Organic Chemistrywebpages.iust.ac.ir/naimi/Lectures/Organic Chemistry/Ch. 6... · Chapter 6....
Transcript of Organic Chemistrywebpages.iust.ac.ir/naimi/Lectures/Organic Chemistry/Ch. 6... · Chapter 6....
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Organic Chemistry
M. R. Naimi-Jamal
Faculty of Chemistry
Iran University of Science & Technology
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Chapter 6. Stereochemistry
Based on McMurry’s Organic Chemistry, 6th edition
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Stereochemistry
Some objects are not the same as their mirror
images (they have no plane of symmetry)
A right-hand glove is different than a left-hand
glove
The property is commonly called “handedness”
Many organic molecules (including most biochemical
compounds) have handedness that results from
substitution patterns on sp3 hybridized carbon
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Enantiomers – Mirror Images
Molecules exist as three-dimensional objects
Some molecules are the same as their mirror
image
Some molecules are different than their
mirror image
These are stereoisomers called
enantiomers
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Enantiomers and the Tetrahedral Carbon
Enantiomers are molecules that are not the same
as their mirror image
They are the “same” if the positions of the atoms
can coincide on a one-to-one basis (we test if they
are superimposable, which is imaginary)
This is illustrated by enantiomers of lactic acid
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Examples of Enantiomers
Molecules that have one carbon with 4 different
substituents have a nonsuperimposable mirror
image – enantiomer
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Mirror-image Forms of Lactic Acid
When H and OH
substituents match
up, COOH and CH3
don’t
when COOH and CH3
coincide, H and OH
don’t
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The Reason for Handedness: Chirality
Molecules that are not superimposable with
their mirror images are chiral (have
handedness)
A plane of symmetry divides an entire
molecule into two pieces that are exact mirror
images
A molecule with a plane of symmetry is the
same as its mirror image and is said to be
achiral
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Chirality
If an object has a plane of symmetry it is
necessarily the same as its mirror image
The lack of a plane of symmetry is called
“handedness”, chirality
Hands, gloves are prime examples of chiral
object
They have a “left” and a “right” version
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• The flask has a mirror plane, or plane of symmetry
• There is no mirror plane for a hand
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Chirality Centers
A point in a molecule where four different
groups (or atoms) are attached to carbon is
called a chirality center
There are two nonsuperimposable ways that
4 different groups (or atoms) can be attached
to one carbon atom
A chiral molecule usually has at least one
chirality center
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Chirality 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
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Problem: Chirality Centers?
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Problem: Same structure or Enantiomers?
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Optical Activity
Light restricted to pass through a plane is plane-polarized
Plane-polarized light that passes through solutions of achiral compounds remains in that plane
Solutions of chiral compounds rotate plane-polarized light and the molecules are said to be optically active
Phenomenon discovered by Biot in the early 19th century
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Measurement of Optical Rotation
A polarimeter measures the rotation of plane-polarized that has passed through a solution
The source passes through a polarizer, and then is detected at a second polarizer
The angle between the entrance and exit planes is the optical rotation.
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Optical Activity
Light passes through a plane polarizer
Plane polarized light is rotated in solutions of
optically active compounds
Measured with polarimeter
Rotation, in degrees, is []
Clockwise rotation is called dextrorotatory
Anti-clockwise is levorotatory
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A Simple Polarimeter
Measures extent of rotation of plane polarized light
Operator lines up polarizing analyzer and measures angle between incoming and outgoing light
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Specific Rotation
To have a basis for comparison, define specific rotation, []D for an optically active compound
[]D = observed rotation/(pathlength x concentration) = /(l x C) = degrees/(dm x g/mL)
Specific rotation is that observed for 1 g/mL in solution in cell with a 10 cm path using light from sodium metal vapor (589 nanometers)
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Specific Rotation and Molecules
Characteristic property of a compound that is optically active – the compound must be chiral
The specific rotation of the enantiomer is equal in magnitude but opposite in sign (or direction).
