Presentation on theme: "STEREOCHEMISTRY Dr. Clower CHEM 2411 Spring 2014 McMurry (8 th ed.) sections 5.1-5.12, 7.5."— Presentation transcript:
STEREOCHEMISTRY Dr. Clower CHEM 2411 Spring 2014 McMurry (8 th ed.) sections 5.1-5.12, 7.5
Stereochemistry Branch of chemistry concerned with the 3D arrangement of atoms in molecules Stereoisomers: Same molecular formula Same connectivity Different 3D orientation (cannot be converted via bond rotation) Previously: Cis and trans Now: Stereochemistry at tetrahedral centers Enantiomers, diastereomers E and Z
Chirality “handedness” Has a mirror image that is nonsuperimposable Example: hand
Chirality A molecular example: Try this with your model kit
Enantiomers Chiral molecules form enantiomers Nonsuperimposable mirror images Result from tetrahedral C (sp 3 ) with 4 different substituents This C is called a chirality center (or stereocenter, or asymmetric center) and are often marked with *
Achiral molecules Are superimposable on their mirror images Contain a plane of symmetry Cuts through the middle of the molecule so that one half reflects the other half achiralchiral
MoleculeStereocenter? Plane of Symmetry? Chrial?
MoleculeStereocenter? Plane of Symmetry? Chrial? Not all molecules with stereocenters are chiral!
Enantiomer Similarities and Differences Same molecular formula, connectivity Different 3D arrangement Same physical properties (mp, bp, solubility) Same spectroscopic properties (IR, NMR, etc.) Same reactivity, in general Products will have different stereochemistry Only one will react with an enzyme (like a hand fitting in a glove) Different designations (R vs. S) Different optical activity
Optical Activity Rotation of plane-polarized light; seen in chiral molecules = degree of rotation; measured by the polarimeter One enantiomer rotates light to the left degrees Levorotatory (-) The other rotates light to the right degrees Dextrorotatory (+)
Optical Rotation Depends on polarimeter pathlength (l) and sample concentration (c) Specific rotation [ ] D is observed under standard conditions = 589.6 nm l = 1 dm (10 cm) c = 1 g/cm 3 (-)-Lactic acid has a [ ] D of -3.82 (+)-Lactic acid has a [ ] D of +3.82 What is [ ] D of a 50:50 mixture of (-) and (+)-lactic acid?
R and S designations Used to describe 3D configuration about a chirality center Not related to direction of optical rotation (+) and (-) To designate R and S need to assign priorities to each group bonded to the stereocenter Cahn-Ingold-Prelog convention
Priority Rules 1. Higher atomic number (of atom bonded to C*) = higher priority -Br > -Cl > -OH > -NH 2 > -CH 3 > -H 2. If 2 of the same atom are bonded to C*, look at atomic number of the next set of atoms
Continue process until first point of difference Some more examples:
Priority Rules 3. Atoms in double bonds count twice; atoms in triple bonds count three times
Which substituent has the higher priority? a) -Br -Cl b) -CH 2 CH 3 -CH(CH 3 ) 2 c) -CH=CH 2 -CH 2 CH 3 d) -CHO -CO 2 H e) -CH 2 OH -CH 2 CH 2 OH
To designate R or S: 1. Locate chirality center 2. Assign priority to the 4 groups (1 = highest; 4 = lowest) 3. Orient molecule so substituent 4 is point away from you (with model or on paper) 4. Read the other groups 1→2→3 (draw arrow on paper) 5. Groups read clockwise = R; counterclockwise = S
Rotating a Tetrahedral Carbon To rotate a carbon and not accidentally change the R/S designation, keep one substituent in the same place, and rotate the other three. Make sure all three groups are rotating in the same direction Do not switch two groups; this changes the R/S designation
Determine whether the two structures in each pair represent constitutional isomers, enantiomers, or identical compounds. a) b)
Fischer Projections Another way of drawing tetrahedral carbons Horizontal lines = out of page Vertical lines = into page Frequently used for chirality centers, especially if a molecule has more than one chiral center
What is the relationship between these two molecules?
Molecules With Multiple Stereocenters Maximum # stereoisomers = 2 n where n = # stereocenters # Stereocenters# StereoisomersStereoisomers 12 RSRS 24 (R,R) (S,S) (R,S) (S,R)
Example: 2,3-Pentanediol Draw Fischer projections for the 4 stereoisomers Carbon chain vertical, C1 at top
Relationships A and B are enantiomers C and D are enantiomers A and C, A and D, B and C, B and D are diastereomers
Diastereomers Stereoisomers that are not mirror images of each other Different physical properties With tetrahedral carbons, require at least 2 stereocenters Cis-trans stereoisomers are also diastereomers
Meso Compounds Maximum # stereoisomers = 2 n where n = # stereocenters The # stereoisomers will be less than 2 n when there is a meso compound Meso compound An achiral compound which contains chirality centers Not optically active The chirality centers typically are identical (have the same 4 substituents) and reflect each other in a plane of symmetry Example:
Another Example: 2,3-Butanediol A = (2R,3R)-2,3-butanediol B = (2S,3S)-2,3-butanediol C = D = meso-2,3-butanediol C and D are superimposable mirror images (the same molecule) Relationship between enantiomers and meso? Diastereomers
Racemic Mixtures aka Racemate, + pair, or d,l pair 50% mixture of two enantiomers Not optically active Separation of enantiomers is difficult React with chiral compound to convert to a pair of diastereomers which can be separated by distillation, recrystallization, etc. Separate on chiral column Separate with enzyme
Applications of Stereochemistry 1. Stereochemistry of reactions If a product has a stereocenter, is the stereochemistry all R, all S, or a mixture? To understand details, need to look at mechanism (next)
Applications of Stereochemistry 2. Reactions with enzymes Receptors/enzymes react with only one enantiomer (like a handshake) Limonene R = orange odor S = pine odor Ibuprofen R = inactive S = active -Decalactone R = porcupine emits to alert predators S = coconut
Thalidomide How many chirality centers? How many stereoisomers? How was the drug administered? What effect did this have on patients who used thalidomide? Francisco Goya
Alkene Stereochemistry Previously, cis-trans stereoisomers Now, E,Z-designation of alkenes Use E,Z instead of cis-trans when More than two substituents on C=C Heteroatoms on C=C To assign E or Z: Rank the two groups on each carbon of the C=C according to the Cahn-Ingold-Prelog priority rules If the higher priority groups are on the same side of the C=C, the alkene has Z geometry If the higher priority groups are on opposite sides of the C=C, the alkene has E geometry