MOLECULARITY OF REACTION

Molecularity is defined in terms of a number which is equal to the number of molecules or atoms that must collide simultaneously to give the products.

The stoichiometric equation is essential to decide the molecularity of a reaction. Since molecularity is determined by postulating the mecha nism for a reaction, it must be a small number (integer), and cannot be a fraction or zero.

Unimolecular Reactions

In these reactions, molecularity is one, i.e., one type of molecules stoichiometrically participates in the reaction.

Example 1: Isomerisation of trans-stilbene to cis-stilbene.

Example 2 : Conversion of all trans-vitamin A to all cis-vitamin A. 

Example 3: Reversible isomerisation of tetracycline at pH range of 2 to 6. The reaction is epimerisation.

The natural configuration of tetracycline has the N(CH3)2 group below the plane and H group above the plane of the paper (A). Under acidic conditions. A is converted reversibly to epi-isomer B (only that part of the structure of tetracycline molecule that is undergoing a change is represented in the above formula).

Bimolecular Reactions

In these reactions, molecularity is two, i.e., two types of molecules are stoichiometrically involved in the reaction. Bimolecular reactions are of two categories, depending on whether the two molecules undergo ing a change are of the same type or different

Example 1: Same type of reactants. Oxidation of hydrogen peroxide.

                        2H202  ➡️  2H20 + 02

Example 2 : Different type of reactants.

1. Alkaline hydrolysis of ethylacetate

CH3COOC2Hs + NaOH  ➡️ CH3COONA + C2H5OH

2. Decomposition of prednisolone in presence of alkali.

Termolecular Reactions

Reactions of termolecular and other higher molecularity observed. This is because three or more molecules havi 6/33 kinetic energy must meet simultaneously in the same region u. -puj to yield a product. The occurrence of such an event is quite unlikely.

The rate equations for the above reactions can be written using the law of mass action. The concept of molecularity is of theoretical relevance, but in practice, order of a reaction is important, because it is obtained by experimentation.

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