Lecture and Chapter 5 Notes

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University of Toronto St. George
Thavarajah/ Woolley/ Nitz

CHM138H1 Jasmyn Lee Chapter 5: Stereochemistry at Tetrahedral Centers 5.1 Enantiomers and the Tetrahedral Carbon 5.2 The Reason for Handedness in Molecules: Chirality Chirality  Chiral – objects that do not have a plane of symmetry; not identical too its mirror image  Achiral – objects that contain a plane of symmetry Properties of Chiral Objects  A chiral object and its mirror image cannot be superimposed  Chirality Center – tetrahedral carbon, bonded to four different groups o Stereocenter, stereogenic center, asymmetric center  Enantiomers (also called optic isomers) – molecules that are mirror images and non-superimposable o Occur when carbon is attached to 4 different groups o Have identical physical properties (melting point, boiling point, IR, NMR…) - unless enantiomers are interacting with another chiral molecule or environment  Biological molecules often have different tastes, smells, toxicities and optical activity o Differ in direction in which their solutions rotate plane-polarized light 5.3 Optical Activity Optical Activity  Jean-Baptiste Biot  A beam of ordinary light consists of electromagnetic waves that oscillate in an infinite number of planes at right angles to the direction of light travel  Polarizer – device that only allows light in a single plane to pas through; all other planes are blocked out o Light is said to be “polarized”  Optically Active Compounds – when light is passed through, the plane of polarization is rotated through an angle  Optically active compounds must be chiral  The specific rotation of the enantiomer is equal in magnitude but opposite in sign  (+) Enantiomer – rotates light to the right  dextrorotatory  (-) Enantiomer – rotates light to the left  levorotatory 1 CHM138H1 Jasmyn Lee  Chirality is important for binding to biological receptors  Eg/ S-ibuprofen bound to prostaglandin H2 synthetase-1 (COX-1) Indicating Chirality  (+)–alanine and (-)–alanine o Refers to the measured optical activity  D-alanine and L-alanine o Historical: refers to the Fischer projection  R – alanine and S – alanine o Cahn-Ingold-Prelog Rules o R and S refer to the configuration around the chirality center o R = Rectus (Right) o S = Sinister (Left) 5.5 Sequence Rules for Specifying Configuration 1. Find the chirality center you wish to define 2. Define the priorities of the groups attached to the chirality center (Cahn-Ingold-Prelog Rules) 3. Rotate molecule to position lowest priority group to the back 4. Connect priorities with arrows o R (rectus) configuration – arrows go clockwise o S (sinister) configuration – arrows go counter clockwise 2 CHM138H1 Jasmyn Lee Cahn-Ingold-Prelog Rules 1. Rank atoms attached to the double bond in decreasing atomic number o Br > Cl > F > O > N > C … > H o Highest atomic number (first) to lowest atomic number (fourth) o Heavier isotopes first 2. If the first atoms in the substituent are the same, look at the 2 , the 3 , the 4 , … 3. Multiple-bonded atoms are equivalent to the same number of single bonded atoms 56 Diasteromers  More than one chirality center o A molecule with n chirality centers can have up to 2 stereoisomers  Diastereomers – stereoisomers that are not mirror images o Have different chemical properties (Boiling point, solubility, melting point, spectra, etc)  Enantiomer vs. Diastereomer o Enantiomers – opposite configurations at all chirality centers o Diastereomers – opposite configurations at some (one or more)
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