Parallel or Competing Reaction 

The reactions in which a substance reacts or decomposes in more than one way are called parallel or side reactions.

If we assume that both  of them are first order, we get.

k₁ = fractional yield of B × k_av

k₂ = fractional yield of C × k_av

If k₁ >  k₂ then

A → B main and

A → C is side reaction

Let after a definite interval x mol/litre of B and y mol/litre of C are formed.

i.e

This means that irrespective of how much time is elapsed, the ratio of concentration of B to that  of C from the start (assuming no B  and C in the beginning ) is a constant equal to k₁/k₂.

Refer to the following video for parallel reactions:

Example of Parallel Reactions