LiAlH4 Reducing Agent: An Explanation

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Discussion Overview

The discussion revolves around the mechanism by which lithium aluminum hydride (LiAlH4) acts as a reducing agent, exploring its chemical properties, the role of hydride ions, and comparisons with other reducing agents. The scope includes theoretical explanations, chemical reactivity, and practical implications in reduction reactions.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants mention that LiAlH4 is a source of hydride ions, with aluminum's low electronegativity leading to a polarized Al-H bond, which contributes to its reactivity.
  • One participant questions whether aluminum is reactive or if the hydrogen is primarily responsible for the reactivity, suggesting that aluminum may not be actively participating in the reduction.
  • Another participant introduces the idea that the reactivity of LiAlH4 is influenced by lithium acting as a Lewis acid and the instability of a formal negative charge on aluminum.
  • A detailed mechanism is proposed where lithium coordinates to a carbonyl, allowing the aluminate to deliver an H- ion for reduction, with a mention of alane (AlH3) as a subsequent reducing agent.
  • One participant notes that alkali hydrides are generally not good reducing agents, often acting as strong bases instead, and expresses uncertainty about this behavior.
  • Another participant provides keywords related to alkali metal hydrides for further exploration, discussing their nomenclature and availability of information.

Areas of Agreement / Disagreement

Participants express differing views on the reactivity of aluminum versus hydrogen in LiAlH4, and there is no consensus on the effectiveness of alkali hydrides as reducing agents. The discussion remains unresolved regarding the specific roles of the components in the reduction process.

Contextual Notes

Some claims about the reactivity of aluminum and the behavior of alkali hydrides depend on specific conditions and definitions that are not fully explored in the discussion.

Saoist
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can anyone explain how this little baby works as a reducing agent? I've heard rumours of H- ions, but that's just weird
 
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Saoist said:
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 :-p .
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).
 
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So aluminum is not very reactive? The hydrogen is then just doing the reacting and the aluminum just sits there?
 
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.
 
Cool. I sort of get it.
 
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).
 
Cesium said:
Weird? Weird is when you have Na-,K-, Rb-, and Cs-. And yes compounds with these species do exist :-p .

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