Acid catalysis

Strong acids catalyze the hydrolysis and transesterification of esters, e.g. for processing fats into biodiesel. In terms of mechanism, the carbonyl oxygen is susceptible to protonation, which enhances the electrophilicity at the carbonyl carbon.

Solid acid catalysts

In industrial scale chemistry, many processes are catalysed by "solid acids". Solid acids do not dissolve in the reaction medium. Well known examples include these oxides, which function as Lewis acids: silico-aluminates (

A particularly large scale application is alkylation, e.g., the combination of benzene and ethylene to give ethylbenzene. Another major application is the rearrangement of cyclohexanone oxime to caprolactam.^[4] Many alkylamines are prepared by amination of alcohols, catalyzed by solid acids. In this role, the acid converts, OH⁻, a poor leaving group, into a good one. Thus acids are used to convert alcohols into other classes of compounds, such as thiols and amines.

Mechanism

Two kinds of acid catalysis are recognized, specific acid catalysis and general acid catalysis.^[5]

Specific catalysis

In specific acid catalysis, protonated solvent is the catalyst. The reaction rate is proportional to the concentration of the protonated solvent molecules SH⁺.^[6] 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. This kind of catalysis is common for strong acids in polar solvents, such as water.

${\displaystyle {\ce {S + AH -> SH+ + A-}}}$

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.

${\displaystyle {\text{rate}}=-{\frac {{\text{d}}[{\ce {R^1}}]}{{\text{d}}t}}=k{\ce {[SH+] [R^1] [R^2]}}}$

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

General catalysis

In general acid catalysis all species capable of donating protons contribute to

${\displaystyle {\text{rate}}=-{\frac {{\text{d}}[{\ce {R^1}}]}{{\text{d}}t}}=k_{1}{\ce {[SH+] [R^1] [R^2]}}+k_{2}{\ce {[A^1H] [R^1] [R^2]}}+k_{3}{\ce {[A^2H] [R^1] [R^2]}}+...}$

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. When reactions are conducted in nonpolar media, this kind of catalysis is important because the acid is often not ionized.

Enzymes catalyze reactions using general-acid and general-base catalysis.

References