the word chiral derives from the greek word ceir (cheir), meaning hand. our hands are chiral - the...
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The word chiral derives from the Greek word ceir (cheir), meaning hand.
Our hands are chiral - the right hand is a mirror image of the left -
as are most of life's molecules such as (R)-alanine and (S)-alanine,
which are mirror images of each other.
OPTICAL ISOMERISM DEALS WITH CHIRALITY
NOT ALL OBJECTS EXHIBIT CHIRALITY, BUT MOST OBJECTS** HAVE MIRROR IMAGES.
EXCEPTIONS ARE: VAMPIRES,THE DEVIL, FALLEN MEN
OPTICAL ISOMERISM
FAUST GOUNOD
HOFFMAN (TALES OF HOFFMAN) HAD HIS REFLECTION STOLEN BY ANTONIO WHO GAVE IT TO DAPERTULU (ASORCEROR).
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Alone in his study, the aged Dr. Faust despairs that his lifelong search for a solution to the riddle of life has been in vain. Twice he raises a goblet of
poison to his lips but falters when the songs of young men and women outside his window re-awaken the unfulfilled passions and desires of his youth. Cursing life and human passion, the envious philosopher calls on Satan for help. The Devil appears, and Faust tells him of his longing for
youth and pleasure; Méphistophélès replies that these desires can be realized if he will forfeit his soul. Faust hesitates until the Devil conjures up a vision of a lovely maiden, Marguerite. A magic potion transforms
Faust into a handsome youth, and he leaves with Méphistophélès in search of Marguerite (Duet: "A moi les plaisirs").
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SO HOW ONE DISTINGUISHED MOLECULES THAT EXHIBIT CHIRALITY AND MOLECULES THAT LACK CHIRALITY (CALLED ACHIRAL MOLECULES)?
MIRROR IMAGES OF CHIRAL MOLECULES ARE NON-SUPERPOSABLE (THAT IS THEY ARE DIFFERENT)
MIRROR IMAGES OF A CHIRAL MOLECULES ARE SUPERPOSABLE (THAT IS THEY ARE THE SAME)
CHIRAL OBJECTS: FEET, SCREWS, GOLF CLUBS, STUDENTS ARM CHAIRS, ENGLISH AND AMERICAN CARS
ACHIRAL OBJECTS: SPOONS, KNIVES, FORKS
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Alice’s father was Dean of Christ Church, Oxford
Mathematic Don at Christ Church, Oxford University
Amateur Photographer
Real name was Charles Dodgson
Alice and Lewis Carroll
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WHAT DO CHIRAL MOLECULES LACK THAT ACHIRAL HAVE?
CHIRAL MOLECULES LACK SYMMETRY
A CHIRAL MOLECULES POSSESS SYMMETRY
A symmetry element is a plane, a line or a point in or through an object, about which a rotation or reflection leaves the object in an orientation indistinguishable from the original.
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CONSIDER 1,2-DIMETHYLCYCLOPENTANE
Me Me Me Me
SAME, thus achiral
each has a plane of symmetry
Me
Me
Me
Me
different mirror images
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ASYMMETRIC CARBON ATOM (CHIRALITY CENTER) OR (STEREOCENTER)
MOLECULE THAT HAS ONE OF THE ABOVE WILL BE CHIRAL
THAT IS, LACKS SYMMETRY
REQUIREMENT: 4 DIFFERENT GROUPS
A
D BC
A
CB D
BD
A
C
DB
A
C
NOTE: EXCHANGE ANY TWO GROUPS TO GO FROM ENANTIOMERTO THE OTHER ENANIOMER!!
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EXAMPLES
H3C H
OMe
OH
O
H NH2 CHO
CH2OH
H OH
Cl OMeO
Me
O
Me
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(R), (S) Nomenclature
• Different molecules (enantiomers) must have different names.
C
CO OH
H3CNH2
H
natural alanine=>
•Usually only one enantiomer will be biologically active.
