Transcript Document
Chapter 5 Stereochemistry
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Assignment for March 13
th
Class.
1.
What is the difference between cellulose and starch?
2. Number of stereoisomers in compounds with molecular formular C 8 H 18 ?
There are 18 structural isomers!
4 of them have mirror images (enantiomers) 1 has three stereoisomers ( enantiomers and diastereomers).
There are 11 stereoisomers!!
Stereochemistry
The Two Major Classes of Isomers
•
constitutional (structural) isomers and stereoisomers .
Constitutional/structural isomers have different connectivity : different IUPAC names, different physical and chemical properties.
Stereoisomers have the same connectivity but different oriention in space : identical IUPAC names (except for a prefix like
cis
or
trans
).
• Configuration : A particular three-dimensional arrangement.
Stereoisomers differ in configuration .
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Stereochemistry
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•
Importance of the stereochemistry
Example Starch : glucose polymer,
a food source to provide energy for growth. It is a major component of human diet: when cereals (barley, rice, wheat etc) and vegetables (potato, sweet potato, etc) are eaten, enzymes break down the starch molecules to glucose
Cellulose : glucose polymer,
gives rigidity to tree trunks, plant stems and leaves and it is resistant to hydrolysis by chemical or biological agents. It is indigestible to humans.
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Stereochemistry
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Stereochemistry
Chiral and Achiral Molecules
• •
Although everything has a mirror image, mirror images may or may not be superimposable .
Some molecules are like hands. Left and right hands are mirror images, superimposable .
but they are not identical, or
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Stereochemistry
Achiral Molecules
•
A molecule or object that is superimposable on its mirror image is said to be achiral .
• Two socks from a pair are mirror images that are superimposable.
A sock and its mirror image are identical.
•
Chiral :
hand in Greek
So, we say right-handed / left-handed molecules But which one is which?
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Stereochemistry
Chiral and Achiral Molecules
•
Chirality check
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Stereochemistry
Chiral and Achiral Molecules
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Stereochemistry
Chiral and Achiral Molecules
• A and B are stereoisomers—specifically, they are
enantiomers.
• A carbon atom with four different groups is a tetrahedral stereogenic center (chiral center) .
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Stereochemistry
Chiral and Achiral Molecules
• With one stereogenic center , a molecule will always be chiral .
• With two or more stereogenic centers , a molecule
may or may not
be chiral.
• In general, a molecule with no stereogenic centers will not be chiral.
There are exceptions to this.
• Achiral molecules usually contain a plane of symmetry but chiral molecules do not.
• A plane of symmetry : a mirror plane that cuts the molecule in half, so that one half of the molecule is a reflection of the other half.
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Stereochemistry
Stereogenic Centers
•
locate a stereogenic center:
examine each tetrahedral carbon atom in a molecule, and look at the four groups —not the four atoms— bonded to it.
• Always omit from consideration all C atoms that cannot be tetrahedral stereogenic centers. These include
CH 2 and CH 3 Any sp or sp 2 groups hybridized C
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Stereochemistry
Stereogenic Centers
•
Larger organic molecules can have two, three or even hundreds of stereogenic centers.
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Stereochemistry
Stereogenic Centers
•
To draw both enantiomers
CH 3 CH 2 CH 3 H OH CH 3 CH 2 CH 3 OH H 15
Stereochemistry
Stereogenic Centers
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Stereochemistry
Stereogenic Centers in Cyclic Compounds
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Stereochemistry
Stereogenic Centers
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Stereochemistry
Importance of Stereogenic Centers
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Stereochemistry
Labeling Stereogenic Centers with
R
or
S
•
Enantiomers are distinguished by adding the prefix
S
to the IUPAC name of the enantiomer.
• • This is called the Cahn-Ingold-Prelog system.
How to designate enantiomers as R or S:
R
or priorities
must be assigned to each group bonded to the stereogenic center,
order of decreasing atomic number
the highest priority (1).
