Haloalkanes and Haloarenes Class 12 Notes | Reactions, Properties & Mechanisms

Haloalkanes and Haloarenes Class 12 Notes | Reactions, Properties & Mechanisms

Haloalkanes and Haloarenes is an important chapter in Class 12 Chemistry that explores the chemistry of halogen-substituted aliphatic and aromatic compounds. This chapter covers their classification, IUPAC nomenclature, methods of preparation, physical and chemical properties, and crucial reaction mechanisms like nucleophilic substitution (SN1 and SN2), elimination, and electrophilic substitution. These compounds also have industrial and environmental significance, especially in the context of CFCs and pesticides. This comprehensive guide provides easy-to-understand notes, reaction mechanisms, and key concepts to help students score high in board exams and competitive entrance tests like NEET and JEE.

1. SN1 Reaction (Unimolecular Nucleophilic Substitution)

🔸 Mechanism:

SN1 occurs in two steps:

Step 1: Formation of carbocation
R–X → R⁺ + X⁻ (slow step – rate-determining)

Step 2: Nucleophile attacks the carbocation
R⁺ + Nu⁻ → R–Nu

🔸 Characteristics:

Feature	SN1 Reaction
Reaction order	First order (rate depends only on R–X)
Mechanism steps	Two-step
Intermediate	Carbocation
Substrate type	Tertiary > Secondary >> Primary
Stereochemistry	Racemization (loss of optical activity)
Solvent	Polar protic solvents (e.g., H₂O, alcohol)
Example	(CH₃)₃CBr + H₂O → (CH₃)₃COH + HBr

🧪 2. SN2 Reaction (Bimolecular Nucleophilic Substitution)

🔸 Mechanism:

SN2 occurs in one step:

Nu⁻ attacks carbon while X⁻ leaves → Both happen simultaneously

R–X + Nu⁻ → [Transition state] → R–Nu + X⁻

🔸 Characteristics:

Feature	SN2 Reaction
Reaction order	Second order (rate depends on R–X & Nu⁻)
Mechanism steps	One-step (concerted)
Intermediate	No carbocation; forms a transition state
Substrate type	Primary > Secondary >> Tertiary
Stereochemistry	Inversion of configuration (Walden inversion)
Solvent	Polar aprotic (e.g., DMSO, acetone)
Example	CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻

⚖️ SN1 vs SN2 – Summary Table

Feature	SN1	SN2
Reaction Order	1st order	2nd order
Number of Steps	2	1
Intermediate	Carbocation	Transition state
Favored Substrate	Tertiary	Primary
Stereochemistry	Racemization	Inversion (Walden)
Solvent	Polar protic	Polar aprotic
Rate Depends On	Only substrate	Both substrate and nucleophile