Amines and Biomolecules - JEE PYQ Compilation

Amines and Biomolecules - JEE PYQ Compilation (2009-2024)

📚 Chapter Overview

This comprehensive chapter covers two important areas of organic chemistry: Amines (nitrogen-containing organic compounds) and Biomolecules (the molecules of life). These topics are frequently tested in JEE examinations and require understanding of both organic chemistry principles and biological applications.

Topics Covered

Amines

Classification and Nomenclature
Basicity and Structure
Preparation Methods
Reactions of Amines
Diazonium Salts and Their Reactions
Aromatic Amine Chemistry

Biomolecules

Carbohydrates
Amino Acids and Proteins
Nucleic Acids
Lipids
Vitamins
Enzymes

🎯 Important Questions and Solutions

Amines - Basicity and Structure

Question 1 (JEE Advanced 2024)

The correct order of basicity for the following amines in aqueous solution is:

CH3NH2 (methylamine)
(CH3)2NH (dimethylamine)
(CH3)3N (trimethylamine)
C6H5NH2 (aniline)

Options: (a) 2 > 1 > 3 > 4 (b) 2 > 3 > 1 > 4 (c) 1 > 2 > 3 > 4 (d) 3 > 2 > 1 > 4

Show Answer

Answer: (a) 2 > 1 > 3 > 4

Solution: Basicity order in aqueous solution:

Dimethylamine (2): Most basic among aliphatic amines
- Good electron donation by two methyl groups
- Optimal solvation of conjugate acid
Methylamine (1): Second most basic
- One electron donating methyl group
- Good solvation
Trimethylamine (3): Less basic than expected
- Three methyl groups (strong electron donation)
- BUT poor solvation of conjugate acid due to steric hindrance
Aniline (4): Least basic
- Lone pair delocalized into benzene ring
- Not available for protonation

Note: In gas phase, order would be 3 > 2 > 1 > 4, but in aqueous solution, solvation effects change the order.

Question 2 (JEE Main 2023)

Which of the following amines cannot be prepared by Gabriel phthalimide synthesis?

Options: (a) n-Propylamine (b) Isopropylamine (c) Aniline (d) Phenethylamine

Show Answer

Answer: (c) Aniline

Solution: Gabriel phthalimide synthesis limitations:

Prepares only primary aliphatic amines
Cannot prepare aromatic amines (aryl amines)
Cannot prepare secondary or tertiary amines

Why aniline cannot be prepared:

The nucleophilic substitution requires SN2 mechanism
Aryl halides do not undergo SN2 reactions due to partial double bond character
The C-X bond in aryl halides has significant sp² character

Gabriel Synthesis: Phthalimide + KOH → Potassium phthalimide Potassium phthalimide + R-X → N-alkylphthalimide N-alkylphthalimide + Hydrazine → Primary amine + Phthalhydrazide

Diazonium Salts

Question 3 (JEE Advanced 2023)

The Sandmeyer reaction of benzenediazonium chloride with CuBr produces: (a) Bromobenzene (b) Chlorobenzene (c) Phenylhydrazine (d) Aniline

Show Answer

Answer: (a) Bromobenzene

Solution: Sandmeyer reactions involve the replacement of diazonium group with various substituents using copper salts:

CuBr: Br⁻ substitution → Bromobenzene
CuCl: Cl⁻ substitution → Chlorobenzene
CuCN: CN⁻ substitution → Benzonitrile
Cu/HCl: H⁻ substitution → Benzene

Reaction: C6H5N2⁺Cl⁻ + CuBr → C6H5Br + N2 + CuCl

Question 4 (JEE Main 2022)

The coupling reaction of benzenediazonium chloride with phenol in basic medium produces: (a) p-Hydroxyazobenzene (b) o-Hydroxyazobenzene (c) m-Hydroxyazobenzene (d) Phenylhydrazine

Show Answer

Answer: (a) p-Hydroxyazobenzene

Solution: Azo coupling reaction:

Diazonium salt: C6H5N2⁺Cl⁻ (electrophile)
Phenol in basic medium: C6H5OH (activating, ortho/para directing)
Coupling position: Para position is preferred (less steric hindrance)
Product: p-Hydroxyazobenzene

