5 Stereochemistry

Organic Chemistry: Structure and Function

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1 Stereochemistry Part 1 Lecture supplement: take one handout from the stage
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2 Stereochemistry: What is It? Isomers Molecules with same chemical formula but different spatial arrangement of atoms Constitutional isomers Differ in sequence of atom connectivity Urea CH 4 N 2 O H 2 N NH 2 O Ammonium cyanate CH 4 N 2 O N H H H H O C N Jöns Jakob Berzelius, 1830
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3 Stereochemistry: What is It? Isomers Same sequence of connectivity, but can be interconverted by rotation around a single bond rotate around C2-C3 bond Butane C 4 H 10 Eclipsed conformation Conformational isomers Are other isomers possible? Tetrahedral carbon... click on movie to play Butane C 4 H 10 Staggered conformation
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4 Historical Background Timeline A: Light 1678: Christiaan Huygens discovers plane-polarized light many vibrational planes nonpolarized light one vibrational plane plane-polarized light light beam Iceland spar crystal (natural CaCO 3 )
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5 Historical Background Timeline A: Light 1815: Jean Baptiste Biot notes some natural substances rotate plane-polarized light polarization plane shifted tube of liquid organic compound or solution plane-polarized light Optically active : the ability to rotate plane-polarized light Optically inactive : does not rotate plane-polarized light
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6 Historical Background Timeline A: Light Dextrorotatory : rotates plane-polarized light in a clockwise direction (+) Levorotatory : rotates plane-polarized light in a counterclockwise direction (-) Optical activity (-)-Nicotine N N H CH 3 (+)-Methamphetamine CH 3 N H 3 C H H
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7 Historical Background Timeline B: Tartaric Acid Tartaric acid Racemic acid OH OH HO HO O O 1828: Joseph Louis Gay-Lussac shows tartaric acid and racemic acid are isomers 1819: Paul Kester isolates racemic acid from tartar From Latin racemus : bunch of grapes 1769: Carl Wilhelm Scheele examines tartar (deposited in casks during wine fermentation); isolates tartaric acid
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8 Historical Background Timeline B: Tartaric Acid 1838: Biot notes racemic acid is optically inactive 1832: Jean Baptiste-Biot notes tartaric acid is optically active 1847: Louis Pasteur separates ammonium sodium salt of racemic acid into (+) and (-) crystals
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9 Historical Background Timeline B: Tartaric Acid O Na O NH 4 HO HO O O Ammonium sodium racemate optically inactive (+)-tartaric acid optically active identical to Scheele’s tartaric acid (-)-tartaric acid optically active Pasteur’s separation of racemic acid Quantity: equal Optical activity: equal but opposite Conclusion: Racemic acid is a 1:1 mixture of two optically-active substances separate crystals
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10 Historical Background Timeline B: Tartaric Acid 1853: Pasteur investigates mesotartaric acid Mesotartaric acid OH OH HO HO O O Artificial, optically-inactive isomer of tartaric acid Cannot be separated into (+) and (-) forms Tartaric acid isomers have different biological properties 1854: Pasteur notes a certain plant mold metabolizes (+) but not (-)-tartaric acid
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11 Historical Background Timeline C: Tetrahedral Carbon A molecule having a tetrahedral carbon atom with four unequal attachments exists as a pair of isomers.
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  • Enantiomer, Tartaric acid, stereoisomers

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