Haloalkanes Haloarenes Notes PDF – Chemistry Revision

Haloalkanes and haloarenes are organic compounds containing halogen atoms. They play a vital role in synthetic chemistry, pharmaceuticals, and agrochemicals. This chapter explores their structure, nomenclature, preparation, properties, and reactions.

Classification

Based on Number of Halogen Atoms

• Mono-halo compounds: One halogen atom (e.g., CH₃Cl)
• Di-halo compounds: Two halogen atoms (e.g., CH₂Cl₂)
• Tri-halo compounds: Three halogen atoms (e.g., CHCl₃)

Based on Type of Carbon

Type	Example	Hybridization
Haloalkanes	CH₃CH₂Cl	sp³ (alkyl)
Haloarenes	C₆H₅Cl	sp² (aryl)

General Formula

Haloalkanes: R–X (R = alkyl group, X = halogen)

Haloarenes: Ar–X (Ar = aryl group)

Nomenclature

IUPAC Rules

• Prefix: Halogen name (fluoro-, chloro-, bromo-, iodo-)
• Root: Longest carbon chain
• Numbering: Lowest locant for halogen

Examples:

• CH₃CH₂Cl → Chloroethane
• C₆H₅Br → Bromobenzene

Preparation Methods

 From Alcohols

R–OH + HX → R–X + H₂O Reagents: PCl₅, SOCl₂, HBr

Halogenation of Alkanes

CH₄ + Cl₂ → CH₃Cl + HCl (in sunlight)

Sandmeyer Reaction (for haloarenes)

Ar–NH₂ → Ar–X via diazonium salt

Physical Properties

Property	Haloalkanes	Haloarenes
Boiling Point	Increases with mass	Higher than alkyls
Solubility	Insoluble in water	Soluble in organics
Density	Higher than water	Moderate

Chemical Reactions

Nucleophilic Substitution

SN1: Two-step, tertiary alkyl halides

SN2: One-step, primary alkyl halides

Example: CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻

Elimination Reaction

Dehydrohalogenation: CH₃CH₂Br + alc. KOH → CH₂=CH₂ + HBr

Electrophilic Substitution (Haloarenes)

Nitration: C₆H₅Cl + HNO₃ → C₆H₄ClNO₂

Sulphonation: C₆H₅Cl + H₂SO₄ → C₆H₄ClSO₃H

Mnemonics

Halogen Series

F, Cl, Br, I

Mnemonic: "Funny Clowns Bring Ice"

Reactivity Order (SN1)

3° > 2° > 1°

Mnemonic: "Tigers Roar Louder"

Applications

Haloalkanes: Refrigerants (CFCs), solvents, anesthetics

Haloarenes: Dyes, pesticides, pharmaceuticals

Environmental Impact

CFCs: Cause ozone depletion

Some haloarenes: Toxic and persistent

Quick Revision Table

Feature	Haloalkanes	Haloarenes
Carbon Type	sp³ (alkyl)	sp² (aryl)
Reactivity	High (SN reactions)	Low (due to resonance)
Substitution Type	SN1/SN2	Electrophilic
Preparation	From alcohols, alkanes	From diazonium salts
Uses	Solvents, anesthetics	Dyes, drugs

Final Tips for Students

• Focus on reaction mechanisms and naming rules
• Practice SN1 vs SN2 distinctions
• Use mnemonics for halogen order and reactivity
• Understand resonance effects in haloarenes