.
in
The atom of highest atomic number gets 20
Stereochemistry
Labeling Stereogenic Centers with R or S
•
If two atoms on a stereogenic center are the same
assign priority based on the atomic number of the atoms bonded to these atoms.
One
atom of higher atomic number determines the higher priority.
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Stereochemistry
Labeling Stereogenic Centers with R or S
•
Among isotopes: assign priorities in order of decreasing mass number.
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Stereochemistry
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Labeling Stereogenic Centers with R or S Assigning a priority to an atom that is part of a multiple bond:
treat a multiply bonded atom as an equivalent number of singly bonded atoms.
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Stereochemistry
Labeling Stereogenic Centers with R or S
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Stereochemistry
Labeling Stereogenic Centers with R or S
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Stereochemistry
Labeling Stereogenic Centers with R or S
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Stereochemistry
Labeling Stereogenic Centers with R or S
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Stereochemistry
Labeling Stereogenic Centers with R or S
Figure 5.7
Examples: Orienting the lowest priority group in back 28
Stereochemistry
Labeling Stereogenic Centers with R or S
Stereochemistry
Labeling Stereogenic Centers with R or S
more examples
H OH HOOC Lactic acid CH 3 H OH O HOOC C H 2 MethoxyLactic acid CH 3 HO H 3 C O H H O O H O O H O
S
OH CH 3 H O O H O OH C H 2 O CH 3
S
Stereochemistry
Labeling Stereogenic Centers with R or S
more examples
HO H O HO H H 2 C O C CH 2 O C H 3 C O H O H 3 C O H H 2 C O C CH 2 O C
S R
Stereochemistry
Labeling Stereogenic Centers with R or S
more examples
3 3 3 3
H 3 C I
3 2 3 2 1
H
3 2 1 3 2 3 3 3 3
Stereochemistry Diastereomers
•
For a molecule with n stereogenic centers, the number of stereoisomers is 2
n
.
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possible stereoisomers of 2,3-dibromopentane.
maximum Diastereomers are stereoisomers that are not mirror images of each other.
Stereochemistry
• •
Meso Compounds stereoisomers of 2,3-dibromobutane.
the maximum number of stereoisomers is 4.
Meso Compounds
Stereochemistry
• •
Compound C contains achiral .
a plane of symmetry so that they possess two identical halves.
, and is Meso compounds generally contain a plane of symmetry
•
or center of symmetry
Br Cl Cl Br
Meso Compounds
Stereochemistry
Stereochemistry
•
When a compound has more than one stereogenic center, R and S configurations must be assigned to each of them.
One stereoisomer of 2,3-dibromopentane The complete name is (2S,3R)-2,3-dibromopentane
Stereochemistry
Identical CH 3 H Br (
S
) (
R
) C C CH 2 CH 3 H Br A Enantiomers CH 3 CH 2 H Br (
S
) C (
R
) C CH 3 H Br B CH 3 Diastereoisomers (
R
) (
R
) CH 2 CH 3 Br H CH 3 (
S
) (
R
) C C Br H CH 2 CH 3 CH 3 CH 2 (
S
) (
S
) CH 3 H Br
Stereochemistry
Disubstituted Cycloalkanes
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1,3-dibromocyclopentane.
Stereochemistry
Summary—Types of isomers
Stereochemistry
Determining the relationship between two nonidentical molecules
Stereochemistry
Physical Properties of Stereoisomers
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enantiomers
have except in their
identical chemical and physical properties
interaction with chiral substances and plane-polarized light .
• Plane-polarized (polarized) light : light that has an electric vector that oscillates in a single plane. Plane-polarized light arises from passing ordinary light through a polarizer.
• A polarimeter : measures of the degree to which an organic compound rotates plane-polarized light
.
Stereochemistry
Physical Properties of Stereoisomers —Optical Activity
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achiral compounds : optically inactive --- the light that exits the sample tube remains unchanged.
