CHAPTER 15
The 3-D Shape of Molecules
General, Organic, & Biological Chemistry
Janice Gorzynski Smith
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CHAPTER 15: The 3-D Shape of Molecules
Learning Objectives:
Understand the difference between Chiral and Achiral
molecules
Identify chiral centers
Draw the enantiomers & diastereomers of a chiral
molecule (using dashes & wedges to show geometry)
Draw fischer projections of chiral molecules
Explain the relevance of chiral centers in Biochemistry
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Smith. General Organic & Biological Chemistry 2nd Ed.
ISOMERS
Same chemical formula
Stereoisomers
Constitutional
Isomers
Spatial Isomers
Structural Isomers
Different connectivity between atoms
Different spatial arrangement of atoms but with
the same connectivity
Enantiomers
Diastereomers
Mirror images of a chiral molecule
that are not superimposable
All stereoisomers that are not mirror
images of one another
Cis/Trans
Isomers
Conformational
Isomers
Rotation around C-C single bonds
R
R
R
R
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Stereoisomers
Chiral & Achiral
•Left hands and right hands are mirror images of
each other.
•A molecule that is not superimposable on its mirror
image is chiral.
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Smith. General Organic & Biological Chemistry 2nd Ed.
Stereoisomers
Chiral & Achiral
•Two socks from a pair are mirror images that are
superimposable.
•A molecule that is superimposable on its mirror
image is achiral.
Smith. General Organic & Biological Chemistry 2nd Ed.
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Stereoisomers
Chiral & Achiral
To test whether a molecule is chiral or achiral:
1. Draw the molecule in 3 dimensions.
2. Draw its mirror image.
3. Try to align all bonds and atoms.
•To superimpose a molecule and its mirror image
you can perform any rotation but you cannot
break bonds.
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Smith. General Organic & Biological Chemistry 2nd Ed.
Stereoisomers
Chiral & Achiral
For CH2BrCl:
Rotate the
molecule to
align bonds:
ACHIRAL
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Smith. General Organic & Biological Chemistry 2nd Ed.
Stereoisomers
Chiral & Achiral
For CHBrClF:
CHIRAL
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Smith. General Organic & Biological Chemistry 2nd Ed.
Stereoisomers
Chiral & Achiral
Identify the chiral centers in…
cholesterol
glucose
cysteine
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Chiral
Molecules
Enantiomers
Many drugs are chiral, and often they must interact with a
chiral receptor to be effective.
Thalidomide was an anti-nausea drug sold to pregnant
in the 60’s as a mixture with its enantiomer. It was found
that the enantiomer interacted with the body differently,
and caused catastrophic birth defects.
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Smith. General Organic & Biological Chemistry 2nd Ed.
Chiral
Molecules
Enantiomers
Ibuprofen is an active anti-inflammatory agent
whose enantiomer is inactive, and it is sold as a
racemic mixture.
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Smith. General Organic & Biological Chemistry 2nd Ed.
Chiral
Molecules
Enantiomers
Two different enantiomers can interact differently
with smell receptors in the brain and have
different perceived odors.
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Smith. General Organic & Biological Chemistry 2nd Ed.
Stereoisomers
Multiple Chiral Centers
Draw the Enantiomer of the
amino acid Glutamate
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Chiral
Molecules
Multiple Chiral Centers
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Smith. General Organic & Biological Chemistry 2nd Ed.
Stereoisomers
chemwiki.ucdavis.edu
Multiple Chiral Centers
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Chiral
Molecules
Fischer Projections
A Fischer Projection takes a 3D tetrahedral shape,
and re-draws it:
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Smith. General Organic & Biological Chemistry 2nd Ed.
Chiral
Molecules
Fischer Projections
We can draw the cross and Fischer projection
for both enantiomers of 2-butanol:
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Smith. General Organic & Biological Chemistry 2nd Ed.
Chiral
Molecules
Fischer Projections
Fischer Projections with multiple chiral centers:
A&C; A&D; B&C; B&D are all Diastereomers
Smith. General Organic & Biological Chemistry 2nd Ed.
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Chiral
Molecules
Fischer Projections
Draw a Fischer Projections of the carbohydrate erythrose’s
stereoisomers (all 4).
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Chiral
Molecules
Optical Properties
•Two enantiomers have identical physical properties
(melting pt., boiling pt., solubility, etc.)
•They differ in how they interact with plane-polarized
light.
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Smith. General Organic & Biological Chemistry 2nd Ed.
Chiral
Molecules
Optical Properties
•An achiral compound does not change the direction
of plane-polarized light and is optically inactive.
•A chiral compound rotates the plane of polarized light
through an angle α and is optically active.
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Smith. General Organic & Biological Chemistry 2nd Ed.
Chiral
Molecules
Optical Properties
Assign Priority to the substituents bonded to the chiral carbon 1-highest, 4-lowest, to determine R or S rotation
1. The higher the atomic number of the immediate substituent atom, the higher the priority.
For example, H– < C– < N– < O– < Cl–.
2. If two substituents have the same immediate substituent atom, evaluate atoms progressively further away from the
chiral center until a difference is found.
For example, CH3– < C2H5– < ClCH2– < BrCH2– < CH3O–.
3. If double or triple bonded groups are encountered as substituents, they are treated as an equivalent set of singlebonded atoms.
For example, C2H5– < CH2=CH– < HC≡C–
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Chiral
Molecules
Optical Properties: R & S
Enantiomers have
opposite optical
properties: ie, R,R vs S, S
rotation of light
One of the chiral centers of
diastereomers will rotate
light in the same way: ie,
R,R vs R,S
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chemwiki.ucdavis.edu
Chiral
Molecules
Optical Properties: L & D
L is “left handed”
(equivalent to “S”)
D is “right handed”
(equivalent to “R”)
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