LiAlH4 vs. NaBH4 — What's the Difference?

LiAlH4 (Lithium Aluminium Hydride) and NaBH4 (Sodium Borohydride) are both widely used reducing agents in organic chemistry, but they differ significantly in strength and application. LiAlH4 is more potent, capable of reducing a wide range of functional groups, including esters, carboxylic acids, amides, aldehydes, and ketones to alcohols. It is reactive towards water and requires anhydrous conditions and inert solvents like diethyl ether. On the other hand, NaBH4 is less reactive, primarily used for the reduction of aldehydes and ketones to alcohols, and can tolerate mild aqueous conditions.

LiAlH4's aggressive reactivity makes it unsuitable for selective reductions when multiple functional groups are present that could be reduced. It demands careful handling due to its violent reaction with water, producing hydrogen gas. NaBH4 offers a milder alternative, allowing for selective reduction in the presence of other sensitive functional groups and is safer to handle in typical laboratory settings due to its stability in water.

The choice between LiAlH4 and NaBH4 depends on the desired outcome of the reaction and the nature of the substrate. For complex molecules requiring specific group reductions without affecting other functional groups, NaBH4 is often the preferred choice. Conversely, for more comprehensive reductions, LiAlH4 is utilized despite its handling precautions.

While LiAlH4 is typically used in anhydrous solvents to prevent decomposition, NaBH4's compatibility with aqueous solutions expands its utility in more varied reaction conditions. This characteristic makes NaBH4 more versatile for reductions that can tolerate or benefit from aqueous environments.

The distinct reactivity profiles of LiAlH4 and NaBH4 highlight the importance of selecting the appropriate reducing agent in synthesis strategies. Understanding their differences is crucial for successful organic transformations, impacting both the efficiency and safety of chemical reactions.