Ch.8 - Reactions of Alkenes

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch.8 - Reactions of Alkenes

Problem 72

Chapter 8, Problem 72

Propose a mechanism for reaction of the first three propylene units in the polymerization of propylene in the presence of a peroxide.

Verified step by step guidance

Step 1: Initiation - The peroxide (ROOR) undergoes homolytic cleavage under high pressure to form two alkoxy radicals (RO•). This step generates the reactive species needed to initiate the polymerization process.

Step 2: Radical Addition - One of the alkoxy radicals (RO•) reacts with the double bond of the first propylene molecule (CH₂=CHCH₃). The radical attacks the π-electrons of the double bond, forming a new carbon-centered radical on the propylene unit.

Step 3: Propagation - The carbon-centered radical formed in Step 2 reacts with a second propylene molecule. The radical attacks the double bond of the second propylene, forming a new bond and generating another carbon-centered radical on the growing chain.

Step 4: Addition of Third Propylene Unit - The radical formed in Step 3 reacts with a third propylene molecule in the same manner, extending the chain further and creating a new radical at the end of the chain.

Step 5: Termination (Optional) - The polymerization process can terminate when two radicals combine, either by coupling or disproportionation, to form a stable product. However, in this case, the chain continues to grow until the reaction conditions or reactants are exhausted.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Polymerization Mechanism

Polymerization is a chemical process that combines small molecules called monomers into a larger, more complex structure known as a polymer. In the case of propylene, the polymerization involves the addition of multiple propylene units through a chain reaction, typically initiated by heat, pressure, or chemical initiators like peroxides. Understanding the mechanism is crucial for predicting the structure and properties of the resulting polymer.

Radical Initiation

In the presence of peroxides, polymerization often begins with radical initiation, where the peroxide decomposes to form free radicals. These radicals are highly reactive species that can initiate the polymerization process by attacking the double bond of propylene, leading to the formation of a new radical that can further react with additional monomers. This step is essential for starting the chain reaction that leads to polymer formation.

Chain Growth Polymerization

Chain growth polymerization is a type of polymerization where the growth of the polymer chain occurs through the successive addition of monomer units to an active site, typically a radical. This process continues until the active site is terminated, either by combination with another radical or by other means. In the case of propylene, understanding this concept helps in predicting how the polymer structure evolves as more propylene units are added.

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Chain-Growth Polymers Example 1

Related Practice

Textbook Question

Ozonolysis can be applied selectively to different types of carbon–carbon double bonds. The compound shown below contains two vinyl ether double bonds, which are electron-rich because of the electron-donating alkoxy groups. Ozone reacts more quickly with electron-rich double bonds and more slowly with hindered double bonds. At −78 °C, this compound quickly adds two equivalents of ozone. Immediate reduction of the ozonide gives a good yield of a single product. Show the expected ozonolyis product, and label the functional groups produced, some of which are not typical from ozonolysis of simple alkenes.

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Textbook Question

When styrene (vinylbenzene) is commercially polymerized, about 1–3% of 1,4-divinylbenzene is often added to the styrene. The incorporation of some divinylbenzene gives a polymer with more strength and better resistance to organic solvents. Explain how a very small amount of divinylbenzene has a marked effect on the properties of the polymer.

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Textbook Question

The cationic polymerization of isobutylene (2-methylpropene) is shown in Section 8-16A. Isobutylene is often polymerized under free-radical conditions. Propose a mechanism for the free-radical polymerization of isobutylene.

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Textbook Question

The following cyclization has been observed in the oxymercuration-demercuration of this unsaturated alcohol. Propose a mechanism for this reaction.

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Textbook Question

Predict the product of formula C₇H₁₃BrO from the reaction of this same unsaturated alcohol with bromine. Propose a mechanism to support your prediction.

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Textbook Question

A graduate student attempted to form the iodohydrin of the alkene shown below. Her analysis of the products showed a good yield of an unexpected product. Propose a mechanism to explain the formation of this product.

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