Why does reduction using sodium borohydride require a protic solvent?
Answers
Among many reducing agents are LiAlH4 and NaBH4; the former being stronger because it can reduce a wide variety of compounds including carboxylic acids, esters, nitriles ,amides, aldehydes and ketones at room temperature while the latter reduces only aldehydes and ketones at room temperature.
As H (2.10) is more electronegative than Al (1.61), H carry a signifant negative charge and LiAlH4 reacts violently with protic solvents like H2O and ROH to form flammable H2.So we have to use inert/ anhydrous/ nonprotic solvents like R2O and THF.
NaBH4 reacts very slowly with protic solvents at room temperature because B (2.04) and H electronagativities are comparable .So reactions with NaBH4 can be easily conducted using protic solvent like ROH. However, NaBH4 decomposes with -COOH which need to be protected by forming its Na salts by reacting with NaOH .In this way only –CHO or -CO- groups are reduced while –COOH can be regenerated from its Na salt.