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,
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,
Aldehydes and ketones react with alcohols reversibly in the presence of acid catalysts to
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