Everything about Acid Catalysis totally explained
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.
Use in synthesis
Acid catalysis is mainly used for
organic chemical reactions. There are many possible chemical compounds that can act as sources for the protons to be transferred in an acid catalysis system. A compound such as
sulphuric acid (H
2SO
4) can be used. Usually this is done to create a more likely
leaving group, such as converting an OH group to a H
2O
+ 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
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.
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.
Further Information
Get more info on 'Acid Catalysis'.
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