definitions o stereochemistry refers to the 3-dimensional properties and reactions of molecules. o...
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Definitionso Stereochemistry refers to the 3-dimensional
properties and reactions of molecules.o It has its own language and terms that need to be
learned in order to fully communicate and understand the concepts.
o Stereoisomers are compounds with the same connectivity, different arrangement in
space.
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A Brief Review of Isomerism• The flowchart summarizes the types of isomers.
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Constitutional Isomers(Structural)
• Different order of connections gives different carbon backbone and/or different functional groups
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Stereoisomers(Configurational)
• Same connections, different spatial arrangement of atoms.– Enantiomers: (nonsuperimposable mirror
images)– Diastereomers: (all other stereoisomers)
• Includes cis, trans isomers.
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Constitutional Isomers vs Stereoisomers
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Enantiomers – Mirror Images• Molecules exist as three-dimensional objects.• Some molecules are the same as their mirror
image.• Some molecules are different than their mirror
image. – These are stereoisomers called enantiomers.
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Enantiomers and the Tetrahedral Carbon
• Enantiomers are molecules that are not the same as their mirror image.
• This is illustrated by enantiomers of lactic acid.
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Mirror-image Forms of Lactic Acid
• When H and OH substituents match up, COOH and CH3 don’t
• when COOH and CH3 coincide, H and OH don’t .
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Examples of Enantiomers
• Molecules that have one carbon with 4 different substituents have a nonsuperimposable mirror image – enantiomer.
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C CH H
CH3H3C
cis-2-butene trans-2-butene
C CH
H3C
CH3
H
Diastereomers• Stereoisomers that are not mirror images.
• Geometric isomers (cis-trans).
• The cis isomer has two groups on the same side.
• The trans isomer has two groups on opposite sides.
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StereoisomersEnantiomers …. pair of stereoisomers that are related
to each other as non-super-imposable mirror image isomers
Meso compounds … stereoisomers that have more than one chiral center and are super-imposable on their mirror images
Diastereomers …. pair of stereoisomers containing more than one chiral center and are not mirror images of each other
CH3
C2H5
Cl
H
H
Cl
CH3
C2H5
H
Cl
Cl
H
CH3
C2H5
Cl
Cl
H
H
CH3
C2H5
H
H
Cl
Cl
I II III IV
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Chirality• Chiral: from the Greek, cheir, (hand).• If an object has a plane of symmetry it is
necessarily the same as its mirror image• The lack of a plane of symmetry is called
“handedness”, chirality.• Hands, gloves are prime examples of chiral object.
– They have a “left” and a “right” version.
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Chirality Centers• A point in a molecule where four different
groups (or atoms) are attached to carbon is called a chirality center.
• There are two nonsuperimposable ways that 4 different different groups (or atoms) can be attached to one carbon atom.– If two groups are the same, then there is
only one way.• A chiral molecule usually has at least one
chirality center.
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• For a molecule with 1 chiral center, 21 = 2 stereoisomers are possible.
• For a molecule with 2 chiral centers, a maximum of 22 = 4 stereoisomers are possible.
• For a molecule with n chiral centers, a maximum of 2n stereoisomers are possible.
• Larger organic molecules can have two, three or even hundreds of stereogenic centers.
Chirality Centers
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2,3,4-trichlorohexaneHow many stereoisomers?
Cl
Cl
Cl3 asymmetric centers
8 stereoisomers
* **
2n, n= # asymmetric centers (3)
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Two Chiral Centers• Molecules with more
than one chirality center have mirror image stereoisomers that are enantiomers.
• In addition they can have stereoisomeric forms that are not mirror images, called diastereomers.
2R,3S 2S,3R
2R,3R 2S,3S
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• A meso compound is an achiral compound that contains tetrahedral stereogenic centers. All meso compounds contain a plane of symmetry.
• Compound C has two stereogenic centers but it contains a plane of symmetry, and is achiral; C is a meso compound.