•Configuration around the chiral carbon is specified with (R) and (S).
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Cahn-Ingold-Prelog Rules Overview
Assign priorities to each group
View molecule with the lowestpriority away from the viewer
Arrow is drawn from the atom with 1st priority through the atom with 2nd priority to the atom with 3rd priority.
If arrow points clockwise - R(rectus)
If arrow points counter- clockwise - S (sinister)
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Assigning priorities-CASE 1
• Assign a priority number to each atom attached to the chiral carbon.
•Atom with highest atomic number assigned the highest priorities #1.
In case of isotopes, high(er)est priority given tothe isotope with high(er)est mass number.
CASE 1 - four different atoms attached to chiral atom
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3-D Examples Assigning PrioritiesCl
F BrH 1
2
3
4
Cl
FBr H
1
2
34
PERSPECTIVE
BrF
Cl
H
FBr
Cl
H
WEDGE1
2
3
4
1
2
3
4
F Br
H
Cl
F Br
H
Cl
FISHER
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PRIORITY ASSIGNMENT EXAMPLES CASE 2 - In case of ties among 1st atom, go to 2nd
atoms, then to 3rd etc until the tie is broken.Consider straight chain groups
Me < Et<n-propyl etcConsider effect of branching
Et< isopropyl < tert-butyl
Consider effect of hetero atom
Tert-butyl < CH2OMe
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Groups possessing multiple bonds
C=C C-C
C C Carbon doubly bondedto carbon is likened unto a carbon bonded to two carbons
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Assign Priorities
C
CO OH
H3CNH2
H
natural alanine
1
2
3 4
Cl
HCl
H
*
12
34
12
3
4
=>
CC
O
H
CH CH2
CH2OHCH(CH3)2
*C
C
C
CH2OHCH(CH3)2
HO
O
C
CH CH2
C
*expands to
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Assign (R) or (S)• Working in 3D, rotate molecule so that lowest
priority group is in back.• Draw an arrow from highest to lowest priority
group.• Clockwise = (R), Counterclockwise = (S)
=>
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Examples
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Fischer Projections• Flat drawing that represents a 3D molecule
• A chiral carbon is at the intersection of horizontal and vertical lines.
• Horizontal lines are forward, out-of-plane.
• Vertical lines are behind the plane.
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Fischer Rules
• Carbon chain is on the vertical line.
• Highest oxidized carbon at top.
• Rotation of 180 in plane doesn’t change molecule.
• Do not rotate 90!• Do not turn over out of plane! =>
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Fischer Mirror Images
• Easy to draw, easy to find enantiomers, easy to find internal mirror planes.
• Examples:CH3
H Cl
Cl H
CH3
CH3
Cl H
H Cl
CH3
CH3
H Cl
H Cl
CH3=>
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Fischer (R) and (S)• Lowest priority (usually H) comes forward, so assignment
rules are backwards!
• Clockwise 1-2-3 is (S) and counterclockwise 1-2-3 is (R).
• Example:
CH3
H Cl
Cl H
CH3
(S)
(S) =>
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Properties of Enantiomers• Same boiling point, melting point, density• Same refractive index• Different direction of rotation in polarimeter• Different interaction with other chiral molecules
– Enzymes– Taste buds, scent
=>
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Optical Activity• Rotation of plane-polarized light• Enantiomers rotate light in opposite directions, but
same number of degrees.
=>
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Polarimetry• Use monochromatic light, usually sodium D
• Movable polarizing filter to measure angle
• Clockwise = dextrorotatory = d or (+)
• Counterclockwise = levorotatory = l or (-)
• Not related to (R) and (S)
=>
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Biological Discrimination
=>
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Racemic Mixtures
• Equal quantities of d- and l- enantiomers.
• Notation: (d,l) or ()
• No optical activity.
• The mixture may have different b.p. and m.p. from the enantiomers!
=>
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Racemic Products
If optically inactive reagents combine to form a chiral molecule, a racemic mixture of enantiomers is formed.
