Addition of two atoms of hydrogen t

Addition of two atoms of hydrogen to benzoin or of four atoms of hydrogen to benzil gives a mixture of stereoisomeric diols, of which the predominant isomer is the nonresolvable (2R,3S)-hydrobenzoin,the meso isomer, accompanied by the enantiomeric (2R,3R)and (2S,3S) compounds. The reaction proceeds rapidly at room temperature; the intermediate borate ester is hydrolyzed with water to give the product alcohol.4RC=O + Na*BH~ -> (R,CHO),B-Na*(R,CHO)B-Na* + 2 H,0 ->4R,CHOH + Na*BO,The procedure that follows specifes use of benzil rather than benzoin because you can then follow the progress of the reduction by the discharge of the yellow color of the benzil.ProcedureIn a 50-mL Erlenmeyer flask, dissolve 0.5g of benzil in S mL of 95% ethanol and cool the solution under the tap to produce a fine suspension. Then add 0.l gof sodium borohydride (large excess).The benzil dissolves, the mixture warms up, and the yellow color disappears in 2-3 min. Aftera totalof 10 min, add S mLofwater, heattothe boilingpoint, filterincase the solution is not clear, dilute to the point of saturation with more water (10 mL), and set the solution aside to crystallize, meso-Hydrobenzoin separates in lustrous thin plates, mp 136-137C;yieldisabout 0.35g. Cleaning Up The aqueous filtrate should be diluted with water and neutralized with acetic acid (to destroy borohydride) before fushing the mixture down the drain.