Q.86 Chemistry ⭐ 360° Option Elimination Active

NEET 2025 Chemistry Question 86 Solution

Topic: Alcohols, Phenols and Ethers · Official NTA Booklet Code 45

Question

Total number of possible isomers (both structural as well as stereoisomers) of cyclic ethers of molecular formula CHO is:

(1)6
(2)8
(3)10
(4)11

✓

Verified Against NTA Final Answer Key

Option (3) : 10

📚 Alcohols, Phenols and Ethers Challenging

📋 Given Parameters & Problem Setup

Molecular Formula C₄H₈O

Class of Compounds Cyclic ethers (Heterocyclic compounds with an ether -O- in the ring)

Inclusion Both structural isomers AND stereoisomers (enantiomers and diastereomers)

⚡

The 30-Second Exam Hall Shortcut: ⚡ Ring-Size Systematic Enumeration (1 + 3 + 6 = 10)

⏱️ Save ~60 seconds

DBE = 1 (accounted for by the ring). 5-membered ring = THF (1). 4-membered ring = 2-methyloxetane (chiral C2 ⟹ 2) + 3-methyloxetane (1) = 3. 3-membered ring = 2-ethyloxirane (chiral C2 ⟹ 2) + 2,2-dimethyloxirane (1) + 2,3-dimethyloxirane (cis-meso + trans-d/l ⟹ 3) = 6! Total = 1 + 3 + 6 = 10!

1 Compute DBE: 4 + 1 - 8/2 = 1. The ring accounts for the 1 degree of unsaturation, so all bonds are single.

2 Five-membered ring (Oxolane / THF): Tetrahydrofuran has 1 achiral structure ⟹ 1 isomer.

3 Four-membered ring (Oxetane):

4 - 2-Methyloxetane has a chiral carbon at C2 ⟹ 2 stereoisomers (d + l).

5 - 3-Methyloxetane is symmetrical (achiral) ⟹ 1 isomer.

6 Subtotal for 4-membered = 2 + 1 = 3 isomers.

7 Three-membered ring (Oxirane / Epoxide):

8 - 2-Ethyloxirane has a chiral carbon at C2 ⟹ 2 stereoisomers (d + l).

9 - 2,2-Dimethyloxirane is achiral ⟹ 1 isomer.

10 - 2,3-Dimethyloxirane has two chiral carbons: cis-isomer is meso (1) and trans-isomer is chiral (pair of enantiomers: 2) ⟹ 1 + 2 = 3 isomers.

11 Subtotal for 3-membered = 2 + 1 + 3 = 6 isomers.

12 Total cyclic ethers = 1 (5-ring) + 3 (4-ring) + 6 (3-ring) = 10 isomers!

13 Select Option (3) with complete confidence!

🛡️

Why You Can 100% Trust This Shortcut: Enantiomerism and Optical Activity in Substituted Rings (NCERT Chemistry · Organic Chemistry: Some Basic Principles and Techniques).

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Determine Degree of Unsaturation and Ring Classifications

Calculate the double bond equivalent (DBE) for C₄H₈O:

\text{DBE} = C + 1 - \frac{H}{2} = 4 + 1 - \frac{8}{2} = 1

• Because the question restricts compounds strictly to "cyclic ethers", the 1 degree of unsaturation is entirely utilized by the ring structure.
• Possible ring sizes containing oxygen: 5-membered, 4-membered, and 3-membered.

Step 2

Enumerate 5-Membered and 4-Membered Cyclic Ethers

Systematically count structural and stereoisomers for oxolanes and oxetanes:

\text{5-Membered: Tetrahydrofuran (THF)} \implies 1\text{ isomer}

• 4-Membered (Oxetanes):
• - 2-Methyloxetane: Carbon-2 is bonded to -H, -CH₃, -O-, and -CH₂-. All four groups are distinct, so C2 is a stereocenter, producing a pair of enantiomers (R and S) ⟹ 2 isomers.
• - 3-Methyloxetane: Has an internal plane of symmetry through C3 and oxygen, achiral ⟹ 1 isomer.
• Total 4-membered = 2 + 1 = 3 isomers.

Step 3

Enumerate 3-Membered Cyclic Ethers (Oxiranes) and Calculate Total

Examine substituted oxiranes (epoxides):

\text{3-Membered (Oxiranes): } 2 + 1 + 3 = 6\text{ isomers}

• - 2-Ethyloxirane: C2 is a chiral center, producing (R) and (S) enantiomers ⟹ 2 isomers.
• - 2,2-Dimethyloxirane: Both methyl groups reside on the same carbon, achiral ⟹ 1 isomer.
• - 2,3-Dimethyloxirane: Contains two stereocenters at C2 and C3:
• * (2R, 3S)-2,3-dimethyloxirane is the cis-isomer with an internal plane of symmetry (meso form) ⟹ 1 isomer.
• * (2R, 3R) and (2S, 3S) are the chiral trans-enantiomers ⟹ 2 isomers.
• * Oxiranes subtotal = 2 + 1 + 3 = 6 isomers.
• Grand Total = 1 (THF) + 3 (Oxetanes) + 6 (Oxiranes) = 10 isomers.
• Matches Option (3) 10 perfectly.

⭐

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

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Corresponds to counting structural isomers only without stereoisomers.

⚠️

Student Fallacy (Why candidates pick this trap): Ignoring stereoisomers completely.

2 B

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Omitting enantiomers of either oxetane or oxirane.

⚠️

Student Fallacy (Why candidates pick this trap): Missing chiral centers in 2-methyloxetane and 2-ethyloxirane.

3 C

✓ 100% Correct

Verdict & Scientific Reason:

100% correct. Rigorous stereochemical enumeration yields exactly 10 isomers.

💡

Why this is airtight: None; masterclass organic stereochemistry.

4 D

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Overcounting meso form as two separate enantiomers.

⚠️

Student Fallacy (Why candidates pick this trap): Treating meso-2,3-dimethyloxirane as a pair of enantiomers.

⚠️

NTA Trap Alert: Counting Structural Isomers vs. Stereoisomers

There are exactly 6 STRUCTURAL cyclic ethers (THF, 2-methyloxetane, 3-methyloxetane, 2-ethyloxirane, 2,2-dimethyloxirane, 2,3-dimethyloxirane). When stereoisomerism is included, 2-methyloxetane adds +1, 2-ethyloxirane adds +1, and trans-2,3-dimethyloxirane adds +2 (with cis as meso), giving 6 + 4 = 10!

🛡️ How to avoid the -1 mark: Always break down by ring size and check every carbon with distinct substituents for chirality!

📖

Core NEET Syllabus Standard

NEET Chemistry Core Syllabus · Alcohols, Phenols and Ethers Standard Benchmark

Core Scientific Principle Tested:

"Total isomers include all constitutional isomers, enantiomeric pairs, and meso diastereomers."

Syllabus Focus: Isomerism in Ethers: Structural and Stereoisomerism

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

Ring sizes: 5-ring (1) + 4-ring (3) + 3-ring (6) = 1 + 3 + 6 = 10 (Option 3)!

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NEET 2025 Chemistry Question 86 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Ring-Size Systematic Enumeration (1 + 3 + 6 = 10)

Step-by-Step Rigorous Workflow & Mathematical Proof

Determine Degree of Unsaturation and Ring Classifications

Enumerate 5-Membered and 4-Membered Cyclic Ethers

Enumerate 3-Membered Cyclic Ethers (Oxiranes) and Calculate Total

360° Option Elimination Engine Key Feature

NTA Trap Alert: Counting Structural Isomers vs. Stereoisomers

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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