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.

Naming Alkenes

Alkenes are named using a series of rules that are similar to those for alkanes, except with the suffix –ene instead of –ane to identify the functional group.

Step 1: Name the parent hydrocarbon.  Find the longest carbon chain containing a double bond. 

Name the compound based off the number of carbons, using the suffix –ene.

Step 2: Number the carbon atoms in the chain.  Begin at the end closest to the double bond, or if

the double bond is exactly in the center of the chain, begin at the end closer to the first branch point. 

This assures that the double bond receives the lowest number possible.

Step 3: Write the Full Name.  Number the substituents according to their position on the parent

chain, and list alphabetically.  Show the position of the double bond by giving the number of the first

alkene carbon directly before the parent name.  If there is more than one double bond, show the

position of each double bond and use the suffix  –diene, -triene and so forth.

In 1993 IUPAC changed their naming recommendations to place the position of the double bond immediately before the –ene suffix rather than before the parent name.  This change is not widely accepted in the United States, so the old system is used.

Cycloalkenes are named in a similar fashion, but because there is no end to the chain the cycloalkene is named so the double bond is between C1 and C2.  The first substituent has the lowest possible just like in a regular chain.  The position of the double bond is not necessary to mention in the name, because the double bond is always assumed to be located between C1 and C2.  In the new naming system the locant in position right before the suffix in a diene.

Some alkenes names have been used for so long that they are accepted despite the fact that they do not follow the IUPAC naming system.  For example, ethene is the alkene derived from ethane, but the name ethylene has been used for so long that it has been accepted by IUPAC.

Here are a few molecules with more accepted common names:

Naming alkenes are very similar to naming alkanes.  Identifying where the double bond is, is the key to successfully naming the molecule.