Simple And Complex Reactions
SIMPLE AND COMPLEX REACTIONS
A simple reaction takes place in a single step. Simple reactions are also known as elementary reactions. One step reactions are elementary reactions. In some reactions many side reactions occur along with the main reaction involving product formation.
Reactions which do not take place in a single step but take place in a sequence of a number of elementary steps are called as complex reactions.
i. Occurs in single step
ii. Overall order values are small. Total and pseudo order values lie between 0,1,2 and 3.
iii. No side reactions
iv. Products are formed directly from the reactants
v. Experimental rate constant values agree with the calculated values. Theories of reaction rates apply well on simple reactions.
2. Complex reactions
i. Occurs in multi (or) many steps.
ii. Overall order values are large and greater than 3.0. Sometimes fractional orders such as 1/2, 1/3, 3/2 etc. are seen.
iii. Many side reactions are present.
iv. In some complex reactions
v. products are not formed in steps directly involving the reactants.
vi. Experimental overall rate constant values differ from the calculated values. Theories of reaction rates do not agree well on complex reactions.
Types of Complex reaction
The reactions in which the reactant forms an intermediate and the intermediate forms the product in one or many subsequent reactions are called as consecutive or sequential reactions. In such reactions the product is not formed directly from the reactant. Various steps in the consecutive reaction are shown as below :
A --- k1--- > B --- k1 --- > C
A = reactant ; B = intermediate ; C = product. Initially only the reactant A will be present. As the reaction starts, A produces an intermediate B through k1 rate constant. As and when B is formed, it produces the product C through k2 rate constant. After the completion of reaction only 'C' is present and concentrations of A and B will be zero.
Example of consecutive reactions
Saponification of a diester in presence of an alkali :
R'OOC- (CH 2)n-COOR ---k1 --- > R'OOC-(CH 2)n-COOH -- -- k2 --- > HOOC - (CH 2)n - COOH
(ii) Parallel reactions
In these group of reactions, one or more reactants react simultaneously in two or more pathways to give two or more products. The parallel reactions are also called as side reactions.
The reactant A reacts to give products B,C,D separately by following three different reaction pathways involving different k1, k2, k3 rate constants respectively. Among the many side reactions, the reaction in which maximum yield of the product obtained is called as the main or major reaction while the other reactions are called as side or parallel reactions.
Examples of parallel reaction :
Bromination of bromobenzene :
(iii) Opposing reactions
In opposing reactions the products formed react back simultaneously to form the reactants. These reactions are also called as reversible reactions.
A + B -- Kf-- > < --Kr ---C + D
Examples of opposing reactions
(i) Reaction between CO and NO2 gases
(ii) Isomerisation of cyclopropane to propene
(iii) Dissociation of hydrogen iodide in gas phase
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