Motivated by the selective binding of multifunctional organic molecules at the semiconductor surfaces, theoretical calculations of benzonitrile at the Si(I 00)-2 x 1 surface are carried out in order to explore the principles that control the competition and selectivity of surface reactions between the phenyl ring and the cyano group. B3LYP density functional theory with a Si9H12 one-dimer cluster model is used to mimic surface reactions. The possible pathways of C2=C1-C=-N [4 + 2] cycloaddition, CdropN [2 + 2] cycloaddition, Cdouble bondC [2 + 2] cycloaddition, Cdouble bondC [4 + 2] cycloaddition, and C-H dissociation are investigated. The calculations illustrate that CdropN [2 + 2] cycloaddition is the selective pathway for chemisorbed benzonitrile at the Si (100)-2x I surface.