Side reaction

${\displaystyle \text{A}+\text{B}\stackrel{k_1}{\to }{\text{P}}_1^{}}$

${\displaystyle \text{A}+\text{C}\stackrel{k_2}{\to }{\text{P}}_2^{}}$

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).

In organic synthesis

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 a 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 called a parallel reaction, especially in kinetics.

There may also be more complicated relationships; for example, Compound A could reversibly but quickly react to substance B (with speed k₁), or irreversibly but slowly (k₁> k₋₁ >> k₂) to substance C:

${\displaystyle \text{B}\stackrel{-\rightharpoonup }{\leftharpoondown -}\text{A}\stackrel{k_2}{\to }\text{C}}$

Assuming that the reaction to substance C is irreversible, as it is thermodynamically very stable, B is the kinetic and C is the thermodynamic product of the reaction. If the reaction is carried out at low temperatures and stopped after a short time, it is called kinetic control; the kinetic product, B, is the primary substance 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 called thermodynamic control; the thermodynamic product, C, is the primary substance formed.

Conditions for side reactions

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.