Quasiclassical trajectory calculations using quantum mechanical energies and forces generated by the Venus and Gaussian programs provide for the first time a detailed dynamical picture of singlet carbene, CCl(2) and CF(2), cycloadditions to alkenes on the B3LYP/6-31G* surface. For CF(2), B3LYP/6-31G* with exact exchange reduced to 12% HF was also employed to better mimic the high accuracy surface. The range of geometries sampled in reactive trajectories and the timing of bond formation were explored. All trajectories follow the nonlinear approach proposed by Moore and Hoffmann. The reaction of CCl(2) with ethylene is a dynamically concerted reaction, with an average time gap between formation of the two bonds of 50 fs. The reaction of CF(2) with ethylene is dynamically complex with biexponential decay of the diradical species formed from the first bond formation. A general quantitative dynamical classification of cycloaddition mechanisms is proposed, based on the timing of bond formation.