A Rule of Thumb: "Higher Electronegativity seeks bonding with Lower Electronegativity" Before completing my PhD in organic chemistry, with a minor in inorganic, some 20 yrs ago, I realized that the main mechanism for most organic reactions and most inorganic reactions was one of negative seeking a positive. The trick is to identify which part of any molecule or reactive species (ions or radicals) has a negative nature and which part of that molecule or reactive species has a positive nature. This is fairly easy when we determine the electronegativity of the atoms in any molecule or reactive species. Those atoms with more electronegativity behave like negative species. Those atoms with less electroegativity behave like positive species. This is true for neutral molecules as well as ions and radicals. As an example of a reaction that is a bit less obvious: The Diels Alder reaction (4n+2). The diene part is the more negative species because it has more electron density. The ene part is the more positive species. The electron density of the diene is higher at the two ends of the diene because electrons that are not s-orbital type are "freer" to delocalize and due electron-electron repulsion the electron density moves to the end of the chain. I'm not suggesting that this method describes all chemical reactions, but it does describe a great many of them. If you mention steric factors then you are really talking about the electron-electron repulsion of the electrons in the hydrogens, and / or the proton-proton repulsion of the hydrogens. Steric factors are also affected or guided or determined by electron - electron repulsion. Your thoughts on this "rule of thumb" are most welcome.