Chapter 6 Stereochemistry

Chapter 6 Stereochemistry - Chapter 6 Pasteurs Discovery of...

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Chapter 6 Pasteur’s Discovery of Enantiomers 6.5 Pasteur’s Discover of Enantiomers 1. Louis Pasteur began work on a study of crystalline tartaric acid salts derived from wine 2. On recrystallizing a concentrated solution of sodium ammonium tartrate below 28 o C, Pasteur made surprising observation that two distinct kinds of crystals precipitated. Futhermore, the two kinds of crystals were mirror images of each other 3. The crystals were separated into right handed crystals and left handed crystals. 4. The original sample (50:50 mixture of left and right) was optically inactive. Solutions of right/left handed crystals only were optically active and their specific rotations were equal in amount but opposite in sign 5. 50:50 mixture of enantiomers is called a racemic mixture or racemate a. Denoted by ± which indicates that they contain equal amount of dextrorotatory and levorotatory enantiomers b. Is optically inactive because the (+) rotation from one enantiomer, exactly cancels the (-) rotation from the other c. These enatiomers have identical properties in the absence of chiral environment 6. Pasteur stated that there exists an asymmetric arrangement having non superimposible image and the atoms of the other enantiomer possesses precisely the inverse asymmetric arrangement 7. Enantiomers , also called optical isomers, have identical physical properties such as meting and boiling point but differ in the direction in which their solutions rotate plane polarized light 6.6 Sequence Rules for Specifying Configuration 1. method used to verbally specified 3-D arrangement or configuration of substituents around a stereocentre employs the same sequence rules for specifiying E and Z alkene stereochemistry 2. Sequence Rules for assigning priorities to substituents bonded to stereocentre a. RULE 1: Look at four atoms directly attached to the stereocentre and assign priorities in terms of atomic numbers. Atoms with highest atomic number is ranked as highest priority while atoms with lowest atomic number has lowest priority b. RULE 2 : If decision cant be reached by ranking the first atoms in the substituents, look at the second, third or fourth atoms outward until first difference is found c. RULE 3 : Multiple bonded atoms are equivalent to the same number of
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Chapter 6 Stereochemistry - Chapter 6 Pasteurs Discovery of...

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