Going forward, there will be two big factors to keep in mind as you learn each addition reaction. “Regiochemistry” and”Stereochemistry”. Today, let’s talk about regiochemistry.
“Regiochemistry” [reeg-eee-o-chemistry] refers to how certain reactions end up forming one constitutional isomer over the other. Yesterday I said that every addition reaction involves breaking C-C (Pi) and forming a C-X and a C-Y bond. If X and Y are different atoms, then we have the potential to form two different constitutional isomers.
Many, many years ago, Viktor Markovnikoff noticed that when strong acids such as H-Cl add to alkenes, one type of product was preferred: the hydrogen added to the end of the double bond attached to the fewest carbons (“least substituted”) and the halide added to the end of the double bond attached to the most carbons (“most substituted”).
Important: he didn’t know why, at the time. He just made the observation that it happened. [This is how science works! We make observations first. Understanding “why” comes later.]
In our example notice that C-2 of the alkene is attached to one H and one carbon (“R”), whereas C-1 of the alkene is attached to two hydrogens. So C-2 is the “most substituted” carbon. When we add H-Br to this alkene, the major product is the one where Br adds to C-2 (the “most substituted”) and the H adds to C-1 (the “least substituted”). This is “Markovnikoff’s rule”. (In this case we also get a small amount (~10-20%) of the opposite “regioisomer” – the “anti-Markovnikoff” product).
As we’ll see, certain reactions follow Markovnikoff’s rule. In others, Markovnikoff’s rule doesn’t operate, because the two atoms we’re adding to the alkene are exactly the same. Finally, there are even reactions (2 of them) in which the “anti-Markovnikoff” product is formed selectively.
A few days from now we’ll go into why Markovnikoff’s rule operates when we add HBr [as well as HCl, HI, and H3O(+)] to alkenes. For today, just make sure you can recognize which end of an alkene is the “most” and “least” substituted.
Bottom line: when you hear “regioselectivity”, think: “Markovnikoff or anti-Markovnikoff”.
Thanks for reading! James