Acid-Base catalysis (definition – Examples – Mechanism)

– In this topic, we will discuss Acid-Base catalysis: definition, Examples and Mechanism.

Acid-Base catalysis

– A number of homogeneous catalytic reactions are known which are catalysed by acids or bases, or both acids and bases.  These are often referred to as Acid-Base catalysts.

Arrhenius pointed out that acid catalysis was, in fact, brought about by H+ ions supplied by strong acids, while base catalysis was caused by OH– ions supplied by strong bases


(1) Inversion of Cane sugar

(2) Keto-Enol tautomerism of Acetone

Acid-Base catalysis (definition - Examples - Mechanism)

(3) Decomposition of Nitramide

(4) Hydrolysis of an Ester

Acid-Base catalysis (definition - Examples - Mechanism)

General Acid-Base catalysis

– More recently it has been found that :

(a) Not only H+ ions but all Bronsted bases (proton donors) cause acid catalysis.

– Thus the general acid catalysts are: H+, undissociated acids (CH3COOH), cations of weak bases (NH4+) , and water (H2O).

(b) Not only OH ions but all Bronsted bases (proton acceptors) act as base catalyst.

– Thus the general base catalysts are: OH, undissociated bases, anions of weak acids (CH3COO) and water (H2O).

– The catalysis brought about by general acids and bases is termed General Acid-Base catalysis.

– For elucidation, decomposition of nitramide is also catalysed by acetate ions (CH3COO).

Mechanism of Acid-Base catalysis

(a) In acid catalysis

– The H+ (or a proton donated by Bronsted acid) forms an intermediate complex with the reactant, which then reacts to give back the proton. For example, the mechanism of keto-enol tautomerism of acetone is :

(b) In base catalysis

– The OH ion (or any Bronsted base) accepts a proton from the reactant to form an intermediate complex which then reacts or decomposes to regenerate the OH (or Bronsted base).

– For example, the decomposition of nitramide by OH ions and CH3COO ions may be explained as follows :

(i) By OH– ions

Acid-Base catalysis (definition - Examples - Mechanism)

(ii) By CH3COO– ions :

Acid-Base catalysis (definition - Examples - Mechanism)

Leave a Reply

Your email address will not be published. Required fields are marked *