Can someone help me understand the scope of reduction reacti

Can someone help me understand the scope of reduction reactions especially with sodium borohydride. Why does the hydrogen attached to boron have a negative charge? I understand that different reducing agents could be used. Are there particular advantages/disadvantages to sodium borohydride when compared to alternative reducing agents? Will sodium borohydride reduce all functional groups or are there some functional groups where a stronger reducing agent is needed?

Solution

NaBH4 is a mild reducing agent which reduces aldehyde,ketone,acyl chloride to alcohols.NaBh4 remains as Na⁺ and BH4^-.Na⁺coordinates with oxygen atom of carbonyl group and activates the carboyl carbon for nucleophilic attack,and H^- aattacks to the carbonyl oxygen and get reduced to alcohols.

Hydrogen atoms attached with boron atom bears negative charge because valency of B atom is 3. And in BH₄^- has valency of 4 and boron is electro positive so H atom contains negative charge.

NaBH₄ has advantage over LiAlH4 that it is more selective over LiAlH4 but less selective than 1H containing reducing agents.

NaBH4 does not reduce ester,carboxylic acid amide etc. To reducing them,more stronger reducing agent needed,e.g; LiAlH4.