Acids and Acid Catalysis—Homogeneous

Abstract

The concept of acidity has changed tremendously over the years. It has developed from protic acids in the aqueous phase via Lewis acids with no proton at all to superacids. Acids are the most frequent catalysts of organic chemistry in the homogeneous phase and they find application as heterogeneous catalysts as well. In solution, they catalyze a wide variety of transformations ranging from rearrangements of various kinds through alkylations to Friedel–Crafts reactions. Intermediates are carbenium as well as carbonium ions. In many cases, they can be identified and occasionally even isolated. Many laws governing acid‐catalyzed reactions are known to such an extent that mechanistic and quantitative descriptions can be given. In the first part of this review, a general treatment of homogeneous acids and acid catalysis is given through the development of various acid–base theories and acidity scales. This is followed by a short characterization of the most important Brønsted and Lewis acids, including superacids, as well as chiral acids applied in homogeneous catalysis. In the last section, characteristic examples of important acid‐catalyzed processes of organic chemistry, including rearrangements, carbon–carbon bond formation, and breaking and forming of carbon–heteroatom bonds with emphasis on mechanistic and kinetic aspects, are discussed.