Q.71 Chemistry ⭐ 360° Option Elimination Active

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NEET 2021 Chemistry Question 71 Solution

Topic: Coordination Compounds · Official NTA Booklet Code M1

Question

Ethylene diaminetetraacetate (EDTA) ion is:

(1)Hexadentate ligand with four "O" and two "N" donor atoms
(2)Unidentate ligand
(3)Bidentate ligand with two "N" donor atoms
(4)Tridentate ligand with three "N" donor atoms

✓

Verified Against NTA Final Answer Key

Option (1) : Hexadentate ligand with four "O" and two "N" donor atoms

📚 Coordination Compounds Easy

📋 Given Parameters & Problem Setup

Ligand Ethylenediaminetetraacetate ion (EDTA^4-)

⚡

The 30-Second Exam Hall Shortcut: ⚡ EDTA = 2 Amines + 4 Acetates = Hexadentate (2 N + 4 O)

⏱️ Save ~15 seconds

The chemical formula of $EDTA^{4-}$ is $[(^-OOC-CH_2)_2N-CH_2-CH_2-N(CH_2-COO^-)_2]$ . It possesses 2 tertiary nitrogen atoms with lone pairs ( $2\text{ N}$ ) and 4 negatively charged carboxylate oxygen atoms ( $4\text{ O}$ ). Total donor atoms $= 2 + 4 = 6$ , making it a HEXADENTATE ligand! Match Option (1)!

1 Write the structural components of EDTA: Ethylene backbone (

-CH_2-CH_2-

) + 2 Diamino nitrogens (

-N-

) + 4 Acetate groups (

-CH_2COO^-

2 Count nitrogen donor atoms: 2 tertiary amine nitrogens, each donating 1 lone pair (

2\text{ N}

3 Count oxygen donor atoms: 4 carboxylate groups, each donating through 1 oxygen atom (

4\text{ O}

4 Total donor atoms

= 6 \implies

Hexadentate ligand with 4 "O" and 2 "N" donor atoms.

5 Match Option (1).

🛡️

Why You Can 100% Trust This Shortcut: Canonical NCERT Coordination Compounds section on Types of Ligands.

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Chemical Structure of EDTA4-

Ethylenediaminetetraacetate ion ( $EDTA^{4-}$ ) is an important polychelating agent derived from ethylenediaminetetraacetic acid by loss of four carboxylate protons. Its molecular structure is:

(\text{O}^-\text{OC}-\text{CH}_2)_2\ddot{\text{N}}-\text{CH}_2-\text{CH}_2-\ddot{\text{N}}(\text{CH}_2-\text{COO}^-)_2

Step 2

Identifying the Coordinating Donor Atoms

When $EDTA^{4-}$ binds to a central metal cation ( $M^{n+}$ ) to form an octahedral complex, it wraps around the metal using six donor atoms: - Two Nitrogen Atoms: The two tertiary nitrogen atoms of the ethylenediamine core, each containing an unshared pair of electrons ( $2\text{ N}$ ). - Four Oxygen Atoms: One oxygen atom from each of the four deprotonated carboxylate groups ( $-COO^-$ ) ( $4\text{ O}$ ).

Step 3

Classification by Denticity

Because it coordinates simultaneously through six donor sites, it is classified as a hexadentate ligand with four "O" and two "N" donor atoms.

\text{Denticity} = 2\text{ (N donors)} + 4\text{ (O donors)} = 6\;(\text{Hexadentate})

⭐

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

✓ 100% Correct

Verdict & Scientific Reason:

Verbatim NCERT definition of $EDTA^{4-}$ coordination behavior.

💡

Why this is airtight: None; textbook ligand denticity knowledge.

2 2

✗ Incorrect

Verdict & Scientific Reason:

Unidentate ligands bind through only one donor atom (e.g., $Cl^-, H_2O, NH_3$ ).

⚠️

Student Fallacy (Why candidates pick this trap): Confusing multidentate chelate with unidentate ligand.

3 3

✗ Incorrect

Verdict & Scientific Reason:

This describes ethylenediamine ( $en$ , $H_2NCH_2CH_2NH_2$ ), not EDTA.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing ethylenediamine with ethylenediaminetetraacetate.

4 4

✗ Incorrect

Verdict & Scientific Reason:

Describes diethylenetriamine ( $dien$ ), not EDTA.

⚠️

Student Fallacy (Why candidates pick this trap): Misremembering the number of nitrogen donors.

⚠️

NTA Trap Alert: EDTA vs. Ethylene Diamine (en) Trap

Students who focus only on "ethylene diamine" in the name often think of $en$ and choose bidentate with two N donors (Option 3). The suffix "tetraacetate" adds four oxygen donors, making it a hexadentate ligand ( $4\text{ O} + 2\text{ N}$ ).

🛡️ How to avoid the -1 mark: EDTA = Ethylene Diamine (2 N) + Tetra Acetate (4 O) = Hexadentate (6 donors).

📖

Core NEET Syllabus Standard

NCERT Chemistry · Coordination Compounds Standard Benchmark

Core Scientific Principle Tested:

"Ethylenediaminetetraacetate ion (EDTA4-) is an important hexadentate ligand that coordinates via two nitrogen and four oxygen donor atoms."

Syllabus Focus: Definitions of Some Important Terms (Ligands and Denticity)

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NEET 2021 Chemistry Question 71 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ EDTA = 2 Amines + 4 Acetates = Hexadentate (2 N + 4 O)

Step-by-Step Rigorous Workflow & Mathematical Proof

Chemical Structure of EDTA4-

Identifying the Coordinating Donor Atoms

Classification by Denticity

360° Option Elimination Engine Key Feature

NTA Trap Alert: EDTA vs. Ethylene Diamine (en) Trap

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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