When synthesizing complex organic molecules, it is common to have at least a dozen individual transformations whereby the product of one reaction is then used as the starting material for the next reaction. A multistep synthesis starts with inexpensive, readily available benzaldehyde. The sequence will attempt to first the conversion of benzaldehyde to benzoin using the vitamin, thiamin, as a catalyst. In the second step, the benzoin is oxidized to benzil through the use of an oxidizing agent. The third step is a condensation reaction of benzil with dibenzyl ketone (1,3-diphenyl-2-propanone) to produce tetraphenylcyclopentadienone. An alternativethird step is the reduction of benzil to dihydrobenzoin with a reducing agent, sodium borohydride. An additional fourth step is possible converting the tetraphenylcyclopentadienone to a substituted naphthalene via a Diels-Alder reaction (followed bydecarbonylation) using microwaves as the energy source. The first step is the synthesis of benzoin, this reaction is a the conversion of two molecules of an aldehyde to an alpha-hydroxy ketone. The reaction is known as a benzoin condensation (“condensation” because two molecules become condensed to one molecule). This reaction, which requires a catalyst, if often performed with cyanide ion. We will use thiamine as a catalyst. It is heat-sensitive and may decompose if heated too vigorously. Instead of running this reaction at elevated temperatures for a few hours, we will allow the reaction to proceed closer to room temperature for 24 hours or more. Benzaldehyde is easily oxidized to benzoic acid which can impede the desired reaction so freshly distilled benzaldehyde is used. The concentration of reactants and temperatures of solutions are critical to obtaining a good yield so procedures must be followed carefully. Too much water will force benzaldehyde out of solution preventing an efficient reaction. Too little water prevents the thiamine hydrochloride from...
Please join StudyMode to read the full document