What reaction is expectable from the atomized reactants?
The rate for a reaction is dependent upon the rate constant and, depending upon what type of reaction it is, the concentrations of the reactants.  The rate constant can be further broken down into an entropic factor (how likely are the reactants to collide in an orientation that will produce a reaction) and an energy factor (the activation energy).

Assuming for the moment that the reactant concentrations are such that they will not control the direction of reaction, and that the entropic factors don't differ dramatically for one pathway over another (that's usually the case for similar reactions), then the first reaction to happen will be the one involving the reaction with the lowest activation energy.  This will lead to the kinetically controlled product.

If the back (reverse) reaction from the kinetically controlled product also has a low enough activation energy such that it happens rapidly under our reaction conditions, then essentially we are setting up an equilibrium.  In this case, thermodynamics will eventually control the product distribution and we will eventually wind up with the product that has the lowest free energy, the thermodynamically controlled product.

Many reactions first produce the kinetically controlled product, but with time, the thermodynamic product becomes the major product.