Explain intermediate compound formation theory of catalysis with an example.

Explanation

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 :
` A + B to AB` (1)
A + C + AC (intermediate) (2)
C is the catalyst
`AC + B to AB + C` (3)
Activation energies for the reactions (2) and (3) are lowered compared to that of (1). Hence the formation and decomposition of the intermediate accelerate the rate of the reaction.
Example 1 : The mechanism of Fridel crafts reaction is given below
` C_6H_6 + CH_3Cl overset(AlCl_3)(to) C_6H_5CH_3 + HCl`
The action of catalyst is explained as follows
` CH_3 Cl + AlCl_3 to [CH_3]^(+) [AlCl_4]^(-)`
It is an intermediate
` C_6H_6 + [CH_3^(+)] [AlCl_4]^(-) to C_6H_5CH_3 + AlCl_3 + CHl`