- Acid catalysis
In acid catalysis and base catalysis a
chemical reaction is catalyzed by anacid or a base. The acid is often the proton and the base is often ahydroxyl ion. Typical reactions catalysed by proton transfer areesterfication s andaldol reaction s. In these reactions theconjugate acid of thecarbonyl group is a betterelectrophile 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 H2SO4 can be used. Usually this is done to create a more likelyleaving group , such as converting an OH group to a H2O group.With carbonyl compounds such as
ester s, 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 theconcentration of the protonated solvent molecules SH+. The acid catalyst itself (AH) only contributes to the rate acceleration by shifting thechemical 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 thepH of the system but not on theconcentration s of different acids.rate= -frac{d [R_1] }{dt} = k [SH^+] [R_1] [R_2]
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 catalysedaldol 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 examplediazonium coupling reactions.rate= -frac{d [R_1] }{dt} = k_1 [SH^+] [R_1] [R_2] + k_2 [AH^1] [R_1] [R_2] + k_3 [AH^2] [R_1] [R_2] + ...
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.
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