The chemical consequences of the spatial confinement of electrons when an ethylene molecule is trapped into a microscopic cavity was studied. The pi electronic system of ethylene was modeled by means of the Huckel Molecular Orbital Theory in order to provide an easy interpretation of the mathematical results. It was determined that the reactivity of molecules was increased in two ways : An intrinsic one, which accounts for the weaking of the bond by decreasing the pi-pi* energy gap; and in an extrinsic way, which is related to the increase in the energy levels of the guest molecule with respect to those of the catalysts acid sites. This results in an enhancement of the base character of the molecule owing to the confinement. Finally the order of magnitude of the increment of energy produced on the ethylene pi system when it is confined in cavities of different pore sizes was determined. From our results it is expected that, for cavities smaller than 5-6-Angstrom size, the orbital energy levels of the guest molecule are increased by an amount of around 1 eV or greater.