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What is the Markovnikov rule?

The Markovnikov ‘s rule is an empirical rule formulated in 1869 by the Russian chemist Vladimir Markovnikov, used to predict the regioselectivity of the product of electrophilic addition. Even today it is still valid, revealing its simplicity the way in which the mechanism of various organic reactions proceeds.

This rule applies especially to alkenes in their hydrohalogenation reactions. Thus, it allows predicting which will be the major products when alkenes and alkynes react with HX (HF, HCl, HBr and HI).

Suppose for example the hydrobromination of propene or propylene (upper image). Markovnikov’s rule predicts that the majority product will be the one in which the bromine atom binds to the carbon in the center; while in the minor product, the bromine ends up attached to the terminal carbon of the double bond.

Also note that the major product has the hydrogen atom attached to the terminal carbon. This last point is the easiest to memorize: the hydrogen in HX goes to the carbon of the double bond that has more hydrogen or CH bonds.

Electrophilic addition

Molecules such as HX can be added to the double bonds of alkenes or alkynes. HX has an electron-deficient atom, which is H, and an electron-rich atom, which is X.

Therefore, HX can be represented as H δ + -X δ- , with the H δ + region being susceptible to receiving electrons from, in this case, the double bond of an alkene, electrophilic attack taking place (see image below).

Stability of carbocations

Formation of the two carbocations in the hydrobromination of 1-butene. Source: Gabriel Bolívar via MolView.

As seen above, the 1-butene double bond attacks the hydrogen in HBr. In doing so, the electrons of the H-Br bond “open” towards the bromine atom, forming a carbocation and the Br  anion . However, there is the possibility of forming two carbocations: one primary (1st) and the other secondary (2nd).

Each carbocation is an intermediate compound from which the final product will be derived. As there are two carbocations, we will then have two products at the end: 2-bromobutane and 1-bromobutane. Br  acts as a nucleophile attacking either of the two carbocations:

Products of the hydrobromination of 1-butene. Source: Gabriel Bolívar via MolView.

But the proportion of both products is not 50%; 2-bromobutane is produced in much greater quantity than 1-bromobutane.

The rich get richer

Markovnikov’s rule was born from the final observations, and from them the mechanisms illustrated above could be considered to be able to explain them.

Without the need to always capture these mechanisms, or to deduce the stability of the intermediate carbocations, the simple rule is used: in an electrophilic addition, the hydrogen atoms are directed towards the carbon of the double with more hydrogen atoms. Thus, the more hydrogenated carbon becomes even more hydrogenated.

On the other hand, the X atom, in this case the Br, ends on the carbon of the double bond that has more CC bonds, or that is more substituted by other atoms that are not hydrogen. Because in 1-butene its terminal carbon is = CH 2 , this is the one that receives the hydrogen to transform into the -CH 3 group .

Examples

Above we saw the case of the Markovnikov rule applied in the hydrobromination reaction. It is also valid for hydrochlorination, hydroiodination, hydroamination and hydration reactions, whether for alkenes or alkynes.

In the next section you will see some additional examples as exercises.

Solved exercises

Exercise 1

Hydrobromination of 2-methylcyclopentene. Source: Gabriel Bolívar via MolView.

Markovnikov’s rule says that the H of HBr for the upper reaction must go to the carbon of the double bond with more hydrogen atoms. In this case, it corresponds to the carbon above, since the carbon on the right, linked to CH 3 , does not even have hydrogen atoms.

Thus, the hydrogen from the HBr goes to the carbon above, while the Br ends up bound to the carbon on the right (major product). On the other hand, if hydrogen bonds at the carbon on the right, the Br goes to the carbon above (minor product).

Note that the majority product consists of a 3º halide, more stable by definition and stabilization effects of partial positive charges; while the minority product is a halide 2º, less stable.

Both products, the majority and the minority, are obtained in practice, since for steric and random reasons it will always be probable that the hydrogen goes to the carbon of the double bond with less hydrogen. This type of minority product, contrary to the Markovnikov rule, is known as an antimarkovnikov product.

Exercise 2

Hydrobromination of 3-methyl-2-pentene. Source: Gabriel Bolívar via MolView.

Now see the example of 3-methyl-2-pentene.

If you look at it, the right carbon of the double bond has an H, while the left carbon has none. Therefore, the H of the HBr will go towards that carbon, the Br having to go to the carbon in the center to originate product B. Product B is the majority, since again it is a 3º halide, with product A being a 2º halide .

Exercise 3

Example of Markovnikov’s rule in the hydration of a cyclic alkene. Source: Gabriel Bolívar via MolView.

The previous examples would have generated the same products if HCl and HI were used instead of HBr. The same is the same as with H 2 O in an acid and heat catalyzed hydration reaction. The OH comes to replace the X atom of the HX.

Therefore, Markovnikov’s rule tells us that in the above electrophilic addition, the OH will go to the carbon of the most substituted double bond, with less hydrogens.

Thus, product A is the majority, and product B the minority. Note again that product A is a 3rd alcohol, and product B a 2nd alcohol.

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