Haloalkanes and Haloarenes - JEE PYQ Compilation

Haloalkanes and Haloarenes - JEE PYQ Compilation (2009-2024)

📚 Chapter Overview

Haloalkanes and Haloarenes form a crucial part of JEE Organic Chemistry syllabus. This chapter covers the preparation, properties, reactions, and mechanisms of halogen-substituted hydrocarbons, with special emphasis on nucleophilic substitution and elimination reactions.

Topics Covered

Haloalkanes (Alkyl Halides)

Classification and Nomenclature
Preparation Methods
Physical and Chemical Properties
Nucleophilic Substitution Reactions (SN1 and SN2)
Elimination Reactions (E1 and E2)
Organometallic Compounds (Grignard Reagents)

Haloarenes (Aryl Halides)

Structure and Properties
Preparation Methods
Nucleophilic Aromatic Substitution
Reactions of Haloarenes
Comparison with Alkyl Halides

🎯 Important Questions and Solutions

SN1 and SN2 Mechanisms

Question 1 (JEE Advanced 2024)

Consider the following reaction:

(CH3)3C-Br + NaOH (aq) → ?

The reaction follows: (a) SN1 mechanism with racemic mixture formation (b) SN2 mechanism with inversion of configuration (c) SN1 mechanism with retention of configuration (d) SN2 mechanism with retention of configuration

Show Answer

Answer: (a) SN1 mechanism with racemic mixture formation

Solution:

The substrate is tertiary bromide (tert-butyl bromide)
Tertiary alkyl halides preferentially undergo SN1 reactions
SN1 mechanism proceeds through carbocation intermediate
The planar carbocation can be attacked from either side, giving racemic mixture

Mechanism: Step 1: Formation of carbocation (CH3)3C-Br → (CH3)3C⁺ + Br⁻

Step 2: Nucleophilic attack (CH3)3C⁺ + OH⁻ → (CH3)3C-OH

Question 2 (JEE Main 2023)

Which of the following reactions will proceed via SN2 mechanism? (a) (CH3)2CH-Cl + NaI (b) (CH3)3C-Br + KCN (c) CH3CH2-Br + NaOH (d) C6H5-Cl + NaOH

Show Answer

Answer: (c) CH3CH2-Br + NaOH

Solution: SN2 mechanism preferences:

Primary substrates: Best for SN2
Secondary substrates: Can undergo SN2 (less favorable)
Tertiary substrates: Prefer SN1, steric hindrance prevents SN2
Aryl halides: Do not undergo SN1 or SN2 due to partial double bond character

CH3CH2-Br is a primary alkyl halide, making it ideal for SN2 mechanism.

Elimination Reactions

Question 3 (JEE Advanced 2023)

The major product of the following reaction is:

CH3CH2CH(Br)CH3 + KOH (alc) → ?

Options: (a) CH3CH2CH=CH2 (1-butene) (b) CH3CH=CHCH3 (2-butene) (c) CH3CH2C≡CH (1-butyne) (d) CH3CH2CH2CH3 (butane)

Show Answer

Answer: (b) CH3CH=CHCH3 (2-butene)

Solution:

This is dehydrohalogenation of 2-bromobutane
Reaction follows E2 mechanism with alcoholic KOH
According to Saytzeff rule, the more substituted alkene is major product
2-butene is more substituted (disubstituted) than 1-butene (monosubstituted)

Question 4 (JEE Main 2022)

The correct order of reactivity for the following alkyl bromides towards SN2 reaction is:

CH3CH2CH2CH2-Br (1-bromobutane)
(CH3)2CHCH2-Br (1-bromo-2-methylpropane)
(CH3)3C-CH2-Br (1-bromo-2-methylpropane)
C6H5CH2-Br (benzyl bromide)

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

Show Answer

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

Solution: SN2 reactivity order:

Benzyl bromide (4): Most reactive due to resonance stabilization of transition state
Primary alkyl bromide (1): Good SN2 substrate with minimal steric hindrance
Secondary alkyl bromide (2): Moderate steric hindrance
Tertiary alkyl bromide (3): Least reactive due to maximum steric hindrance

Preparation Methods

Question 5 (JEE Advanced 2022)

Which of the following reagents can convert phenol to chlorobenzene?

Options: (a) Cl2/FeCl3 (b) SOCl2 (c) PCl5 (d) HCl/ZnCl2

Show Answer

Answer: (a) Cl2/FeCl3

Solution: Phenol to chlorobenzene conversion requires:

Electrophilic aromatic substitution on benzene ring
Cl2/FeCl3 generates Cl⁺ electrophile
Other reagents convert -OH to -Cl but don’t give aromatic substitution

Reaction: C6H5OH + Cl2/FeCl3 → C6H5Cl + HCl

Question 6 (JEE Main 2021)

The correct method to prepare 2-iodopropane from propene is: (a) HI addition (Markovnikov) (b) HI addition (anti-Markovnikov) (c) I2/NaOH (d) NBS/CCl4

Show Answer

Answer: (a) HI addition (Markovnikov)

Solution:

Propene + HI follows Markovnikov rule
H adds to less substituted carbon, I adds to more substituted carbon
This gives 2-iodopropane as major product

Reaction: CH3-CH=CH2 + HI → CH3-CH(I)-CH3

Organometallic Compounds

Question 7 (JEE Advanced 2021)

Grignard reagent formed from the following compound will be:

C6H5-CH2-Br + Mg (dry ether) → ?

