Sodium borohydride (NABH4) is a reducing agent used in organic chemistry. Commonly, reactions use sodium borohydride to reduce aldehydes and ketones to alcohols. The agent is not successful in reactions attempting to reduce esters, amides or carboxylic acids, and mercuration reactions use sodium borohydride in the second step.
Sodium borohydride is similar to the reducing agent lithium aluminum hydride, but it is less reactive. A reduction using sodium borohydride breaks a carbon-oxygen bond to replace it with a carbon-hydrogen bond. In reactions that reduce aldehydes and ketones, the sodium borohydride almost always forms a carbon-hydroxide bond that results in an alcohol.