The principle of microscopic reversibility (PMR), Curtin-Hammett principle (CHP) and Hammond postulate (HP) are the three pillars of organic reactivity. PMR enables us to construct a reasonable reaction path if the mechanism of the reverse process is known. CHP tells us that stability and reactivity do not necessarily correlate, whereas HP allows us to infer about the transition state structure using intermediates that are sometimes detectable and/or isolable. It is difficult to see what we would do without these fundamentals.
I am teaching my first year organic chemistry course and, while we do not go into the details of PMR and CHP, we do discuss the main elements of HP. The reason HP is useful is that it allows us to weed out the pervasive fallacy “Compound Z must be the reaction product because Z is more stable than the alternative Y”. While I have a habit of saying that it is generally incorrect to state that more stable things are kinetically preferred, it is not necessarily true and here is an example.
Professor Alabugin (http://www.chem.fsu.edu/~alabugin/), whose great talk I just heard at the conference in Moscow, recently published an influential paper in which he re-examined the course of some anionic cyclizations. It appears that there must be a major re-evaluation of some stomping grounds of cyclization mechanisms involving alkynes. The 4-endo/5-exo comparison presents a particularly interesting scenario: the reason for the lower barrier to form the 5-membered ring lies in the exothermicity of the reaction. In other words, the transition state that leads to the formation of the 5-membered ring is lower because the product is more stable. This is an important application of the Marcus theory to polar mechanisms.