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Pasteur’s Discovery of Enantiomers (1849)
Louis Pasteur discovered that sodium ammonium salts of tartaric acid crystallize into right handed and left handed forms
The optical rotations of equal concentrations of these forms have opposite optical rotations
The solutions contain mirror image isomers, called enantiomers and they crystallized in distinctly different shapes – such an event is rare
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Relative 3-Dimensionl Structure
The original method was a
correlation system,
classifying related
molecules into “families”
based on carbohydrates
Correlate to D- and L-
glyceraldehyde
D-erythrose is the mirror
image of L-erythrose
This does not apply in
general
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Sequence Rules for Specification of Configuration
The configuration is specified by the relative
positions of all the groups with respect to each
other at the chirality center
The groups are ranked in an established priority
sequence (the same as the one used to
determine E or Z) and compared.
The relationship of the groups in priority order in
space determines the label applied to the
configuration, according to a rule
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Sequence Rules (IUPAC)
Assign each group priority according to the Cahn-
Ingold-Prelog scheme With the lowest priority
group pointing away, look at remaining 3 groups
in a plane
Clockwise is designated R (from “Rectus”
Latin for “right”)
Counterclockwise is designated S (from “Sinister”
Latin word for “left”)
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R-Configuration at Chirality Center
Lowest priority group is pointed away and direction of higher 3 is clockwise, or right turn
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Examples of Applying Sequence Rules
If lowest priority is
back, clockwise is R
and counterclockwise
is S
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Practice Problem
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Problem: Assign R or S
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Problem: R or S?
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Problem: Same structure or Enantiomers?
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Diastereomers
Molecules with more than one chirality center
have mirror image stereoisomers that are
enantiomers
In addition they can have stereoisomeric forms
that are not mirror images, called
diastereomers
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Problem: Assign configurations
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Problem: R or S?
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Problem: Assign R or S
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Tartaric acid:
Tartaric acid has two chirality centers and
two diastereomeric forms
One form is chiral and the other is achiral,
but both have two chirality centers
An achiral compound with chirality centers is
called a meso compound – it has a plane of
symmetry
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Enantiomers Meso forms
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Another example of a meso compound
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Molecules with More Than Two Chirality Centers
Molecules can have very many chirality centers
Each center has two possible permanent
arrangements (R or S), generating two possible
stereoisomers
The number of possible stereoisomers with n
chirality centers is 2n
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Problem: How many Chirality centers?
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Physical Properties of Stereoisomers
Enantiomeric molecules differ in the direction in
which they rotate plane polarized but their other
common physical properties are the same
Diastereomers have a complete set of different
common physical properties
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Racemic Mixtures and Their Resolution
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)
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Racemic Mixtures and Their Resolution
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
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A Brief Review of Isomerism
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Constitutional Isomers
Different order of connections gives different carbon backbone and/or different functional groups
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Stereoisomers
Same connections, different spatial
arrangement of atoms
Enantiomers (nonsuperimposable mirror
images)
Diastereomers (all other stereoisomers)
Includes cis, trans and configurational
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Stereochemistry of Reactions: Addition of HBr to Alkenes
Many reactions can produce new chirality
centers from compounds without them
What is the stereochemistry of the chiral
product?
What relative amounts of stereoisomers
form?
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Stereochemistry of Reactions: Addition of HBr to Alkenes
Example addition of HBr to 1-butene
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Achiral Intermediate Gives Racemic Product
Addition via carbocation
Top and bottom are equally accessible
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Mirror Image Transition States
Transition states are mirror images and product is racemic
Br
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Stereochemistry pf Reactions: Addition of Br2 to Alkenes
Stereospecific
Forms racemic mixture
Bromonium ion leads to anti (trans) addition
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Addition of Bromine to cis-2-butene
Racemic product
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Addition of Bromine to Trans 2-Butene
Gives meso product (both are the same because of
symmetry)
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Stereochemistry of Reactions: Addition of HBr to a Chiral Alkene
Gives diastereomers in
unequal amounts.
Facial approaches are
different in energy
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Chirality at Atoms Other Than Carbon
Trivalent nitrogen is tetrahedral
Does not form a stable chirality center since it rapidly inverts
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Prilosec (omeprazole): Chiral Sulfur
Racemic (at sulfur); the S enantiomer is
physiologically active
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Nexium (esomeprazole):
Pure (S) enantiomer
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Chirality in Nature
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
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Chapter 6, Questions
22, 23, 25, 26, 29,
33, 39, 47, 57, 60