Q.119 Chemistry ⭐ 360° Option Elimination Active

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NEET 2020 Chemistry Question 119 Solution

Topic: Polymers · Official NTA Booklet Code E1

Question

Which of the following is a natural polymer?

(1)cis-1,4-polyisoprene
(2)poly (Butadiene-styrene)
(3)polybutadiene
(4)poly (Butadiene-acrylonitrile)

✓

Verified Against NTA Final Answer Key

Option (1) : cis-1,4-polyisoprene

📚 Polymers Easy

📋 Given Parameters & Problem Setup

Classification Natural polymer

Candidate Polymers cis-1,4-polyisoprene, poly(Butadiene-styrene), polybutadiene, poly(Butadiene-acrylonitrile)

⚡

The 30-Second Exam Hall Shortcut: ⚡ "Natural Rubber = cis-1,4-Polyisoprene; Buna-S & Buna-N are Synthetic!"

⏱️ 15 seconds

Natural rubber extracted from the latex of rubber trees (Hevea brasiliensis) is chemically cis-1,4-polyisoprene. Poly(butadiene-styrene) is Buna-S, poly(butadiene-acrylonitrile) is Buna-N, and polybutadiene is synthetic rubber. Mark cis-1,4-polyisoprene (Option 1) in 5 seconds!

1 Natural rubber is a natural elastomer.

2 Chemical identity of natural rubber: cis-1,4-polyisoprene.

3 Buna-S [poly(butadiene-styrene)] = Synthetic rubber.

4 Buna-N [poly(butadiene-acrylonitrile)] = Synthetic rubber.

5 Polybutadiene = Synthetic elastomer.

6 Select Option (1) cis-1,4-polyisoprene.

🛡️

Why You Can 100% Trust This Shortcut: NCERT Chemistry · Polymers (Natural Rubber: cis-1,4-polyisoprene)

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Identify Source and Chemistry of Natural Rubber

Natural rubber is a natural polymer obtained as latex from the bark of the rubber tree. It possesses remarkable elastic properties and is classified as an elastomer.

Step 2

Stereochemistry of Natural Rubber

Natural rubber is a linear polymer of isoprene (2-methyl-1,3-butadiene). All the double bonds in the polymer chain possess the cis-configuration, making it cis-1,4-polyisoprene: $\text{Natural Rubber} = \text{cis-1,4-polyisoprene}$ The cis-configuration prevents close crystal packing and keeps intermolecular forces weak, giving rubber its elasticity. (In contrast, trans-1,4-polyisoprene is Gutta-percha).

Step 3

Evaluate Synthetic Rubbers in Options

- poly(Butadiene-styrene): Synthetic copolymer known commercially as Buna-S or SBR. - poly(Butadiene-acrylonitrile): Synthetic copolymer known commercially as Buna-N. - polybutadiene: Synthetic homopolymer synthesized by Ziegler-Natta polymerization of butadiene.

Step 4

Conclude Option 1

cis-1,4-polyisoprene is the only naturally occurring polymer among the choices.

⭐

360° Option Elimination Engine Key Feature

Diagnose every choice: Know exactly why the right option works and why distractors fail.

4-Option Deep Scan

1 A

✓ 100% Correct

Verdict & Scientific Reason:

Factually exact; cis-1,4-polyisoprene is the chemical identity of natural rubber.

💡

Why this is airtight: None; textbook polymer definition.

2 B

✗ Incorrect

Verdict & Scientific Reason:

Incorrect; this is Buna-S, a man-made synthetic elastomer.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing synthetic rubber (Buna-S) with natural rubber.

3 C

✗ Incorrect

Verdict & Scientific Reason:

Incorrect; polybutadiene is an industrially synthesized elastomer.

⚠️

Student Fallacy (Why candidates pick this trap): Assuming simple diene polymers are natural.

4 D

✗ Incorrect

Verdict & Scientific Reason:

Incorrect; this is Buna-N, a synthetic oil-resistant elastomer.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing Buna-N with natural rubber.

⚠️

NTA Trap Alert: cis vs trans Polyisoprene

- cis-1,4-polyisoprene = NATURAL RUBBER (elastic). - trans-1,4-polyisoprene = GUTTA-PERCHA (rigid, non-elastic). - Buna-S & Buna-N = SYNTHETIC rubbers.

🛡️ How to avoid the -1 mark: Natural rubber = CIS-1,4-polyisoprene!

📖

Core NEET Syllabus Standard

Chemistry · Polymers Standard Benchmark

Core Scientific Principle Tested:

Syllabus Focus: Natural vs Synthetic Polymers: Natural Rubber as cis-1,4-Polyisoprene

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

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NEET 2020 Chemistry Question 119 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ "Natural Rubber = cis-1,4-Polyisoprene; Buna-S & Buna-N are Synthetic!"

Step-by-Step Rigorous Workflow & Mathematical Proof

Identify Source and Chemistry of Natural Rubber

Stereochemistry of Natural Rubber

Evaluate Synthetic Rubbers in Options

Conclude Option 1

360° Option Elimination Engine Key Feature

NTA Trap Alert: cis vs trans Polyisoprene

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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