  We show that products of the isotopic substitution reactions in
experimentally accessible molecules such as NaK, RbCs, and SrF are cold
according to their translational energy below hundreds of mK. For these
chemical reactions, molecular products may occupy only the lowest rotational
states. We also discuss the possibility of controlling the chemical reactions
by the electric field in ultracold mixtures of molecules and atoms with low
kinetic energy release, where one of the constituent atoms of colliding
molecule is replaced by its isotope. This letter opens new avenues in
investigating the branching ratios of chemical reactions in ultracold
conditions.
