Question

Calculate the theoretical amount of NaBH4, needed to convert 100 mg quantities of benzil to benzoin, and benzil to hydrobenzoin.

Answer 1

One equivalent of sodium borohydride can reduce 4 equivalent of carbonyl group (aldehyde or ketone) to alcohol.

Benzil contain two carbonyl groups and to convert to benzoin one of the carbonyl needs to be reduced. Therefore 0.25 equivalent of sodium borohydride is sufficient theoretically to convert to benzoin.

While for the conversion to hydrobenzoin, both of the carbonyl of benzyl need to be reduced and hence 0.5 equivalent of NaBH4 is needed for the reduction.

Hence,

Molecular weight of Benzil: 210.07

Molecular weight of Benzoin: 212.08

Molecular weight of hydrobenzoin: 214.10

Molecular weight of NaBH4:37.83

For conversion of Benzil to benzoin 0.25 equivalent of NaBH4 is needed

Thus 210.07 gm of Benzil require (37.83/4= 9.46 gm) of NaBH4 then

Then 100 mg of Benzil will require how much NaBH4

By performing cross multiplication

Amount of NaBH4 = (0.1*9.46)/210.07 = 0.0045 gm of NaBH4 = 4.5 mg of NaBH4

For conversion of Benzil to hydrobenzoin 0.5 equivalent of NaBH4 is needed

Thus 210.07 gm of Benzil require (37.83/2= 18.91 gm) of NaBH4 then

Then 100 mg of Benzil will require how much NaBH4

By performing cross multiplication

Amount of NaBH4 = (0.1*18.91)/210.07 = 0.0090 gm of NaBH4 = 9.0 mg of NaBH4