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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

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Dr. Clower

CHEM 2411

Spring 2014

McMurry (8th ed.) sections 5.1-5.12, 7.5

  • 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
  • “handedness”
  • Has a mirror image that is nonsuperimposable
  • Example: hand
  • A molecular example:
  • Try this with your model kit
  • Chiral molecules form enantiomers
    • Nonsuperimposable mirror images
  • Result from tetrahedral C (sp3) with 4 different substituents
  • This C is called a chirality center (or stereocenter, or asymmetric center) and are often marked with *
achiral molecules
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



enantiomer similarities and differences
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
Optical Activity
  • Rotation of plane-polarized light; seen in chiral molecules
  • a = degree of rotation; measured by the polarimeter
  • One enantiomer rotates light to the left a degrees
    • Levorotatory (-)
  • The other rotates light to the right a degrees
    • Dextrorotatory (+)
optical rotation
Optical Rotation
  • Depends on polarimeterpathlength (l) and sample concentration (c)
  • Specific rotation [a]D is observed under standard conditions
    • l = 589.6 nm
    • l = 1 dm (10 cm)
    • c = 1 g/cm3
  • (-)-Lactic acid has a [a]D of -3.82
  • (+)-Lactic acid has a [a]D of +3.82
  • What is [a]D of a 50:50 mixture of (-) and (+)-lactic acid?
r and s designations
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
Priority Rules
  • Higher atomic number (of atom bonded to C*) = higher priority

-Br > -Cl > -OH > -NH2 > -CH3 > -H

  • If 2 of the same atom are bonded to C*, look at atomic number of the next set of atoms
priority rules1
Priority Rules
  • Atoms in double bonds count twice; atoms in triple bonds count three times
which substituent has the higher priority
Which substituent has the higher priority?
  • -Br -Cl
  • -CH2CH3 -CH(CH3)2
  • -CH=CH2 -CH2CH3
  • -CHO -CO2H
  • -CH2OH -CH2CH2OH
to designate r or s
To designate R or S:
  • Locate chirality center
  • Assign priority to the 4 groups (1 = highest; 4 = lowest)
  • Orient molecule so substituent 4 is point away from you (with model or on paper)
  • Read the other groups 1→2→3 (draw arrow on paper)
  • Groups read clockwise = R; counterclockwise = S
r and s stereoisomers for 3 methylhexane
R and S stereoisomers for 3-methylhexane:
  • Hints:
    • Switch any two groups to draw the enantiomer
    • When substituent 4 is forward, 1→2→3 clockwise is S
rotating a t etrahedral carbon
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.
fischer projections
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
molecules with multiple stereocenters
Molecules With Multiple Stereocenters
  • Maximum # stereoisomers = 2n where n = # stereocenters
example 2 3 pentanediol
Example: 2,3-Pentanediol
  • Draw Fischer projections for the 4 stereoisomers
  • Carbon chain vertical, C1 at top
  • A and B are enantiomers
  • C and D are enantiomers
  • A and C, A and D, B and C, B and D are 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
Meso Compounds
  • Maximum # stereoisomers = 2n where n = # stereocenters
  • The # stereoisomers will be less than 2n 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
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
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
Applications of Stereochemistry
  • 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 stereochemistry1
Applications of Stereochemistry
  • 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
  • D-Decalactone
    • R = porcupine emits to alert predators
    • S = coconut
  • 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
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 Egeometry
  • Organic reactions