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CHEM 109A Organic Chemistry
https://labs.chem.ucsb.edu/zakarian/armen/courses.html
Chapter 4 Isomers The arrangement of atoms in space
Problem 81. One of the chair conformers of cis-1,3- dimethylcyclohexane is 5.4 kcal/mol less stable than the other. How much strain energy does the1,3-diaxial interaction introduce?
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Isomers Isomers are compounds with different structures but the same molecular formula. isomers
constitutional isomers stereoisomers (different bond connectivity) (same bond connectivity, different configuration (different 3D arrangement)) OH O
C3H8O enantiomers diastereomers (isomers that are (isomers that are not mirror images) enantiomers) E or Z, cis or trans, one asym. center: R vs. S RSR vs RRR, two or more: RR vs. SS etc. RSR vs. SRS
etc. meso compounds
cis–trans: Diastereomers
will always have hydrogens
H H H
H
H H’s are still H there
cis trans
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Which is Cis and Which is Trans?
E–Z: Diastereomers
which group is the higher priority at this carbon? and this carbon?
lower priority
higher priority H’s are still there
Z E
• priority is determined by atomic weight; if same, move to the next heaviest atom
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E and Z Isomers
E and Z Isomers
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Chiral and Achiral Objects
A chiral object has a nonsuperimposable mirror image.
An achiral object has a superimposable mirror image.
Chiral and Achiral Objects no plane or center of symmetry: chiral
if there is a center of symmetry, or a plane of symmetry: achiral
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Enantiomers
Enantiomers are nonsuperimposable mirror images.
Chiral and Achiral Molecules
A chiral compound has a nonsuperimposable mirror image.
An achiral compound has a superimposable mirror image. (it and its mirror image are identical molecules).
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Enantiomers
The two stereoisomers are called enantiomers. Enantiomers are different compounds: they can be separated. Enantiomers have the same physical and chemical properties. (except: 1. rotation of polarized light; 2. When react with other enantiomeric compounds)
Enantiomers
HO OH
H O H O N N
HO OH
(-)-morphine (+)-morphine binds to opioid receptors in the brain does not binds to opioid receptors in the brain
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Compounds with an Asymmetric Center
Compounds with an Asymmetric Center
Problem 69. How many asymmetric centers does Mecavor have?
HO O
O O
O
Mecavor
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Asymmetric Center versus Stereocenter
Asymmetric center: an atom attached to four different groups. Stereocenter: an atom at which the interchange of two groups produces a stereoisomer.
How to Draw Enantiomers
Perspective formulas
Br Br Skeletal structures
Fischer projections
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R and S configuration. Naming Enantiomers
Assign relative priorities to the four groups.
R and S configuration. Naming Enantiomers
Draw an arrow from 1 to 2 to 3.
If the lowest priority group is on a hatched wedge, then
clockwise = R and counterclockwise = S
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Naming Enantiomers
If the lowest priority group is on a vertical bond, then
clockwise = R and counterclockwise = S
Naming Enantiomers
If the lowest priority group is on a horizontal bond, then
counterclockwise = R and clockwise = S
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Plane-Polarized Light
An Achiral Compound is Optically Inactive
An achiral compound does not rotate the plane of polarization of plane-polarized light.
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A Chiral Compound is Optically Active
A chiral compound rotates the plane of polarization of plane-polarized light.
If it rotates the plane clockwise = (+) If it rotates the place counterclockwise = (–)
R and S versus (+) and (–)
Some R enantiomers are (+) and some are (–).
Some S enantiomers are (+) and some are (–).
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If One Enantiomer is (+), the Other is (–)
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Enantiomeric Excess
Enantiomeric excess tells us how much of an excess of one enantiomer is in a mixture.
For example, if the sample of 2-bromobutane has an observed specific rotation of +9.2, then the enantiomeric excess is 40%. In other words, the excess of one of the enantiomers comprises 40% of the mixture.
Compounds with Two Asymmetric Centers
maximum # of stereoisomers = 2n
(n = # of asymmetric centers)
1 and 2 are enantiomers. 3 and 4 are enantiomers.
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Diastereomers: stereoisomers that are not enantiomers
1 and 2 are enantiomers. 3 and 4 are enantiomers.
Diastereomers are stereoisomers that are not enantiomers.
1 and 3 are diastereomers. 2 and 3 are diastereomers. 1 and 4 are diastereomers. 2 and 4 are diastereomers.
Diastereomers have different physical and chemical properties.
Perspective Formulas of the Four Stereoisomers
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Two Asymmetric Centers, Four Stereoisomers
The cis stereoisomers are a pair of enantiomers. The trans stereoisomers are a pair of enantiomers.
Identifying an Asymmetric Center
An asymmetric center is attached to four different groups.
two asymmetric centers, four stereoisomers
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No Asymmetric Centers
There are only two stereoisomers: cis and trans.
No Asymmetric Centers
There are only two stereoisomers: cis and trans.
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Two Asymmetric Centers: Three Stereoisomers (a Meso Compound and a Pair of Enantiomers)
A Meso Compound Has a Superimposable Mirror Image
Meso compounds are optically inactive even though they have asymmetric centers.
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A Meso Compound Has a Plane of Symmetry
A Meso Compound
A compound with two asymmetric centers that has the same four groups bonded to each asymmetric center will have three stereoisomers:
a meso compound and a pair of enantiomers.
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A Meso Compound
For cyclic compounds with the same substituents bonded to two asymmetric centers, cis = a meso compound and trans = a pair of enantiomers.
Naming Stereoisomers
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Naming Stereoisomers
Naming Stereoisomers
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Physical Properties of Stereoisomers
Nitrogen and Phosphorus Can Be Asymmetric Centers
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Amine Inversion
If one of the four groups attached to N is a lone pair, the enantiomers cannot be separated, because they interconvert as a result of amine inversion.
A Receptor is a Protein Proteins are Chiral Molecules
Because a receptor is chiral, it binds one enantiomer.
A right-handed glove fits only a right hand.
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A Receptor Binds One Enantiomer
Each enantiomer binds to a different receptor in the nose.
Physiological Properties of Enantiomers
Enantiomers can have very different physiological properties.
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Separating Enantiomers
separating by hand
separating by chromatography
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