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Unformatted text preview: Note: Place used disposable pipettes in the "broken glass" container only, never in the trash. Likewise, DO NOT PLACE PAPER TRASH IN THE "BROKEN GLASS" CONTAINER! Experiment 4: A Problem in Diastereoselectivity You must read pages 743-746 and 753-760 in your Organic Chemistry text by P. Y. Bruice, which describes sodium borohydride reduction and acetal formation. You should be familiar with Chapter 5 on stereochemistry. The purpose of this experiment is to determine the stereochemical outcome of a reaction. When a compound contains a chiral center and one performs a reaction to introduce a second chiral center, two diastereomeric compounds may be formed (see your organic text for a discussion on diastereomers). Although two compounds may result from such a reaction, only one of the two may be formed due to various factors, which influence the "diastereoselectivity" of the reaction. In this experiment you will begin with a racemic mixture of the compound benzoin ( 2 ), which is produced by the thiamine-catalyzed benzoin condensation of benzaldehyde. Benzoin contains one chiral carbon. Reduction of benzoin by NaBH 4 introduces a second chiral center such that two diastereomers 3a and 4a are possible. However, only one diastereomer is produced. The goal is to carry out the sodium borohydride reduction of 2 and the ensuing reaction with acetone to form the acetonide (either 3b or 4b ) and then determine whether 3b or 4b is formed. This is best done by analyzing the compound's 1 H and 13 C NMR data, which we will provide for you. Note : The experiment will be divided into two weeks. Part A will be done the first week. CHO OH O N S N N NH 3 + Cl- HO thiamine hydrochloride (catalyst) 3 M NaOH EtOH/H 2 O 2 benzaldehyde 1 benzoin 2 Note: Place used disposable pipettes in the "broken glass" container only, never in the trash....
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- Fall '10
- Organic chemistry