=>
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Chirality of Conformers
• If equilibrium exists between two chiral conformers, molecule is not chiral.
• Judge chirality by looking at the most symmetrical conformer.
• Cyclohexane can be considered to be planar, on average.
=>
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Mobile Conformers
H
BrH
Br
H
BrH
Br
Nonsuperimposable mirror images,but equal energy and interconvertible.
BrBr
H H
Use planarapproximation.
=>
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Nonmobile ConformersIf the conformer is sterically hindered, it may
exist as enantiomers.
=>
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Allenes• Chiral compounds with no chiral carbon
• Contains sp hybridized carbon with adjacent double bonds: -C=C=C-
• End carbons must have different groups.
Allene is achiral.=>
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Fischer Projections• Flat drawing that represents a 3D molecule
• A chiral carbon is at the intersection of horizontal and vertical lines.
• Horizontal lines are forward, out-of-plane.
• Vertical lines are behind the plane.
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Fischer Rules
• Carbon chain is on the vertical line.
• Highest oxidized carbon at top.
• Rotation of 180 in plane doesn’t change molecule.
• Do not rotate 90!• Do not turn over out of plane! =>
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Fischer Mirror Images
• Easy to draw, easy to find enantiomers, easy to find internal mirror planes.
• Examples:CH3
H Cl
Cl H
CH3
CH3
Cl H
H Cl
CH3
CH3
H Cl
H Cl
CH3=>
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Fischer (R) and (S)• Lowest priority (usually H) comes forward, so assignment
rules are backwards!
• Clockwise 1-2-3 is (S) and counterclockwise 1-2-3 is (R).
• Example:
CH3
H Cl
Cl H
CH3
(S)
(S) =>
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Diastereomers
• Stereoisomers that are not mirror images.
• Geometric isomers (cis-trans)
• Molecules with 2 or more chiral carbons. =>
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Alkenes
Cis-trans isomers are not mirror images, so these are diastereomers.
C CH H
CH3H3C
cis-2-butene trans-2-butene
C CH
H3C
CH3
H =>
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Ring Compounds
• Cis-trans isomers possible.
• May also have enantiomers.
• Example: trans-1,3-dimethylcylohexane
CH3
H
H
CH3
CH3
H
H
CH3
=>
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Two or More Chiral Carbons• Enantiomer? Diastereomer? Meso? Assign (R) or (S) to
each chiral carbon.
• Enantiomers have opposite configurations at each corresponding chiral carbon.
• Diastereomers have some matching, some opposite configurations.
• Meso compounds have internal mirror plane.
• Maximum number is 2n, where n = the number of chiral carbons. =>
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ExamplesCOOH
H OH
HO H
COOH
(2R,3R)-tartaric acid
COOH
COOH
HO H
H OH
(2S,3S)-tartaric acid
=> (2R,3S)-tartaric acid
COOH
COOH
H OH
H OH
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Fischer-Rosanoff Convention• Before 1951, only relative configurations could be known.
• Sugars and amino acids with same relative configuration as (+)-glyceraldehyde were assigned D and same as (-)-glyceraldehyde were assigned L.
• With X-ray crystallography, now know absolute configurations: D is (R) and L is (S).
• No relationship to dextro- or levorotatory. =>
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D and L Assignments
CHO
H OH
CH2OH
D-(+)-glyceraldehyde
*CHO
H OH
HO H
H OH
H OH
CH2OHD-(+)-glucose
*
COOH
H2N H
CH2CH2COOH
L-(+)-glutamic acid
*=>
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Properties of Diastereomers
• Diastereomers have different physical properties: m.p., b.p.
• They can be separated easily.• Enantiomers differ only in reaction with
other chiral molecules and the direction in which polarized light is rotated.
• Enantiomers are difficult to separate. =>
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Resolution of Enantiomers
React a racemic mixture with a chiral compound to form diastereomers, which can be separated.
=>
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ChromatographicResolution of Enantiomers
=>
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End of Chapter 5
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