A set of 40 finite temperature ab initio molecular dynamics trajectories is employed to investigate the distribution of addition products and underlying microscopic mechanism of the addition of 1,3-butadiene to the Si(100)-2 x 1 surface. The product yields are in good agreement with recent STM measurements and include a Diels-Alder [4 + 2] adduct with a surface dimer acting as the dienophile, a [4 + 2]-like adduct that bridges two dimers within a row, a [4 + 2]-like adduct that bridges two dimers in adjacent rows, and an interdimer [2 + 2]-like adduct. The trajectories indicate that a common mechanism underlies the distribution and is predominantly a nonconcerted stepwise mechanism that proceeds via an intermediate zwitterion composed of a carbocation bonded to a negatively charged surface dimer.