Meso Compounds
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• Since enantiomers are two different compounds, they need to be distinguished by name. This is done by adding the prefix R or S to the IUPAC name of the enantiomer.
• Naming enantiomers with the prefixes R or S is called the Cahn-Ingold-Prelog system.
Labeling Stereogenic Centers with R or S
* To designate enantiomers as R or S, priorities must be assigned to each group bonded to the stereogenic center, in order of decreasing atomic number. The atom of highest atomic number gets the highest priority (1).
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R-Configuration at Chirality Center
• Lowest priority group is pointed away and direction of higher 3 is clockwise, or right turn.
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• In general, a molecule with no stereogenic centers will not be chiral. There are exceptions to this.
• With one stereogenic center, a molecule will always be chiral.
• With two or more stereogenic centers, a molecule may or may not be chiral, e.g. Meso compound (contains a plane of symmetry or a mirror plane)
• Achiral molecules contain a plane of symmetry but chiral molecules do not.
• A plane of symmetry is a mirror plane that cuts the molecule in half, so that one half of the molecule is a reflection of the other half.
Chiral vs Achiral
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Optical Activity• Light restricted to pass through a plane is
plane-polarized .• Plane-polarized light that passes through
solutions of achiral compounds remains in that plane.
• Solutions of chiral compounds rotate plane-polarized light and the molecules are said to be optically active.
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Plane-Polarized Light through an Achiral Compound
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Plane-Polarized Light through a Chiral Compound
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Optical Activity
• Light passes through a plane polarizer.
• Plane polarized light is rotated in solutions of optically active compounds.
• Measured with polarimeter.
• Rotation, in degrees, is [].
• Clockwise rotation is called dextrorotatory.
• Anti-clockwise is levorotatory.
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• The rotation of polarized light can be clockwise or anticlockwise.
• If the rotation is clockwise, the compound is called dextrorotatory. The rotation is labeled d or (+).
• If the rotation is counterclockwise, the compound is called levorotatory. The rotation is labeled l or (-).
• Two enantiomers rotate plane-polarized light to an equal extent but in opposite directions. Thus, if enantiomer A rotates polarized light +50, the same concentration of enantiomer B rotates it –50.
• No relationship exists between R and S prefixes and the (+) and (-) designations that indicate optical rotation.
Optical Activity
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Specific Rotations of some Common Organic Compounds
Compound [] # * centers
Penicillin V +233.0 3
Sucrose +66.5 10
Camphor +44.3 2
MSG +25.5 1
Cholesterol -31.3 8
Morphine -132.0 5
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Racemic Mixture
• An equimolar mixture of two enantiomers
– because a racemic mixture contains equal numbers of dextrorotatory and levorotatory molecules, its specific rotation is zero.
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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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Chirality in the Biological World
• Except for inorganic salts and a few low-molecular-weight organic substances, the molecules of living systems are chiral.
• Although these molecules can exist as a number of stereoisomers, generally only one is produced and used in a given biological system.
• Enzymes are Chiral and enhance Chirality:– chymotrypsin contains 251 chiral centers.– the maximum number of stereoisomers possible is
2251 .
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A schematic diagram of an enzyme surface capable of binding with (R)-enantiomer but not with (S)-enantiomer.
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* Chiral centers are extremely important in biological processes and drug development.* Properties of drugs depend on stereochemistry.
Biological Activity
CH3
CH3
CH3
O
HOCH3
CH3
CH3
O
HO
(R)-ibuprofen(inactive)
(S)-ibuprofen(active)
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Stereochemistry of Reactions:Addition of HBr to Alkenes
• Many reactions can produce new chirality centers from compounds without them.
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يــأيــها النبي على يصلــون ومالئكتــه الله إنتسليمـــا وسلمـــوا عليه صلــوا آمنوا الذيـن
يــأيــها النبي على يصلــون ومالئكتــه الله إنتسليمـــا وسلمـــوا عليه صلــوا آمنوا الذيـن