This paper reports computational data for the energetics of internal attacks, both in ring-opening reactions (eq 3) where strain energy is released and in model, strain-free systems (eq 4). A comparison is drawn with the corresponding bimolecular processes. The exothermicity of three-membered ring-opening reactions is significantly larger than that of the four-membered ring systems. However, using the Marcus equation, it is shown that the higher reactivity of the three-membered rings is intrinsic to the system and does not stem only from a higher thermodynamic driving force. The intrinsic barriers for the strain-free reactions are shown to be dominated by the position of the nucleophilic and nucleofugic atoms in the periodic table, as in the bimolecular SN2 reactions, although a pi rather than a sigma bond is formed in these reactions.