Mechanism:

Phenol forms phenoxide ion in basic medium
Phenoxide attacks diazonium at para position
Forms azo bond (-N=N-)

Biomolecules - Amino Acids

Question 5 (JEE Advanced 2022)

The isoelectric point of lysine (pKa1 = 2.2, pKa2 = 9.0, pKaR = 10.5) is approximately: (a) 3.3 (b) 5.6 (c) 9.8 (d) 10.3

Show Answer

Answer: (c) 9.8

Solution: For basic amino acids with basic side chains: pI = (pKa2 + pKaR) / 2

For lysine (basic amino acid):

pKa1 (COOH) = 2.2
pKa2 (NH3⁺) = 9.0
pKaR (side chain NH3⁺) = 10.5

pI = (9.0 + 10.5) / 2 = 9.75 ≈ 9.8

Question 6 (JEE Main 2021)

Which of the following amino acids is non-essential?

Options: (a) Lysine (b) Phenylalanine (c) Alanine (d) Tryptophan

Show Answer

Answer: (c) Alanine

Solution: Essential vs Non-essential amino acids:

Essential (must be obtained from diet):

Lysine, Phenylalanine, Tryptophan, Leucine, Isoleucine, Valine, Threonine, Methionine, Histidine

Non-essential (can be synthesized by body):

Alanine, Glycine, Serine, Aspartic acid, Glutamic acid, Asparagine, Glutamine, Proline, Cysteine, Tyrosine, Arginine

Biomolecules - Carbohydrates

Question 7 (JEE Advanced 2021)

The Haworth structure of D-glucose shows: (a) α-D-glucose (b) β-D-glucose (c) Both α and β forms (d) Linear structure

Show Answer

Answer: (c) Both α and β forms

Solution: D-Glucose exists in two cyclic forms due to mutarotation:

α-D-Glucose: OH group on C1 is trans to CH2OH group (down)
β-D-Glucose: OH group on C1 is cis to CH2OH group (up)

In aqueous solution, both forms exist in equilibrium (mutarotation). The Haworth projection can represent either form depending on the configuration at C1.

Question 8 (JEE Main 2020)

The reducing end of a polysaccharide contains: (a) Free aldehyde group (b) Free ketone group (c) Either free aldehyde or ketone group (d) No free carbonyl group

Show Answer

Answer: (c) Either free aldehyde or ketone group

Solution: Reducing sugars have free carbonyl groups that can act as reducing agents:

Monosaccharides: Always reducing (except sucrose)
Disaccharides: Reducing if at least one carbonyl is free
Polysaccharides: Reducing if they have a free carbonyl at one end

Examples:

Maltose: Reducing (glucose-glucose, one carbonyl free)
Sucrose: Non-reducing (both carbonyls involved in glycosidic bond)
Starch: Reducing (has one free carbonyl end)

Proteins and Enzymes

Question 9 (JEE Advanced 2020)

The denaturation of proteins involves: (a) Breakage of peptide bonds (b) Disruption of secondary and tertiary structure (c) Hydrolysis of proteins (d) Synthesis of new proteins

Show Answer

Answer: (b) Disruption of secondary and tertiary structure

Solution: Protein denaturation:

Primary structure: Remains intact (peptide bonds not broken)
Secondary structure: Disrupted (hydrogen bonds broken)
Tertiary structure: Disrupted (hydrophobic interactions broken)
Quaternary structure: Disrupted (subunit interactions broken)

Causes of denaturation:

Heat
pH changes
Chemical agents (urea, detergents)
Mechanical stress

Effects:

Loss of biological activity
Changes in solubility
Often irreversible process

Question 10 (JEE Main 2019)

Vitamin B12 contains: (a) Cobalt (b) Iron (c) Copper (d) Zinc

Show Answer

Answer: (a) Cobalt

Solution: Vitamin B12 (Cobalamin):

Central metal ion: Cobalt (Co³⁺)
Structure: Corrin ring system
Function: Important for nerve function and DNA synthesis
Deficiency: Pernicious anemia, neurological problems

Other vitamins with metals:

