Chemistry%20106

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Chemistry 106

• You need
• Text Lab Book ( all lab safety stuff)
• Online Notes access
• http//learn.roguecc.edu/science/ploozen/

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The 3-D Structure of Molecules -Stereochemistry-
For additional help with stereochemistry, check
out the Organic Chemistry section of the
following website http//www.khanacademy.org/b
rowse
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Isomerism

• Consitutional isomers
• Atoms are bonded to different atoms, therefore,
  different bonding patterns - different IUPAC
  names structural formulas
• Structural
• Positional
• Functional group

• Stereoisomers
• Atoms are oriented differently, but same bonding
  patterns
• Cis-Trans - orientation around a double bond
• Tetrahedral C atom mirror images
• Chiral vs. Achiral

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Chirality
The mirror image of the right hand is the left
hand. Conversely, the mirror image of the left
hand is the right hand.

• Chirality is based on whether a molecule has a
  chiral center.
• Carbon atom with FOUR DIFFERENT attached
  (tetrahedrally) groups (the atom attached atoms
  which are attached to the center carbon)
• Chiral molecules mirror images that are NOT
  superimposable
• (left vs. right handedness)
• Achiral molecules mirror images that are
  superimposable
• (no left vs. right handedness)
• Important Biochemical example
• Left or right handedness of monosaccharides is
  determined by the position of -OH on the chiral
  center.
• Naturally occurring monosaccharides are almost
  always right-handed.
• Plants produce only right-handed monosaccharides

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Chirality
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Chirality
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Stereoisomerism - molecules that have the same
molecular AND structural formulas but different
orientation of atoms

• In order for molecules to exhibit stereoisomerism
  they must have
• A chiral center
• Structural rigidity
• This is the basis for cis-trans isomerism
• Two types of Stereoisomers
• Enantiomers - molecules that are
  nonsuperimposable mirror images
• Ex. Left right handed with single chiral
  centers
• Diastereomers - molecules that are not mirror
  images
• Ex. Cis-trans (possible in some rings and around
  double bonds)

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• Enantiomers Diastereomers

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Thalidomide - introduced as a sedative and
antiemetic in later 1950s and withdrawn in 1961
due to its teratogenicity. an example of a
chiral center in a cyclic compound
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Fischer Projections

• 2- dimensional structural notation showing the
  spatial arrangement of groups around chiral
  centers (to show handedness)
• Tetrahedral geometry
• Vertical lines bonds directed into the page
• Horizontal lines bonds directed out of the page

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Fischer Projections

• Carbon chain is positioned vertically, with the
  carbonyl group at or near the top.
• Ex. glyceraldehyde (2,3-dihydroxypropanal)
• Latin
• Dextro Right
• Levo Left
• Determine D vs L by examining the
• position of the functional group on the
• chiral center

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Compounds with multiple chiral centers

• Naming is complex
• Use the highest chiral C atom in the chain to
  determine D or L.
• If there are 2 or more Ds and 2 or more Ls,
  use different common names for each pair.
• Ex. 2,3,4-trihydroxybutanal

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Number of Stereoisomers possible for a particular
molecule

• General rule
• of isomers 2n (n of chiral centers)
• Sometimes symmetry considerations make some
  mirror images superimposable.

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Properties of Isomers

• Constitutional - differ in most physical and
  chemical properties
• Diastereomers - differ in most physical and
  chemical properties
• Enantiomers - differ in only two properties
• Interactions with plane-polarized light (ppl)
• Interactions with other chiral substances

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Dextrorotatory Levorotatory Compounds

• An enantiomer (chiral cpd) that rotates ppl in
  a clockwise direction is dextrorotatory. ()
• An enantiomer (chiral cpd) that rotates ppl in
  a counterclockwise direction is levorotatory. (-)
• The handedness of enantiomers and the direction
  of rotation are, unfortunately, not related.

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Interactions Between Chiral Compounds

• Enantiomers have the same FP, BP, density, etc.
• Properties depend on IMF
• IMF does not depend on Chirality
• IMF depends on functional groups
• Enantiomers have the same solubility in achiral
  solvents (ethanol), but different solubility in
  chiral solvent (D-2-butanol).
• Rate Extent of Reaction of Enantiomer is the
  same with an achiral reactant but different with
  another chiral reactant.
• Receptor sites for molecules in the body have
  chirality, so enantiomers generate different
  responses.
• Enantiomers react differently to taste buds
• spearmint vs.carroway
• Thalidomide
• beneficial vs. teratogenic

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• D-Epinephrine binds to the receptor at three
  points.
• The human body exhibits a response to the D form
  that is 20 times greater than the response to the
  L form.