The microscopic evolution of a one parameter reaction system in a dense phase is represented, in the diffusion limit, by a Smoluchowski equation with a bistable potential. The WKB techniques recently developed are applied to the calculation of the time dependent reactive fluxk(t), defined as a derivative of the correlation function of the number of particles, in the case of a simplified model of n-butane isomerization already studied by Chandler and other authors. The reactive flux is expressed as a sum of decreasing exponentials; the larger characteristic times and corresponding numerical coefficients are computed explicitly. The results confirm the validity of the phenomenological rate law for times larger than 10-12s.