borohydride

Borohydride usually refers to the borohydride ion, BH4−, or to salts that contain it, most commonly sodium borohydride (NaBH4) and lithium borohydride (LiBH4). The BH4− anion is tetrahedral, with boron bonded to four hydride ligands and an overall negative charge localized on the boron–hydride framework.

Borohydride compounds are widely used as hydride donors in chemistry. They are generally supplied as white to off-white solids and are moisture sensitive. In dry organic solvents they are stable, but on exposure to water or protic media they hydrolyze, releasing hydrogen gas and forming borate species. NaBH4 is soluble in some alcohols and ethers, and it is more commonly used in mild reductions, whereas LiBH4 is more reactive and often employed in reductions that NaBH4 cannot achieve.

Reactivity and scope: NaBH4 efficiently reduces aldehydes and ketones to primary and secondary alcohols. It is typically not effective for esters or carboxylic acids under standard conditions. LiBH4 is stronger and can reduce a broader range of carbonyl substrates, including esters and some carboxylic acids, though often under more forcing conditions. The reductions proceed by transfer of a hydride ion from BH4− to the carbonyl carbon, with the boron hydride being converted to borate species as a by-product.

Applications and safety: Borohydride reagents are staples in organic synthesis, used for selective reductions in pharmaceuticals, natural products, and materials chemistry. They are reducing agents and can react violently with water, acids, or oxidizers; they should be handled dry and under inert conditions to avoid hydrogen evolution and hazardous reactions.