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:^[1]

${\displaystyle {\ce {{A}+B->[{k_{1}}]P1}}}$

${\displaystyle {\ce {{A}+C->[{k_{2}}]P2}}}$

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).

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 competing reaction^[2][3] 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^[4] (especially in the kinetics, see below).

Also there may be more complicated relationships: Compound A could reversibly but quickly react to substance B (with speed k₁) or irreversible but slow (k₁> k₋₁ >> k₂) to substance C:

${\displaystyle {\ce {B<=>A->[{k_{2}}]C}}}$

Thermodynamic and kinetic product.

Assuming that the reaction to substance C is irreversible, as it is thermodynamically very stable. In this case, B is the kinetic and C is the thermodynamic product of the reaction (see also here).^[5][6][7] 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.

Kinetics

Side reactions are also described in the reaction kinetics, a branch of physical chemistry. Side reactions are understood as complex reaction, since the overall reaction (main reaction + side reaction) is composed of several (at least two) elementary reactions.^[9] Other complex reactions are competing reactions, parallel reactions, consecutive reactions, chain reactions, reversible reactions, etc. [10]^{[10]: 280–291}

If one reaction occurs much faster than the other one (k₁ > k₂), it (k₁) will be called the main reaction, the other one (k₂) side reaction. If both reactions roughly of same speed (k₁ ≅ k₂) is spoken of parallel reactions.^[4]

If the reactions ${\displaystyle {\ce {{A}+B->[{k_{1}}]P1}}}$ and ${\displaystyle {\ce {{A}+C->[{k_{2}}]P2}}}$ are irreversibly (without reverse reaction), then the ratio of P₁ and P₂ corresponds to the relative reactivity of B and C compared with A:

${\displaystyle {\frac {\ce {[P1]}}{\ce {[P2]}}}={\frac {k_{1}[{\ce {B}}]}{k_{2}[{\ce {C}}]}}}$

See also

• Rate equation § Parallel or competitive reactions

References