Alkene Addition Reactions: Crash Course Organic Chemistry #16

Summary

In this episode of Crash Course Organic Chemistry, Deboki Chakravarti explores key alkene addition reactions, breaking down patterns that can predict the products without rote memorization. The episode covers halogenation, halohydrin formation, oxymercuration/reduction, and hydroboration, explaining the mechanisms, regioselectivity, and stereochemistry involved in each. Key concepts like Markovnikov's rule, anti-Markovnikov addition, and syn/anti stereochemistry are discussed in depth, with clear notecards summarizing the reactions. The episode emphasizes understanding the chemistry behind the reactions rather than memorizing them, and sets the stage for future exploration of oxidation and reduction in organic chemistry.

Takeaways

• 😀 The exploration of black pepper by Vasco da Gama highlights the role of organic chemicals like piperine in shaping history.
• 😀 Piperine, the compound responsible for the spiciness of black pepper, can undergo an isomerization reaction with sunlight, affecting its heat.
• 😀 Alkenes undergo addition reactions, where two groups add across the double bond, resulting in two new sigma bonds.
• 😀 Three key questions help predict alkene addition reaction products: 1) What is added? 2) Where do the groups add? 3) What is the stereochemistry?
• 😀 Markovnikov's rule guides the regioselectivity of asymmetrical alkene addition reactions, where a proton adds to the carbon with the most hydrogens.
• 😀 Anti-Markovnikov addition occurs with special reagents, where the proton adds to the carbon with fewer hydrogens.
• 😀 In halogenation, halogens (like chlorine or bromine) add across the double bond in an anti addition, with a bromonium ion as an intermediate.
• 😀 Halohydrin formation is a twist on halogenation, using water as a nucleophile, and it results in anti addition with regioselectivity toward the more substituted carbon.
• 😀 Oxymercuration/reduction involves mercury to prevent carbocation rearrangements, adding water or alcohol to alkenes with anti addition and Markovnikov regioselectivity.
• 😀 Hydroboration adds borane to alkenes in an anti-Markovnikov fashion with syn addition, followed by oxidation to replace boron with an alcohol group.

Q & A

What is the general pattern of reactions for alkenes in organic chemistry?

Alkenes are nucleophilic and can attack electrophiles, leading to the breaking of the pi bond and the formation of two new sigma bonds. The addition reactions add two groups across the double bond, one to each carbon.

What are the three key questions to consider when predicting alkene addition reactions?

The three key questions are: 1) What is being added across the double bond? 2) Where will the group add on an asymmetrical molecule? 3) What is the expected stereochemistry of the added groups?

What is Markovnikov's rule?

Markovnikov's rule states that in the addition of a proton to an asymmetrical alkene, the proton will add to the carbon with the most hydrogens, resulting in the most stable product.

What does anti-Markovnikov addition mean?

Anti-Markovnikov addition occurs when the proton adds to the carbon with fewer hydrogens, opposite of the typical Markovnikov addition, and is often seen with specific reactants.

What is the significance of stereochemistry in addition reactions?

Stereochemistry refers to how the groups add in three-dimensional space. If the groups add to the same face of the double bond, it’s syn addition; if they add to opposite faces, it’s anti addition.

How does halogenation of alkenes occur?

In halogenation, a halogen like chlorine or bromine adds across the alkene's double bond. This reaction is typically performed in a non-nucleophilic solvent like carbon tetrachloride and follows anti addition.

What is the role of the bromonium ion in halogenation?

The bromonium ion is a high-energy intermediate that stabilizes the positive charge during halogenation. It blocks one face of the alkene, forcing the bromide ion to attack from the opposite side, leading to anti addition.

What is halohydrin formation, and how does it differ from halogenation?

Halohydrin formation is similar to halogenation but occurs in the presence of water, which acts as a nucleophile. This results in the formation of a halohydrin, an alcohol group bonded to the carbon originally part of the double bond.

What is oxymercuration/reduction in alkene addition reactions?

Oxymercuration/reduction involves adding water across an alkene with the help of mercury (II) acetate. It avoids carbocation rearrangement, adds water to the more substituted carbon, and results in anti addition.

How does hydroboration differ from other alkene addition reactions?

Hydroboration adds boron (BH3) across a double bond in an anti-Markovnikov fashion. It is unique because the addition occurs in a concerted mechanism, and it always results in syn addition of the groups.