Reactions of nitromethane on Si(100) are investigated using density functional theory. Calculations show that nitromethane should strongly chemisorb to the surface dimer by means of a 1,3-dipolar cycloaddition reaction pathway, with no significant activation barrier. Though the 1,3-dipolar cycloaddition results in a strongly bound product, this initial cycloadduct is metastable with respect to rearrangement products formed through oxygen migrations between lattice silicons. Multiple products are likely to be present on the surface, so that it will not be well ordered. This reaction suggests a new route for attaching organic monolayers to Si(100) and provides a model system for bonding in silicon oxynitride films.