogen,
Phosphorus, and Sulfur
 N, P, S commonly found in organic compounds, and can have
chiral centers
 Trivalent nitrogen is tetrahedral
 Does not form a chiral center since it rapidly flips
 Individual enantiomers cannot be isolated = Achiral

27
 Also applies to phosphorus but it flips more slowly
 Can isolate individual enantiomers = Chiral
 Trivalent Sulfur Cations are also Chiral

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9.13 Prochirality
 A molecule that is achiral but that can become chiral by a single alteration is a
prochiral molecule
 Re and Si are used to describe the faces of the prochiral sp2
reactant

29
Prochiral distinctions, paired atoms or groups
 An sp3
carbon with two groups the same is also a prochiral center
 The two identical groups are distinguished by considering either and
seeing if it was increased in priority in comparison with the other
 If the center becomes R the group is pro-R and pro-S if the center
becomes S

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Prochiral Distinctions in Nature
 Biological reactions often involve making distinctions
between prochiral faces or or groups
 Chiral entities (such as enzymes) can always make
such a distinction
 Example: addition of water to fumarate

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Chirality in Nature and Chiral Environments
 Enantiomers have same physical properties, different biological ones
 Stereoisomers are readily distinguished by chiral receptors in nature
 Properties of drugs depend on stereochemistry
 Think of biological recognition as equivalent to 3-point interaction
 Enzymes can make only one enantiomer from an achiral reactant
 In the chiral environment, pro-R and pro-S are chemically different

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