Organohalides

Introduction to Organohalides

Organohalides are organic compounds containing at least one halogen atom (fluorine, chlorine, bromine, or iodine) covalently bonded to a carbon atom. These compounds are important in both natural and synthetic chemistry, serving as intermediates, solvents, pharmaceuticals, and agrochemicals.

• Definition: An organohalide (or alkyl halide) is an organic molecule in which one or more hydrogen atoms have been replaced by halogen atoms.

• General formula: , where R is an alkyl or aryl group and X is a halogen.

• Examples:

  • Halothane (an inhaled anesthetic)

  • Tetrafluoropropene (a refrigerant)

  • Sucralose (an artificial sweetener)

Structures and Nomenclature of Alkyl Halides

Systematic Naming of Alkyl Halides

Alkyl halides are named by identifying the parent hydrocarbon and the halogen substituent(s). The position of the halogen is indicated by a number, and multiple substituents are listed alphabetically.

• Number the carbon chain to give the halogen the lowest possible number.

• Examples:

  • 2-Bromo-2,4-dimethylheptane: The bromine is attached to carbon 2, with methyl groups at carbons 2 and 4.

  • 2-Bromo-4,5-dimethylheptane: Bromine at carbon 2, methyl groups at carbons 4 and 5.

  • 1-Bromo-3-chloro-4-methylpentane: Bromine at carbon 1, chlorine at carbon 3, methyl at carbon 4.

  • 2-Bromo-5-methylhexane: Bromine at carbon 2, methyl at carbon 5. Note: Not named as 5-bromo-2-methylhexane.

Common Names and Alkyl Groups

• Common names use the alkyl group name followed by the halide (e.g., isopropyl chloride for 2-chloropropane).

• Cyclic examples: Cyclohexyl bromide (bromocyclohexane).

Bonding and Reactivity of Alkyl Halides

Electrophilic Carbon

The carbon atom bonded to the halogen is electrophilic due to the polar nature of the C–X bond. The halogen is more electronegative, creating a partial positive charge () on the carbon and a partial negative charge () on the halogen.

• Electrophilic carbon: Susceptible to attack by nucleophiles.

• Bond polarity:

Preparation of Alkyl Halides

Radical Halogenation of Alkanes

Alkyl halides can be synthesized by the radical halogenation of alkanes, typically using chlorine or bromine under UV light.

• Initiation step: Formation of halogen radicals by homolytic cleavage (e.g., ).

• Propagation steps: Halogen radical abstracts a hydrogen atom, forming an alkyl radical, which then reacts with another halogen molecule.

• Termination steps: Combination of radicals to form stable molecules.

Example: Chlorination of Methane

• Overall reaction:

• Further chlorination can yield , , and .

Table: Steps in Radical Chlorination of Methane

• Regioselectivity: The type of hydrogen (primary, secondary, tertiary) affects the product distribution. Tertiary hydrogens are more reactive than secondary, which are more reactive than primary.

• Relative reactivity: Tertiary > Secondary > Primary

Additional info:

• Halogenation is less selective with chlorine (more products) and more selective with bromine (favors substitution at the most substituted carbon).