Hydration of Alkynes = Markovnikov Addition of Water | Keto-Enol Tautomerism

Summary

In this video, Victor, your organic chemistry tutor, explains the hydration of alkynes, a reaction where water is added to alkyne molecules. The process is carried out in the presence of a Bronsted acid or catalytic Mercury ions, forming an enol intermediate that undergoes keto-enol tautomerization to yield a ketone. Victor walks through the acid-catalyzed and mercury-catalyzed mechanisms, providing detailed insights and examples using propene. He also gives shortcuts for predicting products and shares a visual animation to help clarify the process. The video wraps up with a recap and a challenge to draw the mechanisms for practice.

Takeaways

• 😀 Hydration of alkynes involves the addition of water to alkyne molecules, typically in the presence of an acid catalyst or mercury ions.
• 😀 Both acid-catalyzed and mercury-catalyzed reactions produce an unstable enol intermediate, which undergoes keto-enol tautomerization to form a ketone.
• 😀 The acid-catalyzed mechanism starts with electrophilic attack, forming a carbocation (secondary) that is then attacked by water, resulting in the enol intermediate.
• 😀 The enol intermediate is unstable and undergoes keto-enol tautomerization, converting to a ketone as the final product.
• 😀 The mercury-catalyzed mechanism also begins with electrophilic attack by mercury ions, forming a mercurinium ion intermediate.
• 😀 In the mercury-catalyzed reaction, the mercurinium ion opens from the more substituted side, leading to a protonated intermediate that ultimately forms the enol.
• 😀 Both mechanisms (acid and mercury catalysis) involve protonation of the alkyne followed by nucleophilic attack by water.
• 😀 The final product of both reactions is a ketone, as the enol intermediate is highly unstable and rarely the final product.
• 😀 A shortcut for predicting the product is to use the Markovnikov rule: the -OH group adds to the more substituted carbon of the alkyne.
• 😀 The video also provides a visual aid to help students predict the product by remembering the steps of protonation and deprotonation in the mechanism.

Q & A

What is the main reaction discussed in this tutorial?

The main reaction discussed is the hydration of alkynes, which involves the addition of water to alkyne molecules to form an enol intermediate that eventually undergoes keto-enol tautomerization to yield a ketone.

What is the role of the catalyst in the hydration of alkynes?

The catalyst used in the hydration of alkynes can be either sulfuric acid (H2SO4) or a catalytic amount of mercury ions (Hg2+), which facilitate the reaction under milder conditions compared to using pure acid.

What is the enol intermediate, and why is it important in the hydration of alkynes?

The enol intermediate is a molecule in which the oxygen atom is bonded to one of the carbons of the original double bond. It is important because it represents a key intermediate in the hydration process, though it is unstable and will undergo keto-enol tautomerization to form a ketone.

What is the final product of the hydration of alkynes?

The final product of the hydration of alkynes is a ketone, formed after the enol intermediate undergoes keto-enol tautomerization.

What does keto-enol tautomerization mean in this reaction?

Keto-enol tautomerization refers to the equilibrium between the enol (with a hydroxyl group attached to a carbon-carbon double bond) and the ketone (with a carbonyl group). This equilibrium favors the ketone form, making it the final product in this reaction.

How does the Markovnikov rule apply to the hydration of alkynes?

In the hydration of alkynes, the Markovnikov rule dictates that the hydroxyl group (OH) will attach to the more substituted carbon of the alkyne, while the hydrogen atom (H) will attach to the less substituted carbon. This ensures the formation of the more stable intermediate.

What happens during the acid-catalyzed mechanism of alkyne hydration?

In the acid-catalyzed mechanism, the alkyne undergoes an electrophilic attack by a proton (H+) to form a carbocation. This is followed by a nucleophilic attack by water, which leads to the formation of a protonated enol intermediate. Finally, the enol undergoes keto-enol tautomerization to form a ketone.

Why is the primary carbocation rarely formed in the hydration of alkynes?

The primary carbocation is rarely formed because it is highly unstable. In the mechanism, the more stable secondary carbocation is formed instead, as it is lower in energy and more likely to lead to the final product.

What is the significance of the Mercury-catalyzed mechanism?

The Mercury-catalyzed mechanism for the hydration of alkynes is more complex and involves the formation of a mercurinium ion. This mechanism is similar to the oxymercuration of alkenes, but in this case, the mercuric ion facilitates the hydration under milder conditions than the acid-catalyzed mechanism.

How can students predict the product of the hydration of alkynes without going through the entire mechanism?

Students can predict the product of the hydration of alkynes by applying the Markovnikov rule, remembering that the more substituted carbon will attract the hydroxyl group and the less substituted carbon will attract the hydrogen. This allows them to directly write the final product, which is a ketone, without needing to go through the entire mechanism.