Estimating the Strength of Lewis Acids

A quantitative method to determine the strength of Lewis acids and to establish similar scales as discussed in the case of Brpnsted acids would be very useful. However, establishing such a scale is extremely difficult and challenging. Whereas the Brpnsted acid-base interaction always involves proton transfer, which allows a meaningful quantitative comparison, no such common relationship exists in the Lewis acid-base interaction. The result is that the definition of strength has no real meaning with Lewis acids. 

The strength or coordinating power of different Lewis acids can vary widely against different Lewis bases. Thus, for example, in the case of boron trihalides, boron trifluoride coordinates best with fluorides, but not with chlorides, bromides, or iodides. In coordination with Lewis bases such as amines and phosphines, BF3 shows preference to the former (as determined by equilibrium constant measurements).66 The same set of bases behaves differently with the Ag+ ion. The Ag+ ion complexes phosphines much more strongly than amines. In the case of halides (F, CP, Br, and P), fluoride is the most effective base in protic acid solution. However, the order 

The Lewis acidity with respect to strong bases such as NH3 is greater for BC13 than for BF3. In contrast, toward weakbases such as CO, BF3 is a stronger acid than BC13.67 

Despite the apparent difficulties, a number of qualitative relationships were developed to categorize Lewis acids. 

There are many attempts made in the literature72,73 to rate qualitatively the activity of Lewis acid catalysts in Friedel-Crafts-type reactions. However, such ratings largely depend on the nature of the reaction for which the Lewis acid catalyst is employed. 

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