Acid Catalysis

Catalysis provides a reaction a lower energy pathway, so that the reaction proceeds at a greater rate.  Acids can act as catalysts in certain situations. There are two types of acid catalysis; Specific Acid Catalysis and General Acid Catalysis.

Specific Acid Catalysis (SAC)

This is acid catalysis involving only the H30+ (hydroxonium) ion.  Characteristically, it can be detected because the rate is proportional to the concentration of H30+.

The key point to note in specific acid catalysis is that there is rapid reversible protonation before the RDS.

This is found to be the same for all cases of specific acid catalysis.  The addition of other potential proton donors makes no difference to the rate of the reaction so long as the pH is constant.

General Acid Catalysis

This differs from specific catalysis because the reaction can be catalysed by any acid, not just H3O+.  The addition of any proton donor to the reaction mixture will increase the rate.

Above is an example of hydrolysis of an orthoester, which is an example of a reaction of general acid catalysis.

Also, we must note that here, the rate limiting step occurs during the protonation of the substrate. This is characteristic of general acid catalysis.