Haloalkanes & Haloarenes — Class 12 Chemistry

Section 1

⚗️ Introduction to Haloalkanes & Haloarenes

Why Study Haloalkanes?

From the anaesthetic that numbs pain during surgery (halothane: CF₃CHClBr) to the antibiotic chloramphenicol that fights deadly infections — halogen-containing organic compounds are woven into medicine, industry, and everyday life. Understanding their chemistry unlocks an entire world of organic reactions.

What Are Haloalkanes?

A haloalkane (also called an alkyl halide) is an organic compound in which one or more hydrogen atoms of an alkane have been replaced by halogen atoms (F, Cl, Br, or I). The general formula is R—X, where R is an alkyl group and X is a halogen.

What Are Haloarenes?

A haloarene (also called an aryl halide) is a compound in which one or more halogen atoms are directly bonded to an aromatic ring. The general formula is Ar—X, where Ar represents an aryl group such as C₆H₅—.

Why Are These Compounds Important?

Solvents: Dichloromethane (CH₂Cl₂) is widely used in paint strippers and degreasers.
Anaesthetics: Halothane (CF₃CHClBr) was one of the first safe inhaled anaesthetics.
Antibiotics: Chloramphenicol contains a —CHCl₂ group and treats bacterial eye infections.
Refrigerants: Freon-12 (CCl₂F₂) was used in air conditioners before its ban due to ozone depletion.
Synthetic Intermediates: Alkyl halides serve as starting materials for making alcohols, ethers, amines, and countless other compounds.

The Nature of the C—X Bond

The carbon–halogen bond is polar because halogens are more electronegative than carbon. This makes the carbon atom electrophilic (electron-poor), which is precisely why nucleophiles attack it so readily.

In haloalkanes, the carbon bearing the halogen is sp³ hybridised — it has a tetrahedral geometry. In haloarenes (like chlorobenzene), the carbon bonded to the halogen is sp² hybridised — it sits within a planar aromatic ring, which gives these compounds very different reactivity.

If the C—X bond is polar, why don't haloalkanes dissolve in water (which is also polar)? We'll explore this puzzle in Section 4!

Section 2

🏷️ Classification & IUPAC Nomenclature

Classification by Number of Halogen Atoms

Monohaloalkanes: Contain one halogen atom — e.g., CH₃Cl (chloromethane)
Dihaloalkanes: Contain two halogen atoms — e.g., CH₂Cl₂ (dichloromethane)
Polyhaloalkanes: Contain three or more halogen atoms — e.g., CHCl₃ (trichloromethane)

Classification by Type of Carbon Bearing the Halogen

Primary (1°): The halogen is on a carbon bonded to at most one other carbon — e.g., CH₃CH₂Cl
Secondary (2°): The halogen is on a carbon bonded to exactly two other carbons — e.g., CH₃CHClCH₃
Tertiary (3°): The halogen is on a carbon bonded to three other carbons — e.g., (CH₃)₃CCl

Special Halide Types

Allylic halide: Halogen on a carbon adjacent to a C═C double bond — e.g., CH₂═CH—CH₂Cl
Benzylic halide: Halogen on a carbon adjacent to an aromatic ring — e.g., C₆H₅CH₂Cl
Vinylic halide: Halogen directly on a doubly-bonded carbon — e.g., CH₂═CHCl

Don't confuse allylic with vinylic! An allylic halide has the halogen on the carbon next to the double bond, while a vinylic halide has it on the doubly-bonded carbon itself. Vinylic halides are far less reactive in nucleophilic substitution.

IUPAC Naming Rules

Select the longest continuous carbon chain that includes the carbon bearing the halogen.
Number the chain so that the halogen gets the lowest possible locant.
Name the halogen as a prefix: fluoro-, chloro-, bromo-, iodo-.
Use multiplying prefixes (di-, tri-, tetra-) for multiple identical halogens.

Nomenclature Reference Table

Structural Formula	IUPAC Name	Common Name
CH₃CH₂Cl	Chloroethane	Ethyl chloride
CH₃CHBrCH₃	2-Bromopropane	Isopropyl bromide
(CH₃)₃CBr	2-Bromo-2-methylpropane	tert-Butyl bromide
CHCl₃	Trichloromethane	Chloroform
CCl₄	Tetrachloromethane	Carbon tetrachloride
C₆H₅Cl	Chlorobenzene	—

When two different halogens are present, list them in alphabetical order as prefixes. For example, CH₂BrCl is bromochloromethane, not chlorobromomethane.

Section 3

🔧 Methods of Preparation

1. From Alcohols

Alcohols (R—OH) can be converted to haloalkanes by replacing the —OH group with a halogen atom using various reagents:

Using Hydrogen Halides

R—OH + HX → R—X + H₂O
Reactivity order of HX: HI > HBr > HCl

Using Thionyl Chloride (Best Method)

R—OH + SOCl₂ → R—Cl + SO₂↑ + HCl↑
Both by-products are gases — pure product is obtained directly!

Using Phosphorus Halides

R—OH + PCl₅ → R—Cl + POCl₃ + HCl
3R—OH + PCl₃ → 3R—Cl + H₃PO₃

Thionyl chloride (SOCl₂) is often called the best reagent for preparing chloroalkanes from alcohols because both by-products (SO₂ and HCl) escape as gases, leaving behind a pure product with no tedious purification needed.

2. From Alkenes

Addition of Hydrogen Halides (Markovnikov)

CH₃CH═CH₂ + HBr → CH₃CHBrCH₃ (major, 2-bromopropane)
Markovnikov's rule: H adds to C with more H atoms; halogen adds to C with fewer H atoms.

Addition of Halogens

CH₂═CH₂ + Br₂ → CH₂BrCH₂Br (1,2-dibromoethane)

3. Halogenation of Alkanes (Free Radical)

Free Radical Substitution (requires UV light or heat)

R—H + X₂ —^UV→ R—X + HX
Mechanism: Initiation → Propagation → Termination (free radical chain)

4. Named Reactions for Halide Interchange

Sandmeyer Reaction

ArN₂⁺Cl⁻ + CuCl → ArCl + N₂↑
ArN₂⁺Cl⁻ + CuBr → ArBr + N₂↑
Converts diazonium salts to aryl halides using Cu₂Cl₂/Cu₂Br₂

Finkelstein Reaction

R—Cl + NaI —^acetone→ R—I + NaCl↓
NaCl is insoluble in acetone and precipitates, pushing the equilibrium forward (Le Chatelier's principle).

Swarts Reaction

R—Cl + AgF → R—F + AgCl↓
Used to prepare fluoroalkanes — AgCl precipitates, driving the reaction forward.

Notice the clever trick in both the Finkelstein and Swarts reactions: a product precipitates out of solution, continuously pulling the equilibrium toward the product side. Le Chatelier's principle in action!