We study the differential reactivity of alkanethiols with submonolayers of Au on Si(111), where the surface is a mixture of the Au-rich 3×3 structure and the Au-poor 5 × 2 structure. We expose these binary 2D alloy samples to alkanethiols, while monitoring the induced spatial and structural transformations in real time using a low energy electron microscope (LEEM). Energy-dependent diffraction measurements show that, surprisingly, there is no alkanethiol growth on the Au submonolayers. Instead, S-Si interactions induce a disproportionation of the 5 × 2 structure into the unexpectedly inert 3×3 phase and a Au-depleted phase. We show how this process can be used to reduce the dimensionality of a 2D electron system to 1D.