Problem 91 - A Complete Resource Book in Chemistry for JEE Main

Question

What is the order of decreasing reactivity of the following monomers towards anionic polymerization? (1) \(\mathrm{CH}_{2}=\mathrm{CHCN}\) (2) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CH}_{2}\) (3) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}=\mathrm{CH}_{2}\) (a) \(1>2>3\) (b) \(3>2>1\) (c) \(2>3>1\) (d) \(3>1>2\)

Step-by-Step Solution

Answer

(d) 3>1>2

Step 1: Understand Anionic Polymerization

Anionic polymerization involves the reaction of a monomer with a nucleophilic initiator. The monomer that can stabilize the negative charge best will be more reactive.

Step 2: Analyze Structure of Monomer 1

Monomer (1), \( \mathrm{CH}_2=\mathrm{CHCN} \), has a cyano group which is an electron-withdrawing group stabilizing the carbanion through resonance and induction. This increases its reactivity towards anionic polymerization.

Step 3: Analyze Structure of Monomer 2

Monomer (2), \( \mathrm{CH}_3\mathrm{CH}=\mathrm{CH}_2 \), has an alkyl group which slightly donates electron density. It has less reactivity compared to monomers with electron-withdrawing groups.

Step 4: Analyze Structure of Monomer 3

Monomer (3), \( \mathrm{C}_6\mathrm{H}_5\mathrm{CH}=\mathrm{CH}_2 \), has a phenyl group which provides moderate stabilization of carbanion through resonance electron donation. It is more reactive in anionic polymerization than monomers without such stabilization.

Step 5: Determine Reactivity Order

Based on the analysis, monomer (1) is the most reactive due to the strong electron-withdrawing nature of the cyano group. Monomer (3) comes next due to the resonance stabilization by the phenyl group, and monomer (2) is the least reactive. Therefore, the order is \( 1 > 3 > 2 \).

Key Concepts

Monomer Reactivity

In anionic polymerization, the reactivity of a monomer is crucial for the efficiency of the process. Monomer reactivity is determined by how well the monomer can stabilize the negative charge of the carbanion formed during the polymerization.
The structure of the monomer plays a pivotal role in this stabilization. For example, in the provided exercise:

• Monomer 1 \( \mathrm{CH}_2=\mathrm{CHCN} \), is highly reactive due to the presence of an electron-withdrawing cyano group. This group stabilizes the negative charge effectively, enhancing the monomer's reactivity.
• Monomer 2 \( \mathrm{CH}_3\mathrm{CH}=\mathrm{CH}_2 \), is less reactive because its alkyl group does not provide significant negative charge stabilization.
• Monomer 3 \( \mathrm{C}_6\mathrm{H}_5\mathrm{CH}=\mathrm{CH}_2 \), has moderate reactivity due to the phenyl group's ability to stabilize the negative charge through resonance.

The overall reactivity involves a balance between electronic effects, such as resonance, and the inherent properties of the substituent groups attached to the monomer.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) are essential in determining a molecule's behavior in anionic polymerization. These groups pull electron density away from the rest of the molecule, stabilizing the negative charge that develops on the carbanion intermediate.
In the context of our monomers:

• The cyano group \( \mathrm{-CN} \) in Monomer 1 is a strong electron-withdrawing group. It stabilizes the carbanion through both resonance and inductive effects, greatly enhancing reactivity.
• Compared to Monomer 1, Monomer 3 has a phenyl group that also stabilizes through resonance but is not as effective as the cyano group in pulling electron density.

By comparing the strength and type of electron-withdrawing capabilities, we can predict which monomer will likely react more quickly in anionic polymerization.
This highlights the tremendous impact electron-withdrawing groups have on polymer chemistry, influencing both polymer creation and final material properties.

Carbanion Stability

In any polymerization method involving carbanions, the stability of these negatively charged intermediates is fundamental to understanding reactivity. A carbanion is a carbon atom bearing a negative charge, typically highly reactive, but its stability can be influenced by surrounding groups and the molecular structure.
For example, in the given monomers:

• The carbanion formed at Monomer 1 can be effectively stabilized by the cyano group's strong electron-withdrawing effect.
• Monomer 3 benefits from its phenyl group, providing resonance stabilization that allows the negative charge to be delocalized over a larger area, thus enhancing stability.
• On the other hand, Monomer 2 lacks significant stabilizing groups, making its carbanion less stable and therefore less reactive.

Understanding carbanion stability is crucial for predicting the reactivity order in anionic polymerization and for designing monomers that can form robust, high-quality polymers.

Resonance Stabilization

Resonance stabilization is a phenomenon where certain groups can delocalize a negative charge over a larger molecular area, enhancing the stability of intermediates like carbanions. This process involves the overlap of p-orbitals that allows for sharing of electron density.
In the monomers discussed:

• Monomer 3 benefits from resonance stabilization through its phenyl group. The negative charge can move into the aromatic ring, increasing the monomer's reactivity in anionic polymerization.
• The cyano group in Monomer 1 also offers resonance effects, but coupled with strong electron-withdrawing properties, it provides even more effective stabilization.
• Monomer 2 primarily lacks resonance-stabilizing features, limiting its ability to manage a negative charge.

This ability to spread a charge across a molecule not only affects reaction speed but also influences the types of products that are ultimately formed in polymer synthesis. Resonance stabilization plays a central role in designing monomers for targeted polymer applications.