Stereochemistry
Physical Properties of Stereoisomers —Optical Activity
• •
chiral compounds: optically active -- the polarized light through an angle
.
The angle
is measured in degrees ( rotates the plane of
°
), and is called the observed rotation .
Stereochemistry
Physical Properties of Stereoisomers —Optical Activity
• • • •
If the rotation is clockwise – dextrorotatory If the rotation is counterclockwise – --
d
or ( levorotatory +
l
).
or ( Two enantiomers rotate plane-polarized light to an equal extent but in opposite directions .
Thus, if enantiomer A rotates polarized light +5
°
, the same concentration of enantiomer B rotates it –5
°
.
•
No relationship exists between R and S prefixes and the d (+) and l (-) designations that indicate optical rotation.
).
Stereochemistry
Physical Properties of Stereoisomers —Racemic Mixtures
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racemic mixture two enantiomers.
( racemate) : mixture of an equal amount of
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A racemic mixture is optically inactive cancel, and no rotation is observed.
-- the rotations
Stereochemistry
Physical Properties of Stereoisomers
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optical rotation :
concentration, length of the path, temperature dependent • Specific rotation : a standardized optical rotation denoted by the symbol [ ] defined using a specific sample tube length (
l
, in dm), concentration (
c
in g/mL), temperature (25 0 C) , wavelength (589 nm).
• optical rotation v.s. specific rotation – ratio of two enantiomers, purity of one
Stereochemistry
Physical Properties of Stereoisomers
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Since enantiomers have identical physical properties, they cannot be separated by common physical techniques like distillation.
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Diastereomers and constitutional isomers have different physical properties , and therefore can be separated by common physical techniques.
Stereochemistry
• •
Separation of enantiomers Recognition of enationmers differently Fig. 9-13, p. 308
Stereochemistry
Chemical Properties of Enantiomers
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Two enantiomers have exactly the same chemical properties except for their reaction with chiral non-racemic reagents.
•
Many drugs are chiral and often must react with a chiral receptor or chiral enzyme to be effective. One enantiomer of a drug may effectively treat a disease whereas its mirror image may be ineffective or toxic.
Stereochemistry
Physical Properties of Stereoisomers —Optical Purity
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Enantiomeric excess (optical purity) : a measurement of how much one enantiomer is present in excess of the racemic mixture. It is denoted by the symbol
ee
.
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cf. enantiomeric ratio
ee = % of one enantiomer - % of the other enantiomer.
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Example —If a mixture contains 75% of one enantiomer and 25% of the other, the enantiomeric excess is 75% - 25% = 50%.
Thus, there is a 50% excess of one enantiomer over the racemic mixture.
• The enantiomeric excess can also be calculated if the specific rotation [ ] of a mixture and the specific rotation [ ] of a pure enantiomer are known.
ee = ([
] mixture/[
] pure enantiomer) x 100.
Stereochemistry
Physical Properties of Stereoisomers —Optical Purity
•
Enantiomeric excess (optical purity) : a measurement of how much one enantiomer is present in excess of the racemic mixture. It is denoted by the symbol
ee
.
•
cf. enantiomeric ratio
Stereochemistry
Enantiomers and the Sense of Smell
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Because enantiomers interact with chiral smell receptors, some enantiomers have different odors.
Research suggests that the odor of a particular molecule is determined more by its shape than by the presence of a particular functional group.
Cl Cl Cl Cl Cl Cl Camphorelide ordor
Homework 5.31, 5.36, 5.37, 5.43, 5.47, 5.49, 5.51, 5.58, 5.60, 5.62, 5.64
Preview of Chapter 6
Understanding organic reactions
Kind of organic reactions
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substitution, elimination, addition reaction Bond breaking and bond making
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Radicals, carbocations, carbanions
thermodynamics and kinetics of organic reactions