Question

Write notes on consecutive reactions with example.

Answer 1

Hint: Reactions which occur in several steps and involve intermediates are called consecutive reactions. The kinetics for such reactions are complex.

Complete step by step solution:
-Consecutive reactions consist of several steps of reaction in which the product of the first step will be the reactant of the next step and so on.
-Consecutive reactions can also be referred to as complex reactions. Usually the product of the first reaction is called the intermediate. In a three step reaction, the product of first and second reactions is the intermediates, and the final step we obtain the desired product. -Another way to define consecutive reaction is that the reaction in which the reactants form intermediates and then these intermediates react to form the final products.
-In order to explain consecutive reactions let us consider a consecutive occurrence of two first order reversible reactions$$A\to B\to C$$. Here A, B and C are the substances. The change in concentrations of these substances can be calculated by integrating the system of the two kinetic equations. Such calculations show how the concentration of B first increases in the first reaction and then the concentration decreases because it is consumed in the second first order reaction.
-If the reactions are reversible then more complex consecutive reactions are obtained and to calculate the concentration of the intermediate becomes more complex.
-Some of the examples of consecutive reactions are polymerisation, thermal cracking and chlorination of hydrocarbons.
-Chlorination of hydrocarbons steps are
$$C{H}_4\stackrel{C{l}_2,\mathrm{\Delta }}{\to }C{H}_{3}Cl$$
$$C{H}_{3}Cl\stackrel{C{l}_2,\mathrm{\Delta }}{\to }C{H}_{2}C{l}_2$$
$$C{H}_{2}C{l}_2\stackrel{C{l}_2,\mathrm{\Delta }}{\to }CHC{l}_3$$
$$CHC{l}_3\stackrel{C{l}_2,\mathrm{\Delta }}{\to }CC{l}_4$$
From above equations we know that chlorination of methane occurs in 4 steps. Methane when reacted with chlorine in the presence of light we get chloromethane. Chloromethane when reacted with chlorine in the presence of light we get dichloromethane. Same step is repeated: we get chloroform and then again we get carbon tetrachloride.

Note: Usually consecutive reactions can be confused with parallel reactions. Parallel reaction is where two reactions occur simultaneously to get two or more products.