Q.68 Chemistry · Section A ⭐ 360° Option Elimination Active
NEET 2024 Chemistry Question 68 Solution
Topic: Alcohols, Phenols and Ethers · Official NTA Booklet Code T1
Question
Intramolecular hydrogen bonding is present in
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Verified Against NTA Final Answer Key
Option (3) : o-nitrophenol (2-nitrophenol)
📚 Alcohols, Phenols and Ethers Easy
📋 Given Parameters & Problem Setup
Compound 1 m-nitrophenol (3-nitrophenol)
Compound 2 HF (Hydrogen fluoride)
Compound 3 o-nitrophenol (2-nitrophenol)
Compound 4 p-nitrophenol (4-nitrophenol)
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⏱️ Save ~30 seconds The 30-Second Exam Hall Shortcut: ⚡ Ortho-Chelation Chelate Ring Rule
o-nitrophenol places -OH and -NO2 adjacent on the benzene ring, forming a stable 6-membered planar chelate ring via INTRAmolecular H-bonding!
1 Intramolecular hydrogen bonding occurs WITHIN a single molecule when donor and acceptor groups are spatially adjacent.
2 In o-nitrophenol, the hydroxyl hydrogen is in close geometric proximity to the oxygen of the ortho nitro group, forming a 6-membered intramolecular hydrogen-bonded ring.
3 In m-nitrophenol and p-nitrophenol, the functional groups are too far apart to interact intramolecularly; they form INTERmolecular hydrogen bonds between separate molecules.
4 HF exhibits strong INTERmolecular hydrogen bonding, forming zigzag polymeric (HF)n chains.
5 Therefore, intramolecular hydrogen bonding is present exclusively in o-nitrophenol -> Option (3) in 3 seconds!
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Why You Can 100% Trust This Shortcut: NCERT Chemistry · Alcohols, Phenols and Ethers (Physical properties of nitrophenols: steam volatility and hydrogen bonding).
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Step-by-Step Rigorous Workflow & Mathematical Proof
Step 1
Geometric Requirement for Intramolecular Hydrogen Bonding
Intramolecular hydrogen bonding requires the hydrogen donor (e.g. -O-H) and hydrogen acceptor (e.g. -N⁺(=O)O⁻) to be positioned within the same molecule such that a strain-free 5- or 6-membered pseudo-ring (chelate ring) can form:
Step 2
Analysis of o-Nitrophenol
In ortho-nitrophenol (2-nitrophenol), the spatial distance between the phenolic hydrogen and the nitro oxygen is approximately 1.8 to 2.1 Å, facilitating rapid intramolecular proton-chelate formation. This internal bonding prevents association with neighboring molecules, resulting in lower boiling point and steam volatility.
Step 3
Comparison with meta, para Isomers and HF
In m-nitrophenol and p-nitrophenol, the large geometric distance (> 4.5 Å) precludes intramolecular bonding; instead, they associate strongly via intermolecular hydrogen bonding, conferring high boiling points. In HF, each molecule forms intermolecular hydrogen bonds with adjacent molecules:
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4-Option Deep Scan
360° Option Elimination Engine Key Feature
Diagnose every choice: Know exactly why the right option works and why distractors fail.
1 (1)
✗ Incorrect Verdict & Scientific Reason:
Forms intermolecular hydrogen bonds due to unfavorable geometry.
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Student Fallacy (Why candidates pick this trap): Assuming all nitrophenols form intramolecular H-bonds.
2 (2)
✗ Incorrect Verdict & Scientific Reason:
Forms intermolecular hydrogen bonds linking individual HF molecules into polymeric zigzag chains.
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Student Fallacy (Why candidates pick this trap): Confusing diatomic intermolecular bonding with intramolecular bonding.
3 (3)
✓ 100% Correct Verdict & Scientific Reason:
Exhibits internal chelation forming a 6-membered intramolecular hydrogen-bonded ring.
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Why this is airtight: None.
4 (4)
✗ Incorrect Verdict & Scientific Reason:
Forms extensive intermolecular hydrogen bonding resulting in high boiling point and low steam volatility.
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Student Fallacy (Why candidates pick this trap): Failing to recognize that 1,4-positions are diametrically opposite.
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NTA Trap Alert: Intra- vs Inter- Terminology Confusion
Students frequently mix up INTRA (within the same molecule) and INTER (between different molecules).
🛡️ How to avoid the -1 mark: INTRA-murals happen within one school; INTRA-molecular happens within ONE molecule.
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Core NEET Syllabus Standard
Chemistry Core Organic Chemistry · Alcohols, Phenols and Ethers Standard Benchmark
Core Scientific Principle Tested:
"o-Nitrophenol possesses intramolecular hydrogen bonding (chelation) which makes it steam volatile, whereas m- and p-nitrophenols show intermolecular hydrogen bonding."
Syllabus Focus: Phenols: Hydrogen Bonding and Physical Properties
✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗
Ortho is Close, Hugs Itself (Intra); Meta and Para Reach to Neighbors (Inter)!
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