QS Study

Intermediate compound theory:

The essential feature of the theory is that the catalyst chemically combines with one of the reactants to form an unstable active intermediate compound which then reacts further with the other reactants giving the final product and the catalyst is regenerated. In some reactions the intermediate compound can be easily detected and even isolated whereas in some reactions the existence of the intermediate compound can be proved by sophisticated methods alone. Some examples are shown below:

(i) As shown earlier the oxidation of SO2 to SO3 by NO in presence of oxygen or air is known to occur by the following sequence of reactions:

2NO + O2 → 2NO2

2NO2 + 2SO2 → 2SO3 + 2NO

2NO + 2SO2 + O2 → 2SO3 + 2NO     net reaction

However, in the absence of NO the oxidation is extremely slow. NO then may be looked upon as an “oxygen carrier”.

(ii) In the oxidation of iodides by hydrogen peroxide and the oxidation of iodides in acid medium by oxygen, hypoiodous acid (or more correctly speaking IO) is the intermediate active species that is responsible for the reaction.

H2O2 (aq) + I (aq) → H2O (l) + IO (aq)

H2O2 (aq) + IO (aq) → H2O (l) + O2 (g) + I (aq)

(iii) In the reaction between benzene (C6H6) and ethanoyl Chloride (CH3COCl) giving phenyl methyl ketone, C6H5COCH3; anhydrous AlCl3 is used as a catalyst.

C6H6 (l) + CH3COCl → C6H5COCH3 (l) + HCl (g)

AlCl3 first reacts with ethanoyl chloride giving rise to an intermediate species, CH3CO, AlCl4 which then reacts with benzene and phenyl methyl ketone is formed. AlCl3 is regenerated.

CH3COCl + AlCl3 → CH3CO.AlCl4

The intermediate compound theory has been found useful in many reactions, particularly in homogeneous catalysis.

(iii) Acid catalysis of ester hydrolysis is another example where an intermediate is supposed to be formed. The case of hydrolysis of an ester such as methyl acetate in water may be considered.


The reaction is slow but the presence of an acid makes the reaction proceed at a much faster rate.