Description: Addition of sodium borohydride (NaBH4) to aldehydes gives primary alcohols (after adding acid)
Notes: Lots of different acids can be used in the last step. It’s not important which specific acid is used, just that something is present that can form the alcohol.
NaBH4 is a source of hydride (H-) and the reaction begins with the addition of hydride to the carbonyl to the aldehyde (Step 1, arrows A and B). Upon addition of acid, the oxygen is protonated (Step 2, arrows C and D) to give the neutral primary alcohol.
Notes: The choice of H2O / H2SO4 as acid isn’t crucial – this is just an example. Any source of proton (including water) will do.