Halogen Derivatives of Alkanes

B.Sc. 3rd Semester: Halogen Derivatives of Alkanes

Halogen derivatives of alkanes, commonly called haloalkanes or alkyl halides, are organic compounds formed when one or more hydrogen atoms in an alkane are substituted by halogen atoms such as fluorine, chlorine, bromine, or iodine. Their general molecular formula is CnH2n+1X, where X represents the halogen atom.

In haloalkanes, the halogen atom is bonded to a carbon atom that is sp³-hybridized, meaning the carbon has four single bonds arranged tetrahedrally. This carbon-halogen bond (C–X) is a polar covalent bond due to the higher electronegativity of halogens compared to carbon. The polarity of the bond affects the physical and chemical properties of the compound.

Compared to alkanes, haloalkanes have higher boiling points and are generally more reactive, given the polarity of the C–X bond which allows them to undergo various substitution and elimination reactions.

Mono and Dihalo Derivatives of Alkanes

General Methods of Preparation

• Free Radical Halogenation: Alkanes react with halogens (Cl₂, Br₂) in presence of UV light to substitute hydrogen with halogen atoms forming mono and dihaloalkanes.
  RH + X2 → RX + HX
• Addition to Alkenes: Halogens add across double bonds to form vicinal dihaloalkanes.
• From Alcohols: Alcohols react with HX or reagents like PCl₅, SOCl₂ to replace -OH by halogen.
• Halogen Exchange Reactions: Exchange of halogen atoms using metal halides (Finkelstein reaction).

➤ Methods of Preparation in details

Chemical Properties

• Undergo nucleophilic substitution (replacement of halogen) and elimination reactions.
• More reactive than parent alkanes due to polar C-X bond.
• They can undergo further substitution to form dihalo derivatives.

➤ Chemical Properties in details

➤ Mechanism of Substitution Reactions

➤ Mechanism of Elimination Reactions

Uses

• Mono haloalkanes: Used as solvents, refrigerants, and intermediates in organic synthesis.
• Dihaloalkanes: Used in polymer synthesis and pharmaceuticals.
• Some haloalkanes serve as pesticides and anesthetics.