Q.77 Chemistry ⭐ 360° Option Elimination Active

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NEET 2021 Chemistry Question 77 Solution

Topic: Haloalkanes and Haloarenes · Official NTA Booklet Code M1

Question

The correct sequence of bond enthalpy of 'C–X' bond is:

✓

Verified Against NTA Final Answer Key

Option (2) : CH3 - F > CH3 - Cl > CH3 - Br > CH3 - I

📚 Haloalkanes and Haloarenes Easy

📋 Given Parameters & Problem Setup

Homologous Series Methyl halides (CH3-X with X = F, Cl, Br, I)

Target Property Sequence of bond enthalpy of C-X bond

⚡

The 30-Second Exam Hall Shortcut: ⚡ Shorter Bond = Stronger Bond (F > Cl > Br > I)

⏱️ Save ~15 seconds

Down Group 17, halogen atomic radius increases ( $F < Cl < Br < I$ ), so $C-X$ bond length increases ( $CH_3F: 139\text{ pm} \to CH_3I: 214\text{ pm}$ ). Shorter bonds exhibit greater orbital overlap and higher bond dissociation enthalpy: $CH_3-F (452) > CH_3-Cl (351) > CH_3-Br (293) > CH_3-I (234\text{ kJ/mol})$ ! Match Option (2)!

1 Down the halogen group, atomic size of X increases:

F < Cl < Br < I

C-X

bond length increases:

CH_3-F < CH_3-Cl < CH_3-Br < CH_3-I

3 Bond enthalpy is inversely related to bond length.

4 Therefore, bond enthalpy order is:

CH_3-F > CH_3-Cl > CH_3-Br > CH_3-I

5 Match Option (2).

🛡️

Why You Can 100% Trust This Shortcut: Canonical NCERT Haloalkanes fundamental carbon-halogen bond length and orbital overlap trends.

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Periodic Trend in Halogen Atomic Radii

Halogen atoms in Group 17 increase in size as new quantum electron shells are added down the column: $F < Cl < Br < I$ . Carbon uses its $2sp^3$ hybrid orbital to bond with valence $p$ orbitals of increasing principal quantum number ( $2p$ in F, $3p$ in Cl, $4p$ in Br, $5p$ in I).

Step 2

Effect on Orbital Overlap and Bond Length

As orbital size increases, orbital overlap becomes more diffuse and less effective. Experimental bond lengths from NCERT Table 10.2: - $C-F$ : $139\text{ pm}$ - $C-Cl$ : $178\text{ pm}$ - $C-Br$ : $193\text{ pm}$ - $C-I$ : $214\text{ pm}$

Step 3

Bond Dissociation Enthalpy Comparison

Because shorter bonds possess stronger electrostatic attraction between nuclei and shared electrons, bond dissociation enthalpy decreases monotonically down the group:

\Delta_{\text{bond}}H\;(\text{kJ/mol}): \quad \text{CH}_3\text{-F}\,(452) > \text{CH}_3\text{-Cl}\,(351) > \text{CH}_3\text{-Br}\,(293) > \text{CH}_3\text{-I}\,(234)

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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:

This is the order of BOND LENGTH, which is the inverse of bond enthalpy.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing bond length with bond enthalpy.

2 2

✓ 100% Correct

Verdict & Scientific Reason:

Exact sequence: smaller halogen yields shorter, stronger $C-X$ bond with highest dissociation energy.

💡

Why this is airtight: None; textbook bond energy trend.

3 3

✗ Incorrect

Verdict & Scientific Reason:

This is the order of DIPOLE MOMENT ( $CH_3Cl > CH_3F > CH_3Br > CH_3I$ ), NOT bond enthalpy.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing bond enthalpy with dipole moment anomaly.

4 4

✗ Incorrect

Verdict & Scientific Reason:

Places $CH_3Cl$ ahead of $CH_3F$ based on dipole moment, which does not apply to bond enthalpy.

⚠️

Student Fallacy (Why candidates pick this trap): Applying dipole moment exception to bond enthalpy.

⚠️

NTA Trap Alert: Bond Enthalpy vs. Dipole Moment Anomaly Trap

In methyl halides, the DIPOLE MOMENT has an anomaly where $CH_3Cl > CH_3F$ because the larger bond length of $C-Cl$ outweighs the electronegativity of fluorine ( $mu = q imes d$ ). But BOND ENTHALPY has NO anomaly—it strictly decreases monotonically: $CH_3F > CH_3Cl > CH_3Br > CH_3I$ .

🛡️ How to avoid the -1 mark: Dipole moment:

CH_3Cl > CH_3F

. Bond enthalpy:

CH_3F > CH_3Cl > CH_3Br > CH_3I

(no exceptions!).

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Core NEET Syllabus Standard

NCERT Chemistry · Haloalkanes and Haloarenes Standard Benchmark

Core Scientific Principle Tested:

"The C-X bond enthalpy decreases down the group as the size of halogen increases and C-X bond length increases."

Syllabus Focus: Nature of C-X Bond (Bond Enthalpies)

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

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NEET 2021 Chemistry Question 77 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Shorter Bond = Stronger Bond (F > Cl > Br > I)

Step-by-Step Rigorous Workflow & Mathematical Proof

Periodic Trend in Halogen Atomic Radii

Effect on Orbital Overlap and Bond Length

Bond Dissociation Enthalpy Comparison

360° Option Elimination Engine Key Feature

NTA Trap Alert: Bond Enthalpy vs. Dipole Moment Anomaly Trap

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

🔗 Related Questions — Haloalkanes and Haloarenes

Monochlorination Products (Including Stereoisomers) of 2-Methylbutane

Leaving Group Ability and S_N2 Reaction Rates of 1-Iodobutane vs. 1-Chlorobutane

Relative Rates of SN1 Reactions and Carbocation Stability

Halogenation of Alcohols followed by Dehydrohalogenation (Saytzeff Elimination)

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