Acid catalysis

Use in synthesis

Acid catalysis is mainly used for leaving group, such as converting an OH group to a H₂O⁺ group.

With carbonyl compounds such as esters, synthesis and hydrolysis go through a tetrahedral transition state, where the central carbon has an oxygen, an alcohol group, and the original alkyl group. Strong acids protonate the carbonyl, which makes the oxygen positively charged, so that it can easily receive the double bond electrons when the alcohol attacks the carbonyl carbon. This enables ester synthesis and hydrolysis. The reaction is an equilibrium between the ester and its cleavage to carboxylic acid and alcohol. On the contrary, strong bases deprotonate the attacking alcohol or amine, which also promotes the reaction. However, bases also deprotonate the acid, which is irreversible. Therefore, in a strongly basic, aqueous environment, esters only hydrolyze.

Kinetics

Specific catalysis

In specific acid catalysis taking place in solvent S , the chemical equilibrium between solvent S and AH in favor of the SH⁺ species.

S + AH → SH⁺ + A^-

For example in an aqueous concentrations of different acids.

This type of aldol reaction.

General catalysis

In general acid catalysis all species capable of donating protons contribute to reaction rate acceleration. The strongest acids are most effective. Reactions in which proton transfer is rate-determining exhibit general acid catalysis, for example diazonium coupling reactions.

When keeping the pH at a constant level but changing the buffer concentration a change in rate signals a general acid catalysis. A constant rate is evidence for a specific acid catalyst.