The species which can best serve as an initiator for the cationic polymerization is :
For cationic polymerization the best reagent will be AlCl3
Cationic polymerization is a type of chain-growth polymerization where the active center is a carbocation. It requires an initiator that can generate a cationic species, typically by protonating a monomer or by forming a Lewis acid-base complex that creates a positive charge.
Let's evaluate the options:
Option 1: AlCl3 - Aluminum chloride (AlCl3) is a strong Lewis acid. It can accept an electron pair and is commonly used as a catalyst in cationic polymerization, often in combination with a co-initiator like water or HCl to generate protons (H+).
Option 2: BuLi - Butyllithium (BuLi) is a strong base and nucleophile. It is typically used as an initiator for anionic polymerization, not cationic, as it generates carbanions.
Option 3: LiAlH4 - Lithium aluminum hydride (LiAlH4) is a reducing agent. It donates hydride ions (H-), which are nucleophiles, and is not suitable for initiating cationic polymerization.
Option 4: HNO3 - Nitric acid (HNO3) is a strong Brønsted acid that can donate protons (H+). It can initiate cationic polymerization by protonating the monomer to form a carbocation.
Both AlCl3 (a Lewis acid) and HNO3 (a Brønsted acid) can serve as initiators for cationic polymerization. However, AlCl3 is particularly effective and widely used in industrial processes due to its strong Lewis acidity and ability to form highly reactive cationic species.
Final Answer: AlCl3 is the best initiator among the options for cationic polymerization.
Polymerization Mechanisms: Cationic polymerization is one of several mechanisms, including anionic and free radical polymerization. Understanding the nature of the initiator is key to controlling the polymerization process.
Acid-Base Chemistry: The ability of a compound to act as a Lewis acid (electron pair acceptor) or Brønsted acid (proton donor) determines its suitability as an initiator for cationic reactions.
General initiation step for cationic polymerization using a Lewis acid (e.g., AlCl3) with a co-initiator (e.g., H2O):
Followed by:
Where M is the monomer.
For Brønsted acids like HNO3:
Then:
Match the reactions in column I with appropriate types of steps / reactive intermediate involved in these reactions as given in column II
| Column-I | Column-II |
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(p) Nucleophilic substitution |
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(q) Electrophilic substitution |
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(r) Dehydration |
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(s) Nucleophilic addition |
| (t) Carbanion |
Match the chemical conversions in List I with the appropriate reagents in List II and select the correct answer using the code given below the lists:
| List I | List II |
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(1) (i) Hg(OAc)2; (ii) NaBH4 |
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(2) NaOEt |
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(3) Et – Br |
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(4) (i) BH3 ; (ii) H2O2/NaOH |