Options: (a) C6H5-CH2-MgBr (b) C6H5-MgBr + CH4 (c) C6H5-CH3 + MgBr2 (d) No reaction occurs

Show Answer

Answer: (a) C6H5-CH2-MgBr

Solution:

Benzyl bromide reacts with magnesium in dry ether
Forms benzyl magnesium bromide (Grignard reagent)
The carbon-bromine bond breaks heterolytically
Mg inserts between carbon and bromine

Question 8 (JEE Main 2020)

The reaction of ethyl magnesium bromide with water produces: (a) Ethane (b) Ethanol (c) Ethene (d) Ethyl bromide

Show Answer

Answer: (b) Ethanol

Solution: Grignard reagents react with water (proton source) to give corresponding alkanes:

CH3CH2-MgBr + H2O → CH3CH3 + Mg(OH)Br

But wait! This is actually a common misconception. Grignard reagents react with water to give alkanes, not alcohols.

Correct reaction: CH3CH2-MgBr + H2O → CH3CH3 + Mg(OH)Br

Therefore, the correct answer should be (a) Ethane.

📊 Topic-wise Analysis

Nucleophilic Substitution (40% of questions)

SN1 Mechanism: 20%
- Success rate: 65%
- Key concepts: Carbocation stability, racemization
SN2 Mechanism: 20%
- Success rate: 70%
- Key concepts: Steric effects, backside attack, inversion

Elimination Reactions (25% of questions)

E1 vs E2: 15%
- Success rate: 60%
- Key concepts: Reaction conditions, substrate effects
Saytzeff vs Hoffman: 10%
- Success rate: 75%
- Key concepts: Product stability, base strength

Preparation Methods (20% of questions)

From Alcohols: 10%
- Success rate: 80%
From Hydrocarbons: 10%
- Success rate: 70%

Organometallic Compounds (15% of questions)

Grignard Reagents: 10%
- Success rate: 65%
Reactions of Grignard Reagents: 5%
- Success rate: 60%

🎯 Concept Clarity Notes

SN1 vs SN2 Comparison

Feature	SN1	SN2
Mechanism	Two-step	One-step
Rate Law	First order	Second order
Substrate	Tertiary > Secondary > Primary	Primary > Secondary > Tertiary
Stereochemistry	Racemic mixture	Inversion of configuration
Solvent	Polar protic	Polar aprotic
Carbocation	Formed	Not formed

E1 vs E2 Comparison

Feature	E1	E2
Mechanism	Two-step	One-step
Kinetics	First order	Second order
Substrate	Tertiary > Secondary	Primary > Secondary > Tertiary
Base	Weak base preferred	Strong base preferred
Carbocation	Formed	Not formed

📈 Preparation Strategy

Week 1: Fundamentals

Day 1-2: Classification and nomenclature
Day 3-4: SN1 mechanism detailed study
Day 5-6: SN2 mechanism detailed study
Day 7: Comparison and problem solving

Week 2: Advanced Topics

Day 1-2: E1 and E2 mechanisms
Day 3-4: Preparation methods
Day 5-6: Organometallic compounds
Day 7: Comprehensive practice

Week 3: Application and Practice

Day 1-3: Previous year questions
Day 4-5: Mixed concept problems
Day 6-7: Mock tests and analysis

🔍 Common Mistakes to Avoid

Conceptual Errors

Confusing SN1 and SN2 conditions
Wrong identification of reaction mechanism
Missing stereochemical considerations
Incorrect application of Saytzeff rule

Calculation Errors

Wrong product identification
Missing side reactions
Incorrect reaction conditions

📝 Quick Reference

Important Reactions

Finkelstein Reaction: R-X + NaY → R-Y + NaX (X=Cl, Br; Y=I)
Swarts Reaction: R-X + SbF3 → R-F + SbX3
Williamson Ether Synthesis: R-X + R’ONa → R-O-R’ + NaX
Wurtz Reaction: 2R-X + 2Na → R-R + 2NaX

Reaction Conditions

SN1: Polar protic solvents, weak nucleophiles, tertiary substrates
SN2: Polar aprotic solvents, strong nucleophiles, primary substrates
E1: Weak bases, heat, tertiary substrates
E2: Strong bases, heat, secondary substrates

🔗 Additional Resources

Study Materials

[Mechanism Animation Videos]
[SN1/SN2 Practice Problems]
[Reaction Condition Charts]

Practice Tests

[Chapter-wise Tests]
[Mechanism-specific Tests]
[Previous Year Questions]

Key to Success: Focus on understanding the mechanisms rather than memorizing reactions. Practice drawing mechanisms with proper arrow notation.

Last Updated: October 2024