ZLM-chapter 5-stereochemistry

ZLM-chapter 5-stereochemistry - ISOMERS (same no. and kinds...

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LMZ/CHEM109A/ Winter 2010 Chapter 5 Stereochemistry I. Chiral Compounds and Enantiomers (Sec. 5.2-5.3) 1. Molecules that are not superimposable on their mirror images and thus exist in two enantiomeric forms ( enantiomers ) are said to be chiral . A B C D A B C D mirror images 2. A molecule is chiral when there is no plane of symmetry in the molecule (i.e. no imaginary plane that can cut through the molecule so that one half of molecule is not the exact mirror image of the other half--- note: the molecule can rotate ) 3. A carbon atom bonded to four different groups is called a chiral (asymmetric) center . (NOTE: Molecules that possess chiral carbons are usually chiral. Exception: meso- compound or a compound that have a plane of symmetry) Which of the following compounds are chiral? Identify the meso compound(s). CH 3 OH HOOC H CH 3 H HOOC H CH 3 H CH 3 H O Br Br Cl Cl H H e.g. 1 e.g. 2 e.g. 3 e.g. 4 e.g . 5 4. Fisher Projections (Sec 5.5) A tetrahedral carbon atom is represented in Fischer projection by two crossed lines. By convention, the horizontal lines represent bonds coming out of the page, and the vertical lines represent bonds going into the page. 5. Asymmetric center and stereocenters (stereogenic centers) - Asymmetric center: chiral carbon center - stereogenic center: stereoisomers formed when two substituents interchanged. II. Sequence Rules (Cahn-Ingold-Prelog Rules) for Specification of Configuration ( Sec. 5.7) - a verbal method for indicating the 3-D arrangement of atoms (the configuration) at the stereogenic (chiral) center 1 . Look at the four atoms directly attached to the chiral center, - assign priorities in order of decreasing atomic number e.g.1 , I > Br > Cl > S > F > O > N > C > 3 H (T) > 2 H (D) > 1 H - the group with the highest atomic number is ranked first; the group with the lowest atomic number is ranked fourth Br Cl CH 3 H e.g. 2 Br H CH 2 CH 3 CH 3 e.g. 3 2 . If a decision about priority can't be reached by applying rule 1, compare atomic numbers of the second atoms in each substituent, - continuing on as necessary through the third or fourth atoms outward until the first point of difference is reached.
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ZLM-chapter 5-stereochemistry - ISOMERS (same no. and kinds...

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