If you add a strong base to a halide, you get mostly the alkene. If you add a weak base, especialy on primary halides not branched on the β carbon, the product is mostly the substituted. Why is that? 1) The mechanism for the substitution reaction is the heterotytic break of the C-X (where X is the halide) bond, producing the carbocation and X-. The carbocation, though, is more stable as more alkyl groups are added to the α (or β) carbon. In other words, terciary halides and secondary halides produce a much more stable carbocation than primary halides. So why is the primary the only one that can be feasibly substituted? 2) Why a stronger base facilitates the elimination? 3) The reactivity order on substitutions is I>Br>Cl>F. Why?