Q.51 Chemistry ⭐ 360° Option Elimination Active

Vault ↗

NEET 2021 Chemistry Question 51 Solution

Topic: Hydrocarbons · Official NTA Booklet Code M1

Question

Dihedral angle of least stable conformer of ethane is:

✓

Verified Against NTA Final Answer Key

Option (4) : 0°

📚 Hydrocarbons Easy

📋 Given Parameters & Problem Setup

Molecule Ethane (C2H6)

Target Conformer Least stable conformer (Eclipsed conformer)

⚡

The 30-Second Exam Hall Shortcut: ⚡ Eclipsed = 0° Dihedral Angle

⏱️ Save ~20 seconds

The least stable conformer of ethane is the eclipsed conformer due to maximum torsional strain between adjacent C-H bonding pairs. In the eclipsed form, hydrogen atoms directly shadow each other with a dihedral angle of $\theta = 0^\circ$ ! The most stable staggered form has $\theta = 60^\circ$ . Match Option (4)!

1 Recall ethane conformers: Staggered conformer (most stable) and Eclipsed conformer (least stable).

2 Torsional strain is maximal in the eclipsed conformer because electron clouds of adjacent C-H bonds are at minimum distance.

3 Looking down the C-C bond (Newman projection), the dihedral angle between front and back C-H bonds in eclipsed form is

\theta = 0^\circ

4 Match Option (4).

🛡️

Why You Can 100% Trust This Shortcut: Canonical NCERT Hydrocarbons section on Conformations of Ethane.

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Definition of Dihedral (Torsional) Angle

The dihedral angle (also called torsional angle $\theta$ ) in ethane ( $CH_3-CH_3$ ) is the angle between the two intersecting planes formed by the $H-C-C$ bonds of the adjacent carbon atoms when viewed along the carbon-carbon single bond axis.

Step 2

Identifying the Least Stable Conformation of Ethane

Ethane exhibits infinite conformations resulting from free rotation about the $C-C$ $\sigma$ -bond. The two extreme conformations are: - Staggered Conformation: Hydrogen atoms are as far apart as possible (dihedral angle $\theta = 60^\circ$ ). Torsional strain is minimized, rendering it the most stable conformer. - Eclipsed Conformation: Hydrogen atoms on adjacent carbons are closest to each other, resulting in maximum repulsive interaction between adjacent bonding electron pairs (torsional strain of $\approx 12.5\text{ kJ/mol}$ ). Consequently, it is the least stable conformer.

\theta_{\text{eclipsed}} = 0^\circ, \quad \theta_{\text{staggered}} = 60^\circ

Step 3

Conclusion

The dihedral angle of the least stable (eclipsed) conformer is identically $0^\circ$ , corresponding to Option (4).

⭐

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 1

✗ Incorrect

Verdict & Scientific Reason:

At $heta = 120^circ$ , another eclipsed conformation is achieved, but the fundamental dihedral angle defining eclipse is $0^circ$ .

⚠️

Student Fallacy (Why candidates pick this trap): Confusing tetrahedral or planar bond angles (

120^circ

) with the primary dihedral angle.

2 2

✗ Incorrect

Verdict & Scientific Reason:

At $heta = 180^circ$ , the conformer is staggered (anti-periplanar equivalent in butane, highly stable).

⚠️

Student Fallacy (Why candidates pick this trap): Confusing anti-conformation with eclipsed conformation.

3 3

✗ Incorrect

Verdict & Scientific Reason:

This is the dihedral angle of the MOST stable (staggered) conformer of ethane.

⚠️

Student Fallacy (Why candidates pick this trap): Misreading "least stable" as "most stable".

4 4

✓ 100% Correct

Verdict & Scientific Reason:

Exact dihedral angle of the eclipsed conformer where electron-cloud repulsion (torsional strain) is maximal.

💡

Why this is airtight: None; textbook conformation knowledge.

⚠️

NTA Trap Alert: Most Stable vs. Least Stable Reading Trap

Students frequently read "conformer of ethane" and instinctively select $60^\circ$ (Option 3), forgetting that $60^\circ$ corresponds to the MOST stable staggered form, whereas the prompt explicitly asks for the LEAST stable (eclipsed, $0^\circ$ ) form.

🛡️ How to avoid the -1 mark: Highlight the keyword: "LEAST stable"

\implies

Eclipsed

\implies 0^\circ

. "MOST stable"

\implies

Staggered

\implies 60^\circ

📖

Core NEET Syllabus Standard

NCERT Chemistry · Hydrocarbons Standard Benchmark

Core Scientific Principle Tested:

"In the eclipsed conformation of ethane, adjacent C-H bonds have a dihedral angle of $0^\circ$ and experience maximum torsional strain."

Syllabus Focus: Conformations of Alkanes (Ethane)

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

Ready to attempt under real exam pressure?

Test your speed and accuracy in the authentic 200-minute timed NTA simulator.

Launch NEET 2021 CBT Test →

All Chemistry Questions Grid Next (Q.52) →

Sequential Reaction: Anti-Markovnikov HBr Addition, Cyanide Substitution & Mendius Reduction

View Solution →

Spotted a mistake in this solution? Report an error (support@mycompetitiveexams.com)

⚖️ Legal & Fair Dealing Notice: Sourced from publicly released NEET 2021 question papers and NEET answer keys for private study, research, and non-commercial educational practice under Fair Dealing (Section 52(1)(a) of the Indian Copyright Act, 1957). Not affiliated with or endorsed by NTA or NMC.

NEET 2021 Chemistry Question 51 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Eclipsed = 0° Dihedral Angle

Step-by-Step Rigorous Workflow & Mathematical Proof

Definition of Dihedral (Torsional) Angle

Identifying the Least Stable Conformation of Ethane

Conclusion

360° Option Elimination Engine Key Feature

NTA Trap Alert: Most Stable vs. Least Stable Reading Trap

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

🔗 Related Questions — Hydrocarbons

Grignard Reagent Acid-Base Reaction with Primary Amines

Stereospecific Electrophilic & Syn Additions to Alkenes (TAM & CSM Rules)

Stereoselective Hydrogenation of Alkynes (Lindlar vs Birch Reduction)

Sequential Reaction: Anti-Markovnikov HBr Addition, Cyanide Substitution & Mendius Reduction

Solve 4 NEET Questions a Day — One per Subject