Acids Addition to Multiple Bonds

In our discussion of acid catalysis we have up to now restricted our attention to protonic or Bronsted acids. However, the Bronsted acid, defined as a proton donor, calls into existence the Bronsted base, which can act as proton acceptor. But such a base accepts protons by virtue of the fact that it can offer the proton an unshared pair of electrons for bonding, e.g., 

Lewis saw that such a Bronsted base called for a more generalized acid, namely, one that could accept a pair of electrons. This made it possible to discuss acid-base reactions in nonprotonic systems. The reaction of 

BF3 with tertiary amines or with ethers is an example of a Lewis acid reacting with a Bronsted base  

In aqueous solution, metal ions are Lewis acids and the familiar complex ions such as Fe(NO)-+, Cr(H20)i, and AlFi can be looked on as acid-base complexes. Because of their large valence shells, the nonmetallic atoms beyond the second row in the periodic table (S, P, Cl, Br, I, etc.) can show properties of both Lewis acids and bases. 1 can act as a base with metal ions (Lewis acids) to form quite stable complexes such as Ilgir On the other hand I2 can act as an acid in its reaction with electrons donors to form complexes of varying stability. With 1 the reaction I + I2 If proceeds largely to the right in O.l M aqueous solution = 140 liters/mole), Air = -4.0 Kcal. 

This generalization of acid-base properties in terms of electron transfer has been used by the English writers as a basis of classification of reagents as either nucleophilic (electron-donating) or electrophilic (electron-accepting). There is then a classification of reactions in tenns of these categories as well. 

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