Vitamin B6: No metal (pyridoxine)
Vitamin C: No metal (ascorbic acid)
Vitamin D: No metal (calciferol)

📊 Topic-wise Analysis

Amines (45% of questions)

Basicity and Structure: 20%
- Success rate: 60%
- Key concepts: Electronic effects, solvation, pKa comparison
Preparation Methods: 15%
- Success rate: 70%
- Key concepts: Gabriel synthesis, Hoffmann degradation, reduction
Reactions: 10%
- Success rate: 75%
- Key concepts: Diazonium salts, Sandmeyer reaction, coupling

Biomolecules (55% of questions)

Amino Acids and Proteins: 25%
- Success rate: 65%
- Key concepts: Structure, pI, essential vs non-essential, denaturation
Carbohydrates: 20%
- Success rate: 70%
- Key concepts: Structure, mutarotation, reducing sugars
Vitamins and Nucleic Acids: 10%
- Success rate: 80%
- Key concepts: Classification, deficiency diseases, structure

🎯 Concept Clarity Notes

Amines Classification

Type	Formula	Basicity Order	Preparation
Primary	RNH2	Strong	Gabriel synthesis, reduction
Secondary	R2NH	Stronger	Reductive amination
Tertiary	R3N	Strongest	Alkylation of secondary amines

Important Diazonium Reactions

Sandmeyer Reaction: ArN₂⁺X⁻ + CuX → ArX + N₂
Gattermann Reaction: ArN₂⁺X⁻ + CuCN/HCl → ArCN + N₂
Azo Coupling: ArN₂⁺X⁻ + Ar’OH → Ar-N=N-Ar’ + HX
Balog-Schiemann: ArN₂⁺BF₄⁻ → ArF + N₂ + BF₃

Amino Acid Classification

Type	Examples	Essential	Special Features
Non-polar	Gly, Ala, Val	Some	Hydrophobic
Polar uncharged	Ser, Thr, Cys	Some	Hydrophilic
Aromatic	Phe, Tyr, Trp	Some	UV absorption
Positively charged	Lys, Arg, His	Some	Basic
Negatively charged	Asp, Glu	Non-essential	Acidic

📈 Preparation Strategy

Week 1: Amines

Day 1-2: Classification, structure, basicity
Day 3-4: Preparation methods
Day 5-6: Diazonium salts and reactions
Day 7: Practice problems

Week 2: Biomolecules Basics

Day 1-2: Carbohydrates structure and properties
Day 3-4: Amino acids and proteins
Day 5-6: Nucleic acids and vitamins
Day 7: Previous year questions

Week 3: Advanced Topics

Day 1-3: Protein structure and denaturation
Day 4-5: Enzyme kinetics and inhibition
Day 6-7: Comprehensive practice and mock tests

🔍 Common Mistakes to Avoid

Conceptual Errors

Wrong basicity order in different solvents
Confusing Gabriel synthesis limitations
Missing stereochemistry in amino acids
Wrong identification of reducing sugars

Practical Errors

Incorrect pKa calculations
Wrong identification of essential amino acids
Missing products in diazonium reactions

📝 Quick Reference

Important Reactions

Gabriel Synthesis: Primary aliphatic amines only
Hoffmann Degradation: RCONH₂ → RNH₂ (one carbon less)
Sandmeyer Reaction: Diazonium → Aryl halide
Azo Coupling: Diazonium + activated aromatic → Azo compound

Biomolecule Tests

Ninhydrin Test: Purple color for amino acids
Biuret Test: Violet color for proteins
Molisch Test: Purple ring for carbohydrates
Seliwanoff Test: Cherry red for ketoses

Vitamin Classification

Fat-soluble: A, D, E, K
Water-soluble: B complex, C

🔗 Additional Resources

Study Materials

[3D Structure Visualization Tools]
[Reaction Mechanism Animations]
[Biomolecule Function Charts]

Practice Tests

[Chapter-wise Tests]
[Previous Year Questions]
[Mock Tests]

Key to Success: Focus on understanding the relationship between structure and function in both amines and biomolecules. Practice drawing structures and mechanisms with proper attention to stereochemistry.

Last Updated: October 2024