Haloalkanes and Haloarenes

Alkyl halides (haloalkanes) and aryl halides (haloarenes) are formed when one or more hydrogen atoms in an aliphatic or aromatic hydrocarbon are replaced by halogen atoms. In haloalkanes, the halogen atom(s) are bonded to a sp³-hybridized carbon atom of an alkyl group, while in haloarenes, the halogen atom(s) are attached to sp²-hybridized carbon atom(s) of an aryl group.

Haloalkanes and haloarenes are organic compounds in which halogen atoms replace one or more hydrogen atoms in hydrocarbons. The key difference between the two lies in their origin: haloalkanes are derived from open-chain hydrocarbons (alkanes), whereas haloarenes originate from aromatic hydrocarbons.

Haloalkanes, also called alkyl halides, involve halogens bonded to sp³-hybridized carbon atoms. In contrast, haloarenes (aryl halides) have halogens attached to sp²-hybridized carbon atoms in aromatic rings. This difference in hybridisation influences their chemical properties, making haloalkanes and haloarenes more reactive than their parent hydrocarbons. These compounds are extensively used in medicine, industry, and agriculture.

1.0Classification of Haloalkanes and Haloarenes

1. Based on the Number of Halogens

• Mono Haloalkanes and Mono Haloarenes: Contain one halogen atom.
• Poly Haloalkanes and Poly Haloarenes: Contain two or more halogen atoms:
• • Dihalogen Compounds: Two halogen atoms.
  • Trihalogen Compounds: Three halogen atoms.
  • Tetrahalogen Compounds: Four halogen atoms.
  • Further extensions include penta-, hexa-, septa-, octa-, nona-, and deca-halogen compounds.

2. Based on the Hybridization of Carbon

• Halogen Attached to sp³-Hybridized Carbon:
• • Alkyl Halides: Halogen bonded to an alkyl chain.
  • Allylic Halides: Halogen attached to sp³ carbon adjacent to a double bond (C=C).
  • Benzylic Halides: Halogen linked to a benzene ring via sp³ carbon.
• Halogen Attached to sp²-Hybridized Carbon:
• • Vinyl Halides: Halogen bonded directly to sp² carbon in a double bond (R-CH=CHX).
  • Aryl Halides: Halogen is attached directly to the aromatic ring.

3. Based on the Nature of the Carbon-Halogen Bond

• Primary Alkyl Halides: Halogen bonded to a primary carbon atom.
• Secondary Alkyl Halides: Halogen bonded to a secondary carbon atom.
• Tertiary Alkyl Halides: Halogen bonded to a tertiary carbon atom.

2.0Nomenclature of Haloalkanes and Haloarenes

1. IUPAC Nomenclature
The name includes a prefix for the halogen and the parent hydrocarbon’s name, with suffixes like “-ane” (alkane) or “-benzene” (arene).

• Example:
• CH₃Cl: Chloromethane
• C₆H₅Cl: Chlorobenzene

2. Common Names
Common names describe the halogen as a suffix with the parent hydrocarbon.

• Example:
• CH₃Cl: Methyl chloride
• C₆H₅Cl: Phenyl chloride

3.0Preparation of Haloalkanes

1. From Alcohol

• Alcohols react with halogen acids (HX), phosphorus halides (PX₃), or thionyl chloride (SOCl₂) to form haloalkanes.
• Example: R-OH + SOCl₂ → R-Cl + SO₂ + HCl.
• Tertiary alcohols react with HCl at room temperature, while primary and secondary alcohols require zinc chloride as a catalyst.

2. From Hydrocarbons

• Alkanes: Undergo free radical halogenation in the presence of light.
• Alkenes: React with HX via electrophilic addition, following the HI > HBr > HCl > HF reactivity order.

3. By Halogen Exchange (Finkelstein Reaction)

• The reaction of alkyl halides with NaI in dry acetone forms alkyl iodides.
• Example: R-Cl + NaI → R-I + NaCl.

4.0Preparation of Haloarenes

1. From Arenes

• Substitution of hydrogen in aromatic rings with halogens using halogenating agents (e.g., Cl₂, Br₂).

2. Sandmeyer Reaction

• Diazonium salts react with CuCl or CuBr to yield chlorobenzene or bromobenzene.
• Example: C₆H₅N₂⁺Cl⁻ + CuCl → C₆H₅Cl.

5.0Physical Properties of Haloalkanes and Haloarenes

• Haloalkanes are colourless, heavier than alkanes, and slightly soluble in water but dissolve in organic solvents.
• Haloarenes are crystalline solids with boiling points and densities that increase with the number of halogens.
• Order of properties:
• Electronegativity: F > Cl > Br > I.
• Bond Length: C-F < C-Cl < C-Br < C-I.
• Boiling Points: Iodo > Bromo > Chloro > Fluoro derivatives.

6.0Chemical Properties of Haloalkanes and Haloarenes

• Nucleophilic Substitution: Reactions with nucleophiles yield alcohols, ethers, etc.
• Elimination Reactions: Dehydrohalogenation with alcoholic KOH forms alkenes (following Saytzeff’s rule).
• Reaction with Metals: Forms Grignard reagents or higher alkanes in dry ether.
• Reduction: Reducing agents like LiAlH₄ convert haloalkanes to alkanes.

7.0Uses of Haloalkanes and Haloarenes

• Solvents: Dissolve non-polar substances.
• Medicines: Chloramphenicol (typhoid treatment) and chloroquine (malaria treatment).
• Pesticides: DDT.
• Refrigerants: Chlorofluorocarbons (CFCs) used in refrigeration but harmful to the ozone layer