6.4.2 Reactions of Alkenes

• What type of bond is present in alkenes that contributes to their reactivity?

  Carbon-carbon double bond
• Electrophilic addition reactions involve the addition of electrophiles across the carbon-carbon double bond in alkenes.
• Steps in the mechanism of electrophilic addition reactions

  1️⃣ Electrophile approaches the double bond

  2️⃣ Carbocation formation

  3️⃣ Nucleophile attacks the carbocation
• A carbocation is formed as an intermediate in electrophilic addition reactions
• In the final step of electrophilic addition, a nucleophile attacks the carbocation.
• What type of reaction is the addition of halogens to alkenes?

  Electrophilic addition
• In the addition of halogens to alkenes, a three-membered cyclic halonium ion is formed.
• The nucleophilic attack in the addition of halogens to alkenes occurs from the same side as the halonium ion

  False
• What type of product is formed in the addition of halogens to alkenes?

  Dihalide
• Steps in the addition of halogens to alkenes

  1️⃣ Halogen molecule approaches the double bond

  2️⃣ Formation of halonium ion

  3️⃣ Nucleophilic attack by halide ion
• The addition of hydrogen halides to alkenes is an example of electrophilic addition
• In the addition of hydrogen halides to alkenes, the acidic hydrogen atom is attracted to the π electrons.
• What ion acts as the nucleophile in the addition of hydrogen halides to alkenes?

  Halide ion
• Summarize the key steps in the addition of hydrogen halides to alkenes.

  1️⃣ Electrophile (H from HX) attacks the double bond

  2️⃣ Carbocation forms

  3️⃣ Nucleophile (X from HX) attacks the carbocation
• The hydration of alkenes requires an acid catalyst such as sulfuric acid.
• Summarize the key steps in the hydration of alkenes.

  1️⃣ Protonation forms carbocation

  2️⃣ Water attacks carbocation

  3️⃣ Deprotonation regenerates catalyst
• What type of bond characterizes alkenes?

  Carbon-carbon double bond
• Alkenes are saturated hydrocarbons.

  False
• What is the role of an electrophile in electrophilic addition reactions?

  Attacks electron-rich double bond
• Summarize the key steps in the mechanism of electrophilic addition reactions.

  1️⃣ Electrophile approaches and attacks double bond

  2️⃣ Carbocation forms

  3️⃣ Nucleophile attacks carbocation
• What is the product of the addition of halogens to alkenes?

  Dihalide
• The formation of a halonium ion involves a three-membered cyclic structure.
• Summarize the key steps in the addition of halogens to alkenes.

  1️⃣ Electrophile approach

  2️⃣ Formation of halonium ion

  3️⃣ Nucleophilic attack
• Which ion acts as a nucleophile in the addition of hydrogen halides to alkenes?

  Halide ion
• The carbocation formed in the addition of hydrogen halides to alkenes is stable.

  False
• What type of reaction is the addition of hydrogen halides to alkenes?

  Electrophilic addition
• In the first step of the addition of hydrogen halides to alkenes, the acidic hydrogen forms a bond with one of the carbon atoms
• A carbocation intermediate is formed during the addition of hydrogen halides to alkenes.
• What does the halide ion attack in the third step of the addition of hydrogen halides to alkenes?

  Carbocation
• Match the step in the addition of hydrogen halides to alkenes with its description:

  Electrophile attack ↔️ H from HX bonds with C

  Carbocation formation ↔️ Positive charge on carbon

  Nucleophile attack ↔️ X- attacks carbocation
• Arrange the steps in the addition of hydrogen halides to alkenes in the correct order:

  1️⃣ Electrophile attack

  2️⃣ Carbocation formation

  3️⃣ Nucleophile attack
• The addition of water to alkenes requires an acid catalyst
• What type of intermediate is formed when an alkene is protonated during hydration?

  Carbocation
• Water acts as a nucleophile in the hydration of alkenes.
• Match the step in the hydration of alkenes with its description:

  Protonation forms carbocation ↔️ Acid catalyst protonates alkene

  Water attacks carbocation ↔️ Water bonds with carbocation

  Deprotonation regenerates catalyst ↔️ Acid catalyst is reformed
• What does Markovnikov's rule state for the addition of an electrophile to an unsymmetrical alkene?

  Electrophile adds to carbon with more hydrogen
• The stability of carbocations follows the order: tertiary > secondary > primary
• In the addition of $HCl$ to propene, the major product is 2-chloropropane.
• What are two exceptions to Markovnikov's rule?

  Radical addition and steric hindrance
• Arrange the steps in the hydrogenation of alkenes in the correct order:

  1️⃣ Catalyst activation

  2️⃣ Alkene adsorption

  3️⃣ Hydrogen addition

  4️⃣ Alkane release