Side reaction

A side reaction is a chemical reaction that occurs at the same time as the actual main reaction, but to a lesser extent. It leads to the formation of by product, so that the yield of main product is reduced:

{A} + B >[{k 1}] P1

{A} + C >[{k 2}] P2

P1 is the main product if k1> k2. The by product P2 is generally undesirable and must be separated from the actual main product (usually in a costly process).

B and C from the above equations usually represent different compounds. However, they could also just be different positions in the same molecule. A side reaction is also referred to as competing reaction when different compounds (B, C) compete for another reactant (A). If the side reaction occurs about as often as the main reaction, it is spoken of parallel reactions If the reaction is carried out at low temperatures and stopped after a short time, it is spoken of kinetic control, primarily the kinetic product B would be formed. When the reaction is carried out at high temperatures and for long time (in which case the necessary activation energy for the reaction to C is available, which is progressively formed over time), it is spoken of thermodynamic control; the thermodynamic product C is primarily formed.

In organic synthesis, elevated temperatures usually lead to more side products. Side products are usually undesirable, therefore low temperatures are preferred ("mild conditions"). The ratio between competing reactions may be influenced by a change in temperature because their activation energies are different in most cases. Reactions with high activation energy can be more strongly accelerated by an increase in temperature than those with low activation energy. Also, the state of equilibrium depends on temperature. Detection reactions can be distorted by side reactions.