Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

LiAlH4 reducing agent?

  1. Nov 30, 2005 #1
    can anyone explain how this little baby works as a reducing agent? ive heard rumours of H- ions, but thats just weird
     
  2. jcsd
  3. Nov 30, 2005 #2
    Weird? Weird is when you have Na-,K-, Rb-, and Cs-. And yes compounds with these species do exist :tongue: .
    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).
     
    Last edited: Nov 30, 2005
  4. Dec 1, 2005 #3
    So aluminum is not very reactive? The hydrogen is then just doing the reacting and the aluminum just sits there?
     
  5. Dec 1, 2005 #4

    movies

    User Avatar
    Science Advisor

    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.
     
  6. Dec 4, 2005 #5
    Cool. I sort of get it.
     
  7. Dec 4, 2005 #6

    movies

    User Avatar
    Science Advisor

    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).
     
  8. Dec 4, 2005 #7

    DB

    User Avatar

    cesium can u give me some me keywords about these compounds so that i can look them up and read about them? thanks
     
  9. Dec 4, 2005 #8

    movies

    User Avatar
    Science Advisor

    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.
     
  10. Dec 6, 2005 #9
    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/a.../UWS0304/Inorg. Chem. 1982 (21) 1966-1970.pdf
    I'd been interested if you find anything else about these curiousities :approve:
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?