Transcript R,S Configurations

Unit 3
Stereochemistry
Stereochemistry
Chirality and Stereoisomers
Configuration vs. Conformation
(R) and (S) Configurations
Optical Activity
Fischer Projections
Diastereomers
Relative Configurations - The D-L
System
 Resolution of Enantiomers
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Stereochemistry
 Stereochemistry is the study of the
three-dimensional structure of
molecules.
 Central to this study is the notion of
chirality. An object is chiral if it has
a nonsuperimposable mirror image.
 An object is achiral if it is identical to
its mirror image.
Mirror Images and Chirality
From Wikipedia, “Mirror Images”
Chirality in Organic Compounds
 We are, of course, interested in organic
compounds. How do we decide if a
compound is chiral?
 0 asymmetric C atoms* in the compound:
usually achiral.
 1 asymmetric C atom* in the compound:
chiral.
 >1 asymmetric C atoms in the compound:
may or may not be chiral.
*The asymmetric atom can be an element other than C.
Asymmetric C Atoms
 An asymmetric C atom is the
same as a chiral C atom.
 It is a C atom bonded to four
different groups.
 A chiral C atom is designated
by an asterisk (*).
Asymmetric Atom Equals Chiral
Atom
N is the asymmetric
atom here.
Stereocenter
 This is a broader term than chiral
C atom.
 A stereocenter is any atom at
which the interchange of two
groups gives a stereoisomer.
 Chiral C atoms are stereocenters.
 But so are the double-bonded C
atoms in cis-trans isomers.
Stereoisomers
 Isomers are compounds with the
same molecular formula.
 Structural isomers have different
bonding patterns (different connections
between the atoms).
 Stereoisomers have identical bonding
patterns but differ in how the atoms are
oriented in space.
 Enantiomers are mirror image isomers.
 Diastereomers are stereoisomers that are
not mirror images.
Stereocenters
stereocenters
enantiomers
geometric
isomers
(diastereomers)
Drawing a Mirror Image
 Leave the vertical arrangement
alone.
 Switch left and right.
Mirror Plane of Symmetry
 Any molecule with an internal mirror
plane of symmetry cannot be chiral,
even if it has chiral C atoms.
achiral
chiral, enantiomers
achiral
diastereomers
Where is the
plane of
symmetry?
Configuration vs Conformation
 In this unit, we talk about the
configuration of molecules.
 Configuration refers to the 3D
arrangement of atoms in space.
 Conformations come from a single
configuration as groups are rotated
around their sigma bonds.
(R) and (S) Nomenclature of
Asymmetric C Atoms
 Cahn-Ingold-Prelog Convention
 Each asymmetric C atom is assigned (R) or
(S) based on its 3D configuration.
 Each of the four groups on the asymmetric
C atom is assigned a priority: 1(highest), 2,
3, 4(lowest).
 Place group 4 behind the C atom, then draw
a curved arrow from group 1 to group 2 to
group 3.
 If the arrow goes clockwise (CW): (R)
 If the arrow goes counterclockwise
(CCW): (S)
(R) and (S) Nomenclature of
Asymmetric C Atoms
 Assigning priorities
 The higher the atomic number Z, the
higher the priority.
 The isotope with the higher mass
number has higher priority: 2H (or D)
(deuterium) has higher priority than 1H.
 In case of a tie, use the Z of the next
atom along.
 Treat double and triple bonds as if
each were a bond to a separate atom.
(R) and (S) Nomenclature of
Asymmetric C Atoms
 Assigning priorities
 The higher the atomic number Z, the higher
the priority.
3
4
(S)
2
1
3
1
2
Assign priorities.
Put lowest priority in back.
Draw an arrow: 1-2-3.
(R) and (S) Nomenclature of
Asymmetric C Atoms
 Assigning priorities
 The higher the atomic number Z, the higher
the priority.
3
The arrow goes CCW, so the
configuration is (S).
1
2
(S)-1-bromo-1-chloroethane
Nomenclature
 Below is the template you will use to
build the name of ANY organic
compound.
stereomain
functional
substituents
unsaturation
isomerism
chain
group
(R) and (S) Nomenclature of
Asymmetric C Atoms
 Assigning priorities
 In case of a tie, use the next atom along.
C bonded to H, H, C:
tie for 2. But the next
C is also bonded to
H,H,C: priority 2.
C bonded to H, C, C:
priority 1
C bonded to H, H, C:
tie for priority 2.
But the next C is
H,H,H: priority 3.
C bonded to H, H, H:
priority 4
(R) and (S) Nomenclature of
Asymmetric C Atoms
 Assigning priorities
 In case of a tie, use the next atom along.
1
(R)
3
2
4
Put lowest priority in back.
3
2
1
1-2-3 arrow is CW: (R)
(R) and (S) Nomenclature of
Asymmetric C Atoms
 Assigning priorities
 In case of a tie, use the next atom along.
(R)
3
2
1
(R)-3-ethyl-2,3-dimethylhexane
(R) and (S) Nomenclature of
Asymmetric C Atoms
 Assigning priorities
 Treat double and triple bonds as if each
were a bond to a separate atom.
(R) and (S) Nomenclature of
Asymmetric C Atoms
 Assigning priorities
 Treat double and triple bonds as if each
were a bond to a separate atom.
(R)-2,3-dihydroxypropanal
(R) and (S) Nomenclature of
Asymmetric C Atoms
 Assigning priorities
 Treat double and triple bonds as if each
were a bond to a separate atom.
*
(S)-carvone
Drawing the Structure of a
Chiral Compound
 Draw the structure of (S)-2-bromobutane.
1. Identify the chiral C.
2. Assign priorities to the four groups on the C
atom.
3. Put the lowest priority group in back.
Drawing the Structure of a
Chiral Compound
 Draw the structure of (S)-2-bromobutane.
4. Put the highest priority group on top.
5. For (R), put the priority #2 group on the
right and the priority #3 group on the left.
6. For (S), put the priority #3 group on the
right and the priority #2 group on the left.
Drawing the Structure of a
Chiral Compound
 Draw the structure of (S)-2-bromobutane.
7. Double check your structure to make sure
it has the specified configuration.
Drawing the Structure of a
Chiral Compound
 Alanine, NH2CH(CH3)CO2H, is an
amino acid.
 Draw (R)-alanine and (S)-alanine.
 Nearly all naturally-occurring amino acids
have the (S) configuration on the α C.
Name This Compound
 First, get the “nonstereochemical”
name:
 3-bromo-1,1-dimethylcyclopentane
 Then bring in the stereochemistry:
 (R)-3-bromo-1,1-dimethylcyclopentane