Experiment 6-7 - Oxidation, diketone, and cyclic acteal Lab Report

Experiment 6-7 - Oxidation, diketone, and cyclic acteal Lab Report

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Experiment 6 and 7 A. Oxidation of a Primary Alcohol B. Stereoselective Reduction of a Diketone with Sodium Borohydride C. Preparation of a Cyclic Acetal Preparation of a Cyclic Acetal Lut Ming Cheng, 4957880, CHM2123, Section C TA: Wendy Campbell Due Date: November 10, 2008 1 – Theory and Mechanism Oxidation of alcohol is important in the fields of chemistry and biochemistry. Primary alcohols are oxidized to aldehydes and secondary alcohols are oxidized to ketones by using a variety of reagents. Moreover, tertiary alcohols require a different oxidation mechanism to convert the alcohol group to ketone or aldehyde function group. Aldehydes can also be easily oxidized to carboxylic acids in aqueous solution. In the first part of the experiment, oxidation of 4-nitrobenzyl alcohol is carried out with sodium dichromate and sulphuric acid. The brief reaction is shown as below, NO 2 OH [O] NO 2 O H NO 2 O OH + p-nitrobenzyl alcohol p-nitrobenzaldehyde p-nitrobenzoic acid Other than oxidation, there is reduction of ketones and aldehydes. There is a variety of reagents that chemists use to carry out the reduction reaction. However, the use of metal hydrides is preferred due to a lower cost. These hydrides include sodium borohydride and lithium aluminum hydride. Sodium borohydride is more selective than lithium aluminum hydride. Sodium borohydride only reduces aldehydes and ketones to primary and secondary alcohols respectively. On the other hand, lithium aluminum bromide is a more powerful reducing agent. It reduces aldehydes, ketones, esters, carboxylic acids, amides, nitriles and nitro groups. However, it can potentially cause fire when there is a presence of water. In the second part of the experiment, reduction of a diketone is carried out. This reduction reaction is stereoselective. The first chiral center formed directs the reduction of the second carbonyl group. Meso and racemic products are formed at the end of the reaction. Meso diastereomer is the favoered product. The brief reaction is as follow, Ph O O Ph 1. NaBH 4 , EtOH 2. H 3 O + Ph OH OH Ph + Ph OH OH Ph racemic meso Aldehydes and ketones react with alcohols reversibly in the presence of acid catalysts to give acetals. Acetal functions as protecting groups for alcohols, aldehydes or ketones. When functional groups are under protection, they are not reactive to reducing agents and to nucleophilic additions. When the desired reduction is completed, acetal can be converted back to its initial functional group by reacting with acid and excess amount of water. In the last part of the experiment, conversion from meso-1,2-diphenyl-1,2-ethanediol to cyclic ketal is carried out
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by using an excess amount of 2,2-dimethoxypropane. Ph Ph OH OH + C H 3 O C H 3 O CH 3 CH 3 C H 3 SO 3 H CH 2 Cl 2 , r.t.
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