You Know That One Russian Guy…

                 Vladimir Vasilyevich Markovnikov was a Russian organic chemist who worked in the mid 1800’s. During his research working with halides and alkenes, he was looking for any similarities between all of the different reactions.  He noticed a common occurrence that some reactions yielded only one product when they were thought to be able to yield more than one possible product. After examining the reaction more closely, it was noticed that the halide in the only product was bonded to the carbon with a higher degree of substitution. Therefore, the halide would be bonded to, for example, a tertiary carbon rather than a primary or secondary carbon. This and later research led to the formation of the Markovnikov rule, which was widely accepted in the world of organic chemistry. This rule states that in the addition of HX, X being a halide such as bromine and chlorine (excluding fluorine and iodine), to an alkene, the more highly substituted carbocation is formed as the intermediate rather than the less highly substituted one.

                Markovnikov wasn’t the only one to notice the absence of a product after predicting more would occur. This has been a common dilemma for many other researchers as well, including George Kimball and Irving Roberts who conducted research in 1937. During Kimball and Robert’s research with bromine gas and chlorine gas additions to alkenes, they observed only one product, which led them to question why this occurred. The answer to this, they proposed, was instead of the intermediate being a carbocation, like one would normally predict, the intermediate was a bromonium ion or a chloronium ion. These ions would form from a nucleophilic attack from bromine or chlorine. The formation of the ion results in anti-stereochemistry, which is observed in reactions of cycloalkenes with halides. Rather than both a cis and trans product, only trans product was formed. This is because once there’s a bromonium ion on one side of the cycloalkane; the other negatively charged bromine will attack from the opposite side since the large bromine is shielding one whole side. 

                There are also reactions that yield a halohydrin. This involves a reaction taking place between an alkene, X2, and H2O, when X is either Br or Cl. Since this reaction takes place in water, the water molecules are able to compete with the Br- ion as a nucleophile and reacts with the bromonium ion intermediate. This results in the formation of a bromohydrin.

                 In the reaction, once the bromonium ion is formed, water acts as a nucleophile, breaking the ion ring and attaching itself to a carbon. The oxygen becomes positively charged, and as a result of being in water, the oxygen loses a proton in a process called deprotonation, and creates a H3O+.

                 Alkene oxymercuration is closely analogous to halohydrin formation. The reaction is initiated by electrophilic addition of Hg2+ ion to the alkene to give an intermediate mercurium ion, whose structure resembles that of a bromonium ion. Nucleophilic addition of water as in halohydrin formation, followed by deprotonation, then yields a stable organomercury product.

                  The final step, demercuration of the organomercury compound by reaction with sodium borohydrate. Note that the regiochemistry of the reaction corresponds to Markovnikov addition of water, that is, the –OH group attaches to the more highly substituted carbon atom, and the –H attaches to the less highly substituted carbon. The hydrogen that replaces mercury in the demercuration step can attach from either side of the molecule depending on the exact circumstances. All of these are regiospecific reactions, meaning that they produce one structural, or constitutional, isomer over all others.