Quick Links
Science Concentrates
August 29, 2005 - Volume 83, Number 35; p. 27
Explaining LiAlH4, NaBH4 reactivities
Lithium aluminum hydride and sodium borohydride are two common hydride reducing agents, and organic chemists have known that LiAlH4 is more reactive than NaBH4. The origin of this difference has not been thoroughly explored, however. Joseph S. Francisco, Paul G. Wenthold, and colleagues at Purdue University believe they have an explanation based on a new study of the thermodynamic properties and bonding of dialane (J. Am. Chem. Soc. 2005, 127, 11684). Dialane, Al2H6, is structurally similar to diborane, B2H6, but it’s much less stable and hasn’t been well-characterized. The researchers find that the gas-phase hydride affinity of AlH3 to form AlH4–––the energy cost of a hydride reduction––is the same as that for BH3 to form BH4–. This means the two anions should have the same reducing capabilities in the gas phase. The researchers conclude that the difference in reactivities of LiAlH4 and NaBH4 in solution is probably related to the cations and/or solvent properties rather than to the properties of the hydride anions.