The electron-induced reaction of physisorbed vinyl bromide (ViBr) and allyl bromide (AllBr) on Cu(110) at 4.6 K was studied experimentally by scanning tunneling microscopy and theoretically by molecular dynamics. ViBr and AllBr were found to react by two pathways: "Direct", in which the molecule reacted under the tip, and "Delayed", in which reaction occurred spontaneously after the molecule had diffused across the surface away from the tip. The novel pathway of Delayed reaction constituted a major route for both vinyl bromide (68%) and allyl bromide (53%). The observed reaction dynamics for ViBr and AllBr gave evidence of a long-lived vibrationally excited intermediate for both Direct and Delayed reactions. Molecular dynamics simulations with reagent excitation by way of selected vibrational normal modes resulted in either Direct or Delayed reaction, depending on the vibrational mode.