Question

Explain the intermediate compound formation theory of catalysis with an example.

Answer 1

The intermediate compound formation theory:
A catalyst acts by providing a new path with low energy of activation. In homogeneous catalysed reactions a catalyst may combine with one or more reactant to form an intermediate which reacts with other reactant or decompose to give products and the catalyst is regenerated.

Consider the reactions:

\[\ce{A + B -> AB}\] .............(1)

\[\ce{A + C -> AC (intermediate)}\] ..................(2)

C is the catalyst

\[\ce{AC + B -> AB + C}\] .............(3)

Activation energies for reactions (2) and (3) are lower compared to that of (1).

Hence the formation and decomposition of the intermediate accelerate the rate of the reaction.

Example:

The mechanIsm of Fridel crafts reaction is given below

\[\ce{C6H5 + CH3Cl ->[anhydrous AlCl3] C6H5CH3 + HCl}\]

The action of catalyst is explained as follows

\[\ce{CH3Cl + AlCl3 -> [CH3]^+ [AlCl4]^-}\]

It is an intermediate.

\[\ce{C6H6 + [CH3]^+ [AlCl4]^- -> C6H5CH3 + AlCl3 + HCl}\]

This theory describes,

1. The specificity of a catalyst.
2. The increase in the rate of the reaction with an increase in the concentration of a catalyst.

Limitations:

1. The intermediate compound theory fails to explain the action of catalytic poison and activators (promoters).
2. This theory is unable to explain the mechanism of heterogeneous catalysed reactions.