Chemistry and the Markovnikov Rule: Predicting Reaction Outcomes

by Yuvi K - December 17, 2023

Chemistry and the Markovnikov Rule: What is it and How Does it Make Life Easier for a Chemist?

Chemistry can often feel overwhelming at times—not least when you’re trying to predict outcomes of complicated reactions. If you’re ever perplexed by chemical reactions, you can take a moment to be grateful that the Markovnikov Rule exists! Originally proposed by Russian chemist Vladimir Markovnikov in 1869, this biogenic law can be used to easily predict the outcomes of certain chemical reactions. This article will explain how the Markovnikov Rule can help you make better sense of chemical reactions.

What is the Markovnikov Rule?

The Markovnikov Rule (also known as the Markovnikov Principle or Markovnikov’s Law) is a method of predicting the outcome of a chemical reaction involving the addition of two different molecules. By using the Markovnikov Rule, you can accurately decide which molecules will end up together and which of the reacting molecules will bond to form the desired product.

Here’s a quick example of the Markovnikov Rule. Let’s say we have a reaction between an alkene and a hydrogen halide. The Markovnikov Rule states that the hydrogen atom will bond with the carbon in the alkene that has the most hydrogen atoms already attached. This means that the hydrogen will bond with the carbon that already has two hydrogens, rather than the one that has just one. The halide will then bind to the other carbon with only one hydrogen.

The History of the Markovnikov Rule

The Markovnikov Rule is named after Russian chemist and professor at Moscow University, Vladimir Markovnikov. Markovnikov attempted to explain why a certain type of reaction called an “addition reaction” happened the way it did—a puzzle that had puzzled chemists for over a century before his groundbreaking findings.

In 1869, Markovnikov was able to accurately predict the outcomes of addition reactions between an alkene and a halogen. His rules were soon accepted throughout the scientific community, and are still used (with some modifications) today. However, it wasn’t until the 1960s and 1970s that scientists were able to fully understand why Markovnikov’s predictions turned out to be accurate.

The Mechanism Behind the Markovnikov Rule

At its most basic level, the Markovnikov Rule is all about stability. When two different molecules come together to form a new molecule, they create a new “intermediate” molecule with a charge distribution that is different from either of the starting molecules. The intermediate molecule can gain or lose stability depending on the charge distribution of the starting molecules. The Markovnikov Rule helps chemists determine which molecule will gain the most stability, and thus which molecule will end up bonded together in the final product.

For example, when a hydrogen halide is added to an alkene, theMarkovnikov Rule states that the hydrogen will bond with the carbon that already has two hydrogens, and the halide will bond with the other carbon. This is because the intermediate molecule looks similar to the product of a substitution reaction (where the two molecules stay the same, but rearrange their bonding). This intermediate is then more likely to transition into the final product.

Using the Markovnikov Rule in Practice

Reactions between two different molecules can often be difficult to predict. However, by using the Markovnikov Rule, chemists are able to make educated guesses about the outcome of certain reactions. This saves a lot of time and effort for those in the field who can use this rule to their advantage.

To put the Markovnikov Rule into practice, first identify the two molecules that will be reacting. Then, predict the charge distribution of the intermediate molecule and determine which molecules will be the most stable when combined. When you’ve decided which molecules will be most stable, you can accurately predict the outcome of the reaction.

Conclusion

The Markovnikov Rule can be a great tool for predicting outcomes of certain reactions between two different molecules. By making educated guesses about the charge distribution of the intermediate molecule, you can accurately determine which molecules will be the most stable when combined—and thus accurately predict the outcome of the reaction. As you can see, the Markovnikov Rule can take the guesswork out of complicated reactions and make life a lot simpler for the chemist.