Ch9slides - Chapter 9 Stereochemistry three-dimensional...

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1 Chapter 9: Stereochemistry three-dimensional arrangement of atoms (groups) in space Stereoisomers : molecules with the same connectivity but different arrangement of atoms (groups) in space H H 3 C H CH 3 H H 3 C CH 3 H cis-1,2-dimethylcyclopropane trans-1,2-dimethylcyclopropane H 3 C H H CH 3 H 3 C H CH 3 H cis-2-butene trans-2-butene geometric isomers (diastereomers)
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2 Enantiomers: non-superimposable mirror image isomers. Enantiomers are related to each other much like a right hand is related to a left hand Enantiomers have identical physical properties, i.e., bp, mp, etc. Chirality (from the Greek word for hand). Enantiomers are said to be chiral . Molecules are not chiral if they contain a plane of symmetry: a plane that cuts a molecule in half so that one half is the mirror image of the other half. Molecules (or objects) that possess a mirror plane of symmetry are superimposable on their mirror image and are termed achiral . A carbon with four different groups results in a chiral molecule and is refered to as a chiral or asymmetric or stereogenic center.
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3 Achiral Chiral Chiral center (stereogenic, asymmetric) C C C H O H H H H H O O symmetry plane C C C H O H H H H OH O O CH 3 H Not a symmetry plane Optical Rotation: molecules enriched in an enantiomer will rotate plane polarized light are are said to be optiically active. The optical rotation is dependent upon the substance, the concentration, the path length through the sample and the wavelength of light. Polarimeter Plane polarized light: light that oscillates in only one plane
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4 α : angle (# of degrees) plane polarized light is rotated by an optically active sample. Expressed in degrees. Enantiomers will rotate plane polarized light the same magnitude ( α ) but in opposite directions (+ or -) 90% (+) + 10% (-) will rotate light 80% of pure (+) 75% (+) + 25% (-) will rotate light 50% of pure (+) 50% (+) + 50% (-) will be optically inactive 50:50 mixture of enantiomers (+/-): racemate or racemic mixture Each individual molecule is chiral, however the bulk property of the substance is achiral, if it is in an achiral environment. 0 ° + α 0 ° - α dextrorotatory ( d ): rotates light to the right (clockwise) levororotatory ( l ): rotates light to the left (counterclockwise) CH 3 C H HO HO 2 C CH 3 C H HO 2 C HO Specific Rotation [ α ] D : a standardized value for the optical rotation [ α ] λ = T α l • c α = optical rotation in degrees l = path length in dm c = concentration of sample in g/mL T = temperature in °C λ = wavelength of light, usually D for the D-line of a sodium lamp (589 nm) [ α ] D = +14.5° ( c 10, 6N HCl) 20 for alanine: The specific rotation is a physical constant of a chiral molecule The [ α ] D may also depend upon solvent, therefore the solvent is usually specified.
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5 Discovery of chirality in chemistry L. Pasteur (1949) Crystallization of sodium ammonium tartrate. Crystals were mirror Images of each other and separated by tweezers under a microscope.
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This note was uploaded on 03/11/2012 for the course CHEM 220A taught by Professor Sulikowski during the Fall '08 term at Vanderbilt.

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Ch9slides - Chapter 9 Stereochemistry three-dimensional...

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