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

The order is (d) 3>1>2.

Step 1: Understand Anionic Polymerization

Anionic polymerization involves the reaction with a nucleophile, where electron-withdrawing groups stabilize the carbanion formed after the addition of a nucleophile. Monomers with strong electron-withdrawing groups are more reactive because they stabilize the negative charge.

Step 2: Analyze Monomers

Identify electron-withdrawing groups in the given monomers. 1. CH2=CHCN has a strongly electron-withdrawing cyano group (-CN). 2. CH3CH=CH2 lacks electron-withdrawing substituents. 3. C6H5CH=CH2 has a phenyl group, which is less electron-withdrawing compared to the cyano group.

Step 3: Determine Reactivity

The stronger the electron-withdrawing group, the more it stabilizes the anion, increasing reactivity. 1. CH2=CHCN (with the -CN group), is most reactive. 3. C6H5CH=CH2 (with a phenyl group), is less reactive than the CN group. 2. CH3CH=CH2 (with no electron-withdrawing group), is least reactive.

Step 4: Construct Order of Reactivity

Based on the stabilization provided by the electron-withdrawing groups: 1 > 3 > 2 . This sequence indicates that the monomer with the cyano group is most reactive, followed by the phenyl-substituted monomer and then the simple alkene.

Key Concepts

Reactivity of Monomers

In anionic polymerization, the reactivity of monomers is heavily influenced by their ability to stabilize the carbanion formed during the reaction. A monomer's reactivity can be understood by looking at the substituents attached to the double bond. Some key points to consider when evaluating reactivity:

• Monomers with strong electron-withdrawing groups tend to be more reactive.
• These groups help in stabilizing the negative charge on the carbanion, which is a critical step in anionic polymerization.
• The arrangement of atoms or groups also influences the electron density around the reactive site, affecting the monomer's ability to engage in polymerization.

Reactivity decreases as the ability to stabilize the carbanion weakens. For example, a monomer carrying a -CN group will typically be more reactive compared to one that lacks such electron-withdrawing capability.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) play a crucial role in determining the behavior of monomers during anionic polymerization. These groups pull electron density away from the rest of the molecule. This effect results in increased stability of the carbanion formed when a nucleophile attacks the monomer. Common examples of EWGs include:

• Cyano groups (-CN): Known for their strong electron-withdrawing capability, they significantly enhance the reactivity of monomers.
• Phenyl groups: While not as strong as cyanides in electron-withdrawing, they still impart some degree of stability to the carbanion.

The presence and strength of EWGs in a monomer directly influence its polymerization rate and overall reactivity. In comparison, monomers with little to no EWGs tend to be sluggish in anionic polymerization, as they lack the stabilizing effects needed for efficient reaction progression.

Stabilization of Carbanions

The stabilization of carbanions is essential in anionic polymerization. A carbanion is a molecule that contains a negatively charged carbon atom. It arises during the initial steps of polymerization when a nucleophile attacks the double bond of the monomer. To stabilize a carbanion:

• It is important for the molecule to have groups or structures that can distribute the negative charge effectively.
• Electron-withdrawing groups help delocalize the charge, making the carbanion more stable.
• Stronger stabilization results in higher reactivity of the monomer as the polymerization process becomes more favorable energetically.

In the exercise example, the monomer with the cyano group provides excellent stabilization, making it the most reactive. Conversely, without any strong EWGs, stabilization is limited, leading to reduced reactivity as seen with alkenes like CH3CH=CH2.