LiAlH4 Reducing Agent: An Explanation

[Saoist]

can anyone explain how this little baby works as a reducing agent? I've heard rumours of H- ions, but that's just weird

 

[Cesium]

ive heard rumours of H- ions, but that's just weird

Weird? Weird is when you have Na-,K-, Rb-, and Cs-. And yes compounds with these species do exist .
LiAlH4 is an ionic compound comprised of Li+ and AlH4-. It's a great source for hydride ions.
Aluminum has a low electronegativity. Therefore, the Al-H bond is very polarized with Al begin positive and H being negative. This abnormal polarization (oxidation state of -1) for hydrogen results in very high reactivity, especially with atoms that accept electrons (aka are reduced), allowing the hydrogen to become positive again (oxidation state of +1).

 

[P-man]

So aluminum is not very reactive? The hydrogen is then just doing the reacting and the aluminum just sits there?

 

[movies]

Well, the reactivity is a function of the activating ability of Li+ as a Lewis acid and the instability of a formal negative charge on Al.

 

[P-man]

Cool. I sort of get it.

 

[movies]

First the lithium coordinates to the carbonyl, acting as a Lewis acid. Then, the aluminate delivers an H- which reduces the carbonyl. The remaining aluminum species, AlH₃, is called alane and is a pretty good reducing agent in its own right. The alane can go on to reduce more carbonyls then as well. In alane reductions, the alane acts as a Lewis acid by itself because the Al has a vacant coordination site. Lithium could do this too, but I think that the Li probably stays on the alkoxide you get from the first reduction. In most cases you use a fair excess of LiAlH₄, so the alane pathway probably isn't as important as the lithium activated pathway (because LiAlH₄ is more reactive that AlH₃).

 

[DB]

Weird? Weird is when you have Na-,K-, Rb-, and Cs-. And yes compounds with these species do exist .

cesium can u give me some me keywords about these compounds so that i can look them up and read about them? thanks

 

[movies]

The alkali hydrides aren't really good reducing agents. They tend to act as strong bases, not nucleophiles. I'm not entirely sure why though. But I have run a lot of reactions with NaH and it doesn't reduce ketones.

 

[Cesium]

cesium can u give me some me keywords about these compounds so that i can look them up and read about them? thanks

The alkalides are called natrides, katrides, rubides, and cesides. No lithides have been made.
There is relatively little information about these compounds but some articles I've dug up are:
http://www.iupac.org/publications/pac/1993/pdf/6503x0435.pdf
http://www.uni-mainz.de/FB/Chemie/aac/AC1/download/Altdaten/UWS0304/Inorg.%20Chem.%201982%20(21)%201966-1970.pdf
I'd been interested if you find anything else about these curiousities