4. why is it important to use a large excess of sodium borohydride when doing a reduction in aqueous ethanol? (hint: consider what reaction might occur between water and sodium borohydride.)

Hey there!:

Aqueous ethanol contains water , sodium borohydride will react with water and decompose to give hydrogen gas :

NHB4 + 2 H2O => NaBO2 + 4 H2

thus a large excess of borohydride should be used used to take into account losses from the reaction with water , so that there is sufficient sodium borohydride left for the reduction of the chemical compound os interest .

Hope this helps!