[tintin]

what is magnesium hydride? its characteristics and uses?

[Limpet Chicken]

Magnesium hydride is a compound of magnesium and hydrogen, hydrides are usually reducing agents of at least some strength, and  in the case of group I and II hydrides, they react with water to release hydrogen.

The group I and II hydrides are also quite vicious reducing agents, also forming interstitial hydrides with transition metals, such as LiAlH4 and NaBH4, and in the case of sodium borohydride, it also forms a cyano- compound sodium cyanoborohydride which I think, is a more selective reducing agent.

LiAlH4 for example, is pretty much the bigger meaner brother of pretty much any other reducing agent, reacts violently with water, so much so that it is even pyrophoric in the air, and has to be used in anhydrous ether solvents such as ether or THF.

The ordinary hydrides are somewhat less fearsome though, and make for powerful reducing agents.

[movies]

I've never used MgH₂, but I've used NaH and CaH₂.  These actually aren't very good reducing agents, but they are good bases.  NaH is commonly used for deprotonating alcohols and occassionaly for making enolates (it doesn't reduce the ketones).  Calcium hydride works similarly, but isn't quite as reactive.

They are often used for drying reagents that are being distilled.  For example, diisopropyl amine is distilled from CaH₂ prior to use in water sensitive reactions.

NaBH₄ and LiAlH₄ are good reducing agents because you have a negatively charged metal (boron or aluminum), the group I metal doesn't have a very large effect except as a Lewis acid with these reagents.

[budullewraagh]

wouldn't LiBH4 be a more powerful reducer than LiAlH4?

[Limpet Chicken]

Not positive, but I don't think so, judging from the fact that sodium borohydride actually is a weaker reducing agent then LiAlH4, not sure on the finer differences between the two, although NaBH4 is less pyrophoric, and generally dangerous, but, having said that, I would sell my soul for 1kg of either of them

[movies]

LiBH₄ is less reactive than LiAlH₄, but more reactive than NaBH₄.  The latter observation is easy to justify based on how coordinating the metal cation is; Li is a better Lewis acid than Na.  I think the reactivity difference between LAH and LiBH₄ comes from the difference in the bond strengths of the B-H and the Al-H bonds.  The Al-H bond is much more ionic (based on electronegativity) so the hydride bears much more of the negative charge.

BTW, NaBH₄ is hardly pyrophoric at all, which is nice when you need to use it.  It will absorb water and eventually go bad if you don't keep it dry, but the risk of fire/explosion is much, much lower than with LAH.  Not sure about LiBH₄, I've never used it.

Some examples of reactivity:
- any of these three will reduce a ketone or aldehyde to an alcohol in less than 5 minutes (except in exceptional circumstances)
- NaBH₄ cannot reduce an ester (to an alcohol), while the other two can
- LiAlH₄ can reduce acids to alcohols while LiBH₄ generally cannot

[Limpet Chicken]

I read about power cells for cars using NaBH4 recently, I dont suppose if you have heard of any practical use of them have you, in particular, in the UK, I think I might like to dissect a few of those if I can find them, as I have been after NaBH4 for quite some time (from dumped/trashed cars of course)

Have you any info on the reactivity/uses of sodium cyanoborohydride?
I heard it was a lot more selective in doing its thing in general, which could well bee usefull for my sort of chemistry

[movies]

I've heard that there were some plans to use NaBH₄ as a way to conveniently store hydrogen for fuel.  I don't know that it has been implemented anywhere though.

NaCNBH₃ is notably less reactive than NaBH₄.  For example, it generally won't reduce aldehydes or ketones.  Its most common use is in reductive amination.  For this, you mix an amine with a aldehyde/ketone, which then forms an iminium ion in solution.  The iminium is reactive enough to be reduced by the NaCNBH₃, but the starting materials are untouched by the reducing agent.

Another related reagent is NaBH(OAc)₃.  This is a very mild reducing agent which shows a lot of the same reactivity as NaCNBH₃, but without the risk of forming toxic HCN.  NaBH(OAc)₃ will reduce aldehydes, but not isolated ketones (also most of the reductive amination stuff).  A very nice reagent.