Borohydride reduction of a ketone hydrobenzoin from benzil lab report

Multistep Synthesis

Step 1: The Benzoin Condensation: Cyanide Ion Catalyzed

The reaction of two moles of benzaldehyde to form a new carbon-carbon bond is known as the benzoin condensation. It has been catalyzed by two rather different catalysts, cyanide ion and the vitamin, thiamine. Both of which on close examination appear to function in exactly the same way.

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Benzaldehyde MW 106.12

bp 178°C

Benzoin

MW 212.24

mp 135°C

Caution: Do not handle the potassium cyanide if you have open cuts on your hands. Wash your hands immediately after handling the crystals. Never acidify a cyanide solution, HCN gas (the killer) would be given off !

Procedure:

In a round bottom flask place 20 mg of potassium cyanide (POISON!), dissolve the solid in 0.25 mL of water, add 0.50 mL of 95% ethanol and from a micropipette 0.400 mL of pure benzaldehyde. Introduce a stirring bar, and while stirring, reflux the solution gently with the heating block for 30 minutes. Remove the round bottom flask and cool it in an ice bath, and if no crystals appear within a few minutes scratch the inside of the reaction flask to induce crystallization. When the crystallization is complete, remove the solvent by filtration using a Hirsch funnel. Wash the crystals twice with separate 0.4 mL portions of a cold, one to one mixture of 95% ethanol to water. The product is usually colorless crystals with a melting point of 134-135°C. Weigh the dry product and save some solid to take a melting point. 150 mg of dry product is needed for the next step. If your synthesis does not yield over 100 mg, your instructor will provide additional benzoin to make up the difference (a few points will be deducted).

Clean up: This is the most important step in the whole experiment. Discard all filtrate into a waste bottle labeled CYANIDE WASTE in the fume hood. The waste bottle contains 1 % sodium hydroxide and bleach solution (5.25% sodium hypochlorite) that oxidize the cyanide ion.

Step 2: Oxidation of Benzoin to Benzil

Benzoin can be oxidized to the a -diketone, benzil, by a variety of mild oxidizing agents. Too vigorous oxidation by sodium dichromate or potassium permanganate yields chiefly benzaldehyde by cleavage of the bond between the two oxidized carbon atoms, activated by both of the attached phenyl groups. In this procedure, benzoin is oxidized with two equivalents of copper (II) acetate in an acetic acid­-water mixture. Purportedly, this procedure offers the advantage that the benzil obtained is well crystallized and of high purity.

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