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Homework Help: Lewis acid help

  1. Jun 26, 2007 #1
    Just wondering if anyone could post some resources on lewis acids: ie weak to strong lewis acid ranking
    or maybe some table or chart of lewis acids and their reactivity

    I haven't been able to find any resources thus far regarding lewis acids and their reactivity rankings....

    ... thanks!
  2. jcsd
  3. Jun 27, 2007 #2


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    Have you done any research on the problem?
  4. Jun 27, 2007 #3
    well after checking out couple organic synthesis books... yes, and results aren't as clear cut as expected

    I understand that the metal's unoccupied molecular orbitals and the electronegativity of its subsituents affect its ability to accept electrons but all the material I've read categorizes them as either strong or weak lewis acids (vague). There seems to be no numerical way of evaluating the acidity of a lewis acid. So i was wondering if anyone could give a general ranking.

    These are what I might be interested in working with:
    Comments on what would be more acidic than the other and why would be appreciated...
    (To start off i know AlCl3 is a strong lewis acid, and what is typically used for F-C alkyl/acylations...)

    Aluminum isopropoxide ≥98%
    Aluminum chloride anhydrous
    Aluminum bromide anhydrous

    Boron trichloride
    Boron trifluoride
    Boron trifluoride tetrahydrofuran complex

    Iron(III) chloride anhydrous,
    Iron(III) bromide


    Tin(IV) chloride

    Titanium(IV) chloride

    Titanium(IV) isopropoxide
  5. Jun 28, 2007 #4


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    You are actually asking several questions in one. Lewis acids are not usually ranked for strength like mineral acids are but many of the same principles apply. In general, electron withdrawing groups increase the acidity of the metal and electron releasing groups lower the acidity. If the substituted group is bulky and prevents close approach by the reacting species, effective acidity can be lowered. Hard/soft acid analysis is important. Hard acids are acids with high charge and small size while soft acids have lower charge and larger size. Iron(II) chloride is a softer acid than iron(III) chloride, for example. Zinc(II) chloride is a stronger Lewis acid than cadmium(II) chloride.
    Since Lewis acids are electron acceptors, some knowledge of electron affinity is useful as well. Does that help?
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