Theoretical basis of organic reactions

Every organic reaction, at bottom, comes down to one or more of a molecule's covalent bonds being broken while one or more new covalent bonds are simultaneously formed, usually because a second reactant is involved in driving that change. The particular reactant that ends up SUPPLYING the carbon atom for the newly-formed bond is called the substrate; the OTHER reactant, the one actively bringing about the change, is called the reagent -- illustrated by the reaction $CH_4 + Cl_2 \xrightarrow{h\nu} CH_3Cl + HCl$, in which methane is the substrate (it supplies the carbon that ends up in the product), chlorine is the reagent (it drives the substitution), methyl chloride is the product of actual interest, and HCl is simply a byproduct generated alongside it. Because covalent bonds are built from the constituent atoms' own valence electrons, every bond-breaking or bond-forming step in a reaction is really a redistribution of those valence electrons -- and because that redistribution does not happen instantaneously, the overall reaction is typically 'punctuated' along the way by the formation of one or more short-lived, inherently unstable intermediate species, so most real organic reactions genuinely proceed as multi-step processes rather than in one single leap. A reaction MECHANISM is the complete, ordered account of exactly what happens at each individual step of such a multi-step reaction: the electron movement taking place at that step, the bond(s) being broken and/or formed, the accompanying changes in energy and in molecular shape, and the overall rate of the reaction -- and understanding a reaction's mechanism is valuable both for genuinely understanding WHY a given class of organic compound reacts the way it does, and for practically planning out a synthetic route to a target molecule. The rest of section 14.6 develops, in turn: th …