A Multi-step Synthesis. Benzaldehyde to Benzoin. Benzoin to Hydrobenzoin and Benzil.Benzil to Tetraphenylcyclopentadienone.This experiment will be carried out over two lab periods. One report will be written forboth weeks. Include an "Experimental" section as in the Grignard write-up. Goodplanning and execution will be essential for completion of the experiment. IR's, TLC's,and MP's, and final recrystallizations should be done during lulls in the action. Much ofthis can be done during Week 2.Pre-Week 1. The Benzoin Condensation. Benzaldehyde to Benzoin.Benzaldehyde is allowed to react in the presence of sodium hydroxide and thiamine forone week, according to the procedure in Williamson, p. 616, 617. Work-up of thereaction is done at the beginning of the following lab period to yield benzoin which willbe used in subsequent reactions. A sample should be kept for recrystallization andanalysis by mp, ir and tlc. The reflux in the following oxidation must be started as soonas possible.Week 1. The Oxidation and Reduction of BenzoinBenzoin (I), having both a secondary alcohol and a ketone functional group can beoxidized to a diketone, benzil (II), or reduced to a diol, hydrobenzoin (III). In thisexperiment, the commonly used reducing agent, sodium borohydride, is used for thereduction. The oxidation can be accomplished with any of several oxidizing agents,including nitric acid. Nitric acid oxidation however produces toxic nitrogen dioxide gas.In the experiment described herein, a catalytic amount of copper (II) is used as theoxidizing agent, with ammonium nitrate used to regenerate the catalyst.OOHOHOHOOIIIIIICu2NH4NO3NaBH4Both the oxidation and reduction will be carried out on the same day. The oxidationrequires a 1.5 hr reflux period so that reaction should be set up first. While the oxidationis proceeding, the reduction will be carried out. It is important to plan well so thateverything can be done in one lab period.Oxidation of Benzoin. Because the procedure for the oxidation will be taken from anarticle in the library it is essential that you find the article and prepare the prelab outlinewell in advance of the lab. Find the journal article, "The Catalytic Oxidation of Benzointo Benzil", J. Am. Chem. Soc. 1948, 70, 3666 and, using the procedure in that reference(small run), plan a synthesis of benzil starting with 0.0025 mol of benzoin, scaling downthe quantities of all other reagents by the same factor. Note that reaction time remainsthe same. Heat carefully on the heating mantle, with swirling (not shaking) until thevigorous evolution of nitrogen subsides then adjust the heat for a gentle reflux. (On thesmall scale used here, nitrogen evolution may not be vigorous and in fact may not benoticed. Once the reactants are dissolved, assume nitrogen evolution is complete andproceed with the reflux.) While the 1.5 hour reflux is taking place, carry out the nextreaction, " Reduction of Benzoin".After the 1.5 hr reflux, modify the procedure for the work-up as follows: cool thereaction mixture until the flask is just warm to the touch then add it to about 2.5 mL ofice-water, with stirring. Cool the mixture in an ice bath for 5 - 10 min and collect theproduct by suction filtration. Temporarily break the vacuum by lifting the filter slightlyoff of the filter flask, cover the crystals with 2 mL of ice-cold rinse water, andimmediately reattach the vacuum to remove most of the water. Allow the product to dryat least overnight, weigh it, calculate a % yield, obtain an ir spectrum (mull) anddetermine the mp. Save the product for a subsequent reaction.Reduction of Benzoin. In a 50 mL Erlenmeyer flask, dissolve 1.0 g of benzoin in 10 mLof ethanol (all of the benzoin may not dissolve - this is ok). Carefully add, in smallportions, over 5 minutes, 0.20 g of sodium borohydride, with swirling. This reaction isexothermic and care must be taken to not add the borohydride too rapidly. After additionis complete, allow the reaction to proceed at room temperature for 20 min, with frequentswirling. While cooling in an ice bath, add 15 mL water, then 0.5 mL 6M HCl. Addanother 5 mL of water and allow the mixture to stand for 20 min with frequent swirling.Collect the product by suction filtration and rinse with 50 mL of water. Recrystallizeabout 50 mg of the solid by dissolving it in 0.5 mL warm ethanol, adding 1 mL of warmwater and allowing it to cool to rt then in an ice bath for at least 45 minutes, an hour iftime allows. While this is crystallizing, work up the oxidation reaction, which should befinished refluxing at this point. Once the crystallization of hydrobenzoin is complete,separate the crystals from the solvent, rinse, and allow them to dry at least overnight. Amp and ir spectrum (mull) will be determined and a tlc run.On the strereochemistry of the reduction. Reduction of benzoin can lead to differentstereoisomers - a meso compound and a pair of enantiomers. Using the mp of theproduct, decide which, if any, isomer predominates in the reduction reaction. In thediscussion, explain why one isomer might predominate. (Hint: the hydroxyl group inbenzoin can coordinate with the borohydride, thus causing a hydride to add to thecarbonyl from the side on which the hydroxyl group is situated. Using models andconsidering the stability of the benzoin conformers, try to predict on which side of thecarbonyl the hydride will be delivered.)(pws, 4 apr 03)OHOHOHOHmeso - isomerone isomer of anenantiomeric