Addition of water (acid-catalyzed) mechanism | Organic chemistry | Khan Academy

Summary

The script explains the mechanism of an acid-catalyzed hydration reaction of 2-methyl-pent-2-ene. In this reaction, the double bond of the alkene is broken, and a hydrogen and hydroxyl group are added across the double bond. The process follows Markovnikov's rule, with the more stable tertiary carbocation forming, leading to the addition of water. The reaction involves the transfer of a proton and the formation of a carbocation, followed by the nucleophilic attack of water. The result is the conversion of the alkene into an alcohol, demonstrating the acid-catalyzed addition of water.

Takeaways

• 😀 The reaction mechanism involves starting with a methyl group on the second carbon of pentene and ending with a hydroxyl group on the same carbon after an acid-catalyzed reaction.
• 😀 The starting molecule is 2-methyl-pent-2-ene, which undergoes an acid-catalyzed addition of water, involving the addition of hydrogen and hydroxyl groups.
• 😀 In the presence of hydronium, the double bond in the alkene breaks, leading to the formation of a carbocation and the addition of a hydroxyl group and a hydrogen atom.
• 😀 The mechanism follows Markovnikov's rule, which states that the carbon already bonded to more hydrogens will gain more hydrogens, and the carbon with more functional groups will gain those functional groups.
• 😀 The formation of a tertiary carbocation is favored due to its greater stability compared to a secondary carbocation.
• 😀 The reaction progresses through an electrophilic addition, where the double bond reacts with a proton from hydronium to form a carbocation.
• 😀 A water molecule can interact with the carbocation, leading to the addition of a hydroxyl group, and the reaction proceeds in equilibrium.
• 😀 The reaction culminates in the neutralization of the positively charged carbocation by water, which transfers a proton to form hydronium.
• 😀 The final product of the reaction is 2-methylpentan-2-ol, formed by the addition of water (H2O) across the double bond.
• 😀 Acid-catalyzed hydration reactions involve the use of hydronium (H3O+) to facilitate the breaking of the double bond, leading to the formation of an alcohol.

Q & A

What is the starting molecule in this reaction?

The starting molecule is 2-methyl-pent-2-ene, which is a pentene with a methyl group attached to the second carbon and a double bond between the second and third carbons.

What is the key reaction occurring in this process?

The key reaction is an acid-catalyzed addition of water to the alkene, where the double bond is broken, a hydrogen is added to one carbon, and a hydroxyl group is added to the other carbon.

What does Markovnikov's rule explain in this reaction?

Markovnikov's rule states that, in an alkene reaction, the carbon with the most hydrogens will receive more hydrogens, and the carbon with the more functional groups will receive more functional groups, guiding the direction of the addition.

What role does hydronium (H3O+) play in this reaction?

Hydronium (H3O+) acts as the catalyst in this reaction. It donates a proton (H+) to the alkene, initiating the formation of a carbocation intermediate.

Why is the tertiary carbocation more stable than the secondary carbocation?

The tertiary carbocation is more stable because it can distribute the positive charge across more carbon atoms, making it less likely to break apart compared to a secondary carbocation, which can only distribute the charge to fewer carbons.

What happens after the carbocation forms in the reaction?

After the carbocation forms, a water molecule can react with the positively charged carbon (carbocation), leading to the addition of a hydroxyl group, completing the addition of water.

How does the addition of a hydroxyl group occur?

The addition of the hydroxyl group occurs when a water molecule, attracted by the positive charge of the carbocation, donates a lone pair to bond with the carbon, forming a bond with the carbon while the oxygen retains its lone pair.

What is the final step in the reaction mechanism?

The final step involves the removal of a proton from the oxygen of the water molecule, forming neutral water (H2O) and regenerating the hydronium ion, allowing the reaction to be catalyzed again.

How is charge conservation maintained throughout the reaction?

Charge conservation is maintained by the positive charge shifting between intermediates, starting from hydronium (H3O+), transferring to the carbocation, and finally to the oxygen in the water molecule as it becomes hydronium again.

What does 'acid-catalyzed addition of water' mean in the context of this reaction?

'Acid-catalyzed addition of water' refers to the process where an acid catalyst (hydronium) promotes the addition of water to an alkene, resulting in the conversion of the double bond into a single bond and adding a hydrogen and hydroxyl group across the former double bond.