Acid catalysis

From Wikipedia, the free encyclopedia

In acid catalysis and base catalysis a chemical reaction is catalyzed by an acid or a base. The acid is often the proton and the base is often a hydroxyl ion. Typical reactions catalysed by proton transfer are esterfications and aldol reactions. In these reactions the conjugate acid of the carbonyl group is a better electrophile than the neutral carbonyl group itself. Catalysis by either acid or base can occur in two different ways: specific catalysis and general catalysis.

Contents 1 Use in synthesis 2 Kinetics 2.1 Specific catalysis 2.2 General catalysis

[edit] Use in synthesis

Acid catalysis is mainly used for organic chemical reactions. There are mainly possible chemical compounds that can act as sources for the protons to be transferred in an acid catalysis system. A compound such as sulfuric acid (H₂SO₄) can be used. Usually this is done to create a more likely leaving group, such as converting an OH group to a H₂O⁺ group.

[edit] Kinetics

[edit] Specific catalysis

In specific acid catalysis taking place in solvent S , the reaction rate is proportional to the concentration of the protonated solvent molecules SH⁺. The acid catalyst itself (AH) only contributes to the rate acceleration by shifting the chemical equilibrium between solvent S and AH in favor of the SH⁺ species.

S + AH → SH⁺ + A^-

For example in an aqueous buffer solution the reaction rate for reactants R depends on the pH of the system but not on the concentrations of different acids.

This type of chemical kinetics is observed when reactant R₁ in a fast equilibrium with its conjugate base R₁H⁺ which proceeds to react slowly with R₂ to the reaction product for example in the acid catalysed aldol reaction.